Saturation of space weathering in shaping lunar regolith particle morphology

Domain: neuroscience
Published: 2026-01-31
Editorial outcome: minor_revision

Abstract
The lunar mare regolith preserves tripartite records of volcanism, impacting, and space weathering. However, previous studies based on limited soil particle numbers were hindered by issues of sample representativeness. Here we conduct micro-CT scans of bulk soil samples from Chang’e-5 (nearside) and Chang’e-6 (farside), and develop machine learning-based image segmentation and classification methods to identify a vast number of basalt, agglutinate, breccia, and monomineralic particles. The Chang’e-5 basalt exhibits higher plagioclase content than Chang’e-6, while agglutinates from Chang’e-6 have lower void ratios, respectively indicating different lava origins and more intense micrometeorite bombardment for farside Chang’e-6. Despite their contrasting volcanic and impacting histories, the soil particles for these youngest nearside/farside samples exhibit similar morphometric distributions, suggesting that space weathering reached saturation in shaping surficial soil particle morphology in ~2.2 million years or less. These findings may extend to other mare regions and help establish space weathering models for other airless bodies.

Introduction
The lunar mare regolith has been progressively shaped by continuous space weathering since the Moon’s formation1,2. From the sampling efforts of the Apollo, Luna, and Chang’e missions, it is now well established that the mare regolith comprises primarily basalt, meteorite impact melt breccia, micrometeorite impact agglutinates, and monomineralic fragments3–5. The mineralogical and chemical compositions of mare basalts are directly linked to the nature of initial igneous magmatic processes—mantle source melting, the extent of magmatic fractional crystallization in the magma chamber, and degassing during volcanic eruption6–10—while their internal vesicle structures provide clues to volcanic eruption dynamics and the relative distance from source vents11. Following flow emplacement, the mare surface has undergone prolonged modification by meteorite impacts and solar wind irradiation12–14. During impacting, the lunar regolith is imparted with cumulative kinetic energy, which progressively fragments and abrades the lunar bedrock, a process referred to here as “comminution”15. Accompanying impacts, molten glass produced by micrometeorite collisions aggregates finer regolith particles, forming agglutinates16. Agglutinates are irregularly shaped and contain vesicles that not only modulate volatile escape and encode the thermomechanical conditions of micrometeorite bombardment17, but also serve as nanoscale reservoirs capable of storing and mobilizing solar wind-derived hydrogen and water18.

Meanwhile, fragmentation and abrasion continuously reshape particle morphologies19,20, encoding the cumulative effects of space weathering on airless bodies21. On the Moon, the fraction of lunar regolith with particle sizes <1 mm is particularly relevant to surface space weathering22 and warrants detailed investigation. Due to limitations in current analytical techniques as encountered in this study, it remains challenging to obtain reliable morphometric data for particle sizes <20 μm (ref. 23), which are typically classified as “lunar dust”(see Methods). Particles within the 20–20 μm size range, comprising over 60% of the <1 mm fraction, are defined as the “bulk soil” fraction24,25, while the remaining particles (>200 μm) are categorized as “large particles”. Notably, this dominance of the bulk soil fraction is also observed in Apollo samples and can be attributed to the progressive consumption of the lunar dust fraction during agglutinate formation and its subsequent redistribution within the bulk soil fraction22. As such, the bulk soil fraction serves as the primary reservoir of surface evolution signatures on the Moon. The morphology of particles within this fraction not only preserves tripartite records of volcanism, impacting, and space weathering, but also exerts a critical influence on its geomechanical behavior26. Moreover, accurate morphological data serve as a key input for building high-fidelity predictive simulations of the Moon’s surface, enabling predictive simulations of regolith behavior under various exploration scenarios27.

On December 1, 2020, China’s Chang’e-5 (CE5) mission28 landed at 43.06°N, 51.92°W in northern Oceanus Procellarum and returned 1,731 g of regolith (Fig. 1a-b). On June 25, 2024, the Chang’e-6 (CE6) mission24 collected 1,935.3 g of sample materials from the South Pole–Aitken basin (41.625°S, 153.978°W) via both scooping and drilling (Fig. 1a, c). These two missions provided the first paired youngest mare regolith samples (<3 Ga) from the lunar near and far sides29,30, allowing a rare opportunity to construct a unified model of regolith evolution under comparable analytical conditions. Geochronology, geochemistry, and remote sensing data suggest that the CE5 regolith is locally derived and basaltic, with intermediate-Ti basalts10,25, whereas CE6 soils contain a greater proportion of exotic components, including not only local basalts, mainly low- to intermediate-Ti basalts but also noritic and other components24,31. Silicon isotopes further reveal that the CE6 site may have undergone more intense micrometeorite bombardment than the CE5 region32.

Despite extensive study, debate continues over the classification of basalt and the evolution of space weathering in CE5 and CE6 regolith particles. At the heart of these issues are two challenges presented by previous methods: (i) the representativeness of traditional thin-section-based geochemical analyses, which yield only two-dimensional particle data and rely on hand-picked grains; and (ii) the applicability of mineral-scale analyses and interpretations to the evolution of the whole lunar regolith in the region. In recent years, the growing accessibility of micro-computed tomography (micro-CT) has enabled its widespread application in lunar soil research33–37. As a non-destructive technique, micro-CT scanning allows for high-resolution reconstruction of both external particle 3D morphology and internal structures of each individual particle within the bulk soil. It enables precise quantification of the 3D morphometrics of numerous individual particles23,38–40. Recently, machine learning approaches also have also been introduced to segment large numbers of lunar regolith particles from micro-CT radiographs23,39.

It is widely accepted that space weathering exerts a strong influence on the morphology of rock fragments on airless bodies41–43. However, this paradigm may not be directly applicable to the lunar surface, where particle morphology is simultaneously governed by the primary lava composition, impacting history, and the cumulative intensity of space weathering. Variations in primary lava composition give rise to distinct mineralogical assemblages in lunar regolith, which in turn affect particle-scale mechanical responses to space weathering. For instance, plagioclase is more susceptible to brittle fracture than pyroxene, rendering plagioclase-rich basalts and breccias more prone to crack formation and localized abrasion, thereby influencing particle morphology during meteoroid bombardment15,44. At the microscale, different mineral phases also exhibit varying responses to space weathering processes such as solar wind implantation, localized melting, and vaporization45,46, further modifying particle morphology47,48. Additionally, differences in impact energy across the lunar surface produce varying degrees of lithologic fragmentation and regolith gardening effects, both of which significantly affect particle morphology49. Importantly, the mineralogical composition of local regolith is a key factor influencing agglutinate abundance. During rapid, non-equilibrium impact melting, low-melting-temperature mafic minerals such as ilmenite, pyroxene, and olivine are more readily incorporated into impact melts than high-melting-temperature plagioclase50,51. As a result, soils with higher plagioclase contents may produce fewer agglutinates50. The regolith maturity also exerts a strong control on agglutinate abundance, with more mature soils typically containing higher proportions of agglutinates22. Together, these factors influence agglutinate abundance and, consequently, the overall particle-scale morphological characteristics of regolith. Thus, the morphology of lunar regolith particles represents an integrated signature of sampling site geological and space weathering processes. Critically, the lack of comprehensive morphological data across different sampling sites has hindered the ability to disentangle these factors. To rigorously assess the morphological consequences of space weathering, assessing a vast number of particles is required, and comparative study of particle morphology across different lunar regions is urgently needed.

In this study, we apply micro-CT scans to bulk soil from the CE5 (CE5C0400YJFM01001) and CE6 (CE6C0200YJFM008) samples under identical settings, and use machine learning approaches to segment and classify individual particles in the scans. This standardized, non-destructive workflow enables high-precision quantification of 3D morphometrics and internal structures across multiple particle types. Several intrinsic limitations of micro-CT data processing are addressed in our study design. First, segmentation accuracy must be maintained across a wide range of particle sizes to ensure the fidelity of measured morphometrics. Second, the classification algorithm must operate on accurately segmented particles while retaining their internal textures, thereby enabling refined categorization and more refined interpretations of regolith formation and space weathering processes. These challenges are explicitly accounted for in our analysis. Based on this comprehensive dataset, we develop a model for the formation and evolutionary processes of the surficial mare regolith, revealing that space weathering drives morphology saturation at the particle scale across diverse particle types. Our findings provide important insights into the temporal trajectory of lunar surface modification and the convergent morphological pathways induced by prolonged space weathering.

Results
Mineralogical differences between CE5 and CE6 basalts
Based on returned mare soils24,30 and lunar orbiter data52, basalts represent the dominant particle type within lunar mare soils and can also be used to reconstruct the thermochemical volcanic history of the Moon7,53. Among basaltic textures, subophitic and poikilitic types (Fig. 2a, b) are readily distinguished due to their relatively large crystal sizes and dominate the basaltic population, and can also be accurately classified using machine learning algorithms23–25,40. Basalt fragments, which are also prevalent (Fig. 2b), exhibit distinct striped textures that facilitate their recognition. Mineral phase densities in basalts can also be effectively distinguished through CT radiographs (Fig. 2b, c), enabling clear separation into high-density (Hp), medium-density (Mp), and low-density (Lp) phases based on grayscale intensity. Energy-dispersive X-ray spectroscopy (EDS) analyses further confirm that the Hp correspond to ilmenite, the Mp to olivine or pyroxene, and the Lp to plagioclase (Supplementary Fig. 2f, n and o). These three phases constitute the principal mineralogical components of lunar mare basalts30,54.

Based on analyses of 5,605 basalt particles from bulk soils (considering only intact particles, with edge-truncated particles excluded; Supplementary Video 1; Supplementary Video 2), the volume fractions of Hp, Mp, and Lp in CE5 basalts were determined as 6.80%, 53.84%, and 39.36%, respectively, and 7.16%, 61.52%, and 31.32% for CE6 (Fig. 2d). As shown in the ternary plot, the 95% probability density contour encompass all discrete basalt data points previously reported from single-particle geochemical analyses of the CE5 and CE6 samples25,30,54. The large dataset provided here offers an effective supplement to the more limited, discrete basaltic data derived from traditional geochemical methods applied to Apollo and Chang’e series soils. As products of continuous meteoritic impacting of a likely diverse range of cooled lava flows, basalt particles in bulk soils collectively preserve the original mineralogical signatures of lavas, enabling reliable classification of local basalt types. Notably, significant mineralogical differences are observed between CE5 and CE6 basalts, with the latter containing a lower abundance of plagioclase, which contributes to its higher density (Supplementary Fig. 4). Previous studies have suggested that both the CE5 and CE6 lavas experienced high degrees of fractional crystallization10,31. But the observed differences in plagioclase content may reflect varying degrees of this process between the two regions.

Traditionally, mare basalts have been classified based on major-element abundances. Electron probe microanalysis data indicate that TiO2 and Al2O3 are primarily hosted in ilmenite (Hp) and plagioclase (Lp), respectively30,54. For large basalt particles suitable for geochemical analysis and hand-picking, linear relationships exist between ilmenite volume fraction and bulk TiO2 content, and between plagioclase volume fraction and bulk Al2O3 content (Supplementary Fig. 5d, e). Applying these linear relationships, we determined bulk TiO2 and Al2O3 contents of 6.08 wt% and 11.60 wt% for CE5 basalts, and 6.17 wt% and 9.43 wt% for CE6, respectively (Fig. 2e). Compared to CE6 lunar soil, CE5 exhibits considerably elevated Al concentrations, consistent with previous findings31, thus likely categorizing CE5 mare basalts as medium-Ti and high-Al types, whereas CE6 basalts correspond to the medium-Ti and low-Al basalts, based on mare basalt classification criteria55. Both samples are assigned to Eratosthenian-aged mare basalts, whose TiO2 abundances are higher than those of typical low- to medium-Ti Apollo basalts, yet lower than those of high-Ti Apollo basalts (Fig. 2e). This distinction supports the interpretation that late-stage lunar volcanism (<3 Ga) involved distinct lava sources and evolutionary pathways from those recorded by Apollo basalts56. Analysis of the internal pore structures of basalts with equivalent diameters >50 μm reveal that the vast majority of void ratios in both CE5 and CE6 basalts are <2% (Supplementary Fig. 6). Such exceptionally low porosity values are characteristic of solidified, non-vesicular coherent mare basalts11, likely formed as a result of CE5 and CE6 lavas having erupted and been emplaced at considerable distances from the source vents as flat, smooth-surfaced flows with low vesicularity, which subsequently cooled into dense basaltic units11.

Distinct internal vesicles of agglutinates in CE5 and CE6 soils
Agglutinates are special regolith particles consisting of smaller lunar soil fragments (mineral particles, rock clasts, and older agglutinates) bonded together by vesicular, flow-banded glass57. They form primarily through micrometeorite-induced melting, which generates vesiculated agglutinitic glass matrices. The vesicles within these matrices are widely interpreted as the combined textural products of solar wind implantation, which generates small vesicles along particle margins, and volatile gas exsolution during melt generation and quenching, which produces larger internal vesicles17,57. Within glassy matrices, sub-microscopic metallic iron nanoparticles (npFe) form through a combination of micrometeorite impact58, solar wind (induced sputtering and reduction)59, transient thermal-spike (driven reduction)60, and externally triggered Fe2+ disproportionation61, all of which collectively modify the optical and magnetic properties of the soil. As such, agglutinates preserve detailed records of space weathering. Agglutinates occur across a wider range of particle sizes within bulk soils. Larger agglutinates (equivalent diameter >200 μm) can be easily isolated manually, whereas smaller agglutinates in bulk soil (equivalent diameter <200 μm) are typically irregular, fragile, and thus challenging to manually extract and analyze. However, this difficulty is overcome with CT scanning, as the distinctive morphological features and abundant internal porosity of agglutinates are readily identifiable in radiographs.

To further investigate agglutinate internal structure, we performed high-resolution micro-CT scans on 84 manually selected larger agglutinates (42 from CE5 and 42 from CE6). Additionally, 3D models were also acquired of numerous bulk soil agglutinates (236 from CE5 and 555 from CE6). Within the glassy regions, vesicles predominantly exhibit regular geometries (EI < 1.7, FI < 1.7, S > 0.9), whereas at the interfaces between glassy matrix and embedded fragments, vesicles become highly irregular (EI > 1.7, FI > 1.7, S < 0.9) (Fig. 3e; calculation methods for EI, FI, and S morphometrics are detailed in Methods, with illustrative examples shown in Supplementary Fig. 10). This contrast likely arises because vesicles forming within relatively homogeneous glassy regions tend to adopt regular shapes controlled by surface tension, whereas vesicles near fragment-glass interfaces are mechanically constrained by surrounding fragments and thus develop irregular morphologies. This phenomenon is also evident in SE imaging of Apollo 11 agglutinates62. To quantify this distinction, we define the internal regular void fraction (Ir) as the proportion of regular vesicles relative to the total vesicle count of a single agglutinate. As agglutinates are composed of both fragments and a glassy matrix63, the glassy matrix and fragment contents (Fr) are inherently volumetrically complementary. This parameter thus may also relate to the volumetric abundance of internal fragments. To assess this possibility, we manually annotated fragments within selected larger agglutinates (Supplementary Fig. 8a, d), finding a strong inverse correlation between Ir and Fr (Fig. 3b). Based on Ir values, we categorize agglutinates into three classes: (i) those with Ir < 0.35 are fragmentation-defined as high fragment content agglutinates (Fig. 3c), (ii) those with Ir > 0.5 are vitrifaction-defined as low fragment content agglutinates (Fig. 3f), and (iii) those with intermediate Ir values (0.35–0.5) are classified as medium fragment-content agglutinates (Fig. 3d). For the broader population of bulk soil agglutinates, Ir was directly used as a proxy to estimate internal fragment abundance.

Our results show no significant differences in the distribution or mean Ir values between the larger agglutinates of CE5 and CE6, where Ir ≈ Ir = 0.42 (Fig. 3a). However, internal porosity measurements consistently show a considerable lower average void ratio in CE6 agglutinates (e = 0.11) compared to those from CE5 (e = 0.23) (Fig. 3a). For agglutinates in bulk soils, Ir values are higher in CE5 (Ir = 0.49) than in CE6 (Ir = 0.46). Compared to their larger counterparts, the bulk soil agglutinates from CE5 exhibit a modest increase in Ir values, suggesting a relatively lower internal fragment content (Fig. 3a). Given that bulk soil agglutinates likely represent the fragmented products of larger agglutinates64, this trend may reflect inherent structural differences in the initial agglutinates of the two regions, which could in turn influence their subsequent response to space weathering.

Morphological homogenization among similar lunar regolith particle types
The morphology of lunar regolith particles records long-term space weathering processes on the lunar surface23. Using machine learning segmentation and classification methods, we obtain morphometrics—aspect ratio, sphericity, convexity, and roundness—with definitions provided in the Methods and Supplementary Fig. 10, for no fewer than 100 individual effective and intact particles from each particle category from both CE5 and CE6 bulk soil CT scans, ensuring statistically representative datasets for each particle type. The analyzed particle types include basalts, agglutinates, breccias, high-density (Hm), medium-density (Mm), and low-density (Lm) monomineralic particles, as well as glass beads (Fig. 4a). The Hm, Mm, and Lm groups are readily distinguishable in radiographs based on their X-ray attenuation intensities, where Hm is primarily composed of the opaque Fe-oxide mineral ilmenite, Mm mainly represented by pyroxene, olivine, or anisotropic glass, and Lm consists predominantly of plagioclase grains23,65. Breccias were also included; however, due to their limited abundance, their morphometric distributions may not be statistically robust. Glass beads were omitted, given their highly regular, nearly spherical shapes, with morphometrics closely approximating those of ideal spheres (Fig. 4a). To examine potential size-dependent effects, Mm and Lm particles were subdivided into larger (equivalent diameter > 50 μm) and smaller (equivalent diameter < 50 μm) fractions. Our results show that aspect ratio is virtually unaffected by particle size, while sphericity and convexity exhibit only minor size dependence. In contrast, roundness is strongly influenced by particle size, with smaller particles being rounder (Fig. 4c). This trend likely reflects the scale-dependent nature of morphological descriptors and is consistent with observations from Apollo lunar soil studies37.

Our morphology results also indicate remarkable consistency across the distributions of all morphometrics for basalt, breccia, agglutinate, Hm, Mm (both size fractions), and Lm (both size fractions) particles from CE5 and CE6 soils (Fig. 4b, c). Differences in mean morphometrics for corresponding particle categories between the two sites are all within 5% (Supplementary Table 3). The overall regularity (OR) of different particle types exhibits two distinct regimes. More than half of the basalt and monomineralic particles (Hm, Mm, and Lm), in terms of particle number, fall within an OR range of 0.63–0.75 (Fig. 4d). These particle types represent the majority of the bulk soil population (>60 vol%). These particles, characterized by simple internal structure, are herein referred to as “simple particles.” In contrast, breccia and agglutinates display considerably lower OR values than other particle types, indicating a markedly more irregular morphology (Fig. 4d). Owing to their complex internal structure, these particles are categorized as “complex particles.” Using the same methods applied to the lunar regolith, we additionally analyzed the morphometric distributions of a large number of particles from the lunar regolith simulant (LRS: IGG-01), which was produced by mechanically crushing basalt66. Compared to lunar basalt particles, the LRS particles exhibit lower aspect ratios (Fig. 4b). Such characteristics are consistent with the angular-to-subrounded morphologies observed on asteroid regolith surfaces, where particles typically experience short surface residence times and limited abrasion67,68. This similarity likely reflects the fact that both mechanically crushed simulants and asteroid regolith are dominated by relatively “fresh” fragments that have not undergone prolonged regolith gardening.

Building on particle classification, we quantified the relative abundances of each particle type (Fig. 4e). In particular, agglutinate abundance can serve as an indicator of soil maturity57. This parameter correlates positively with the maturity index Is/FeO, but only when Is/FeO < 60, above which agglutinate abundance is known to saturate in highly mature soils69,70. The CE5 bulk soil exhibits an Is/FeO value of ~14, consistent with immature soils71, and an agglutinate abundance of ~8%. In contrast, CE6 soils yield an Is/FeO value of ~56.5, characteristic of mature soils72, together with a higher agglutinate abundance of ~17%. The elevated agglutinate abundance and Is/FeO value of CE6 are consistent with trends established in previous studies70. Furthermore, the Is/FeO index is closely linked to the abundance of npFe, produced on grain rims and within agglutinates by micrometeorite bombardment and solar wind sputtering/redeposition, and therefore reflects the cumulative intensity of these space weathering processes. Another critical component of space weathering is regolith “gardening” driven by meteorite impacts, which is commonly expressed through the surface soil maximum exposure age, where soils with longer exposure durations experience more extensive overturn and reworking73. According to regolith evolution models, CE5 surface soils have a maximum exposure age of ~2.2 Ma, whereas CE6 soils have been exposed since ~3.2 Ma (ref. 74). The slightly longer exposure duration of CE6 regolith suggests a greater degree of surface soil gardening, and together with these maturity indices, consistently demonstrates that CE6 soils have experienced a higher overall degree of space weathering.

Methods
Sample material and micro-CT scans
CE5 lunar sample (CE5C0400YJFM01001) and CE6 lunar sample (CE6C0200YJFM008) were allocated by the China National Space Administration (CNSA) for use in this study. These samples consisted of lunar soil containing various types of regolith particles. All samples were securely stored within a glove box, continuously purged with high-purity dry nitrogen gas (N2 > 99.9999%, H2O < 0.1 ppm, O2 < 0.1 ppm) to prevent contamination and preserve sample integrity.

For micro-CT scans, ~200 mg of lunar soil was carefully transferred within the glove box into a transparent tube with an inner diameter of ~3 mm. After sealing the tube with a protective cap, the sample was removed from the glove box for micro-CT scans. X-ray tomography was performed using a Zeiss Xradia 620 Versa system at an accelerating voltage of approximately 60 kV, achieving a spatial resolution of 1 μm. Initially, a rapid overview scan was carried out at a lower resolution of 7.5 μm (~20 min per scan) to quickly assess the overall particle distribution within the bulk soil sample tube (Supplementary Fig. 1a). Subsequently, to optimize particle extraction efficiency from selected scan regions, six distinct scanning volumes were identified in both CE5 and CE6 samples, intentionally avoiding areas with large particles. Each detailed scan involved a full 360° rotation (~260 min per scan), producing ~1000 2D radiographs per region, covering a cylindrical volume of ~1 mm in diameter and 1 mm in height (Supplementary Video 3; Supplementary Video 4). This non-destructive approach was specifically chosen to maximize sample utilization while minimizing any potential alteration or damage.

A uniquely shaped subophitic-textured CE5 basalt particle and a CE6 breccia were specifically selected from a bulk soil CT scanning region to facilitate subsequent intensity-density calibration (Supplementary Fig. 2). The representative CE6 breccia particle was also used to verify segmentation accuracy of machine learning methods (Supplementary table 1). After micro-CT scans, lunar samples were returned to the glove box and transferred into clean glass dishes. Representative basalt was manually isolated from the CE5 soil sample, whereas breccia was collected from the CE6 sample. To investigate the formation mechanism of lunar agglutinates, 42 relatively larger agglutinates (equivalent diameter > 200 μm) were selected from the CE5 soil, and 42 from the CE6 soil. These 84 particles were mounted onto yellow transparent adhesive tape, and subsequently micro-CT rescanned individually at resolutions between 1.5 and 3.2 μm.

Radiograph intensity-density calibration
Radiograph intensities obtained from micro-CT scans reflect the degree of X-ray attenuation, influenced by multiple parameters such as X-ray energy and material density101. Intensity obtained from a radiograph can be quantitatively correlated with material density according to established relationships23. Prior to density calibration, pretreatment was performed on each set of radiographs using a non-local means filter for noise reduction and an unsharp masking filter for image sharpening via Avizo software (Supplementary Fig. 1a). As the X-ray attenuation characteristics remain stable under fixed scanning parameters, intensity normalization across all scanned regions of CE5 and CE6 bulk soil samples was achieved by standardizing intensity values from micro-CT radiographs to a uniform scale (0–255).

Two representative particles were utilized for the intensity–density calibration (Fig. 2a, Supplementary Fig. 2e, m). These selected particles were cut, and mineral phases within the polished thin section were identified using a Thermo Scientific Quattro S field-emission scanning electron microscope (SEM) equipped with a Thermo Scientific UltraDry energy dispersive X-ray spectroscopy (EDS) detector (Bruker XFlash 6|30). The thin section was mounted directly onto conductive carbon adhesive tabs with either carbon foil or carbon-coated substrates. SEM imaging was conducted under optimized operational conditions: electron-beam accelerating voltages were set at 10–15 kV with electron beam currents ranging from 7–30 pA for high-resolution secondary electron imaging, whereas backscattered electron imaging and elemental mapping utilized voltages of 15–20 kV and beam currents of 50–120 pA.

Mineral distributions on thin sections were obtained (Supplementary Fig. 2e, m). Bulk soil CT scanning radiographs containing these two particles were spatially aligned to precisely match their corresponding SEM planes. Subsequently, four mineral phases of interest—plagioclase (Supplementary Fig. 2a, g), pyroxene (Supplementary Fig. 2b, h-l), olivine (Supplementary Fig. 2c) and ilmenite (Supplementary Fig. 2d)—were identified in the SEM images. Representative regions within each mineral phase were also selected for detailed elemental analyses using EDS, providing precise compositional data for atomic percent (Supplementary Table 2). Corresponding intensity values of these mineral regions were then extracted from aligned radiographs (Supplementary Fig. 2f, n). For density estimation, mineral densities were assigned based on literature values corresponding to mineral phases with elemental compositions closely matching those measured by EDS analysis65 (Supplementary Table 2), enabling accurate linear calibration of the intensity–density relationship for the lunar samples (Supplementary Fig. 2o). It should be noted that although the coefficient of determination reaches 0.99, notable deviation arises in the intensity range > 225, where the linear model becomes less valid due to X-ray attenuation effects in high-density materials23. Nevertheless, all polymineral particles and bulk soil averages presented in the density-related analyses (Supplementary Fig. 4) have mean intensities < 150, within the valid linear range. Therefore, nonlinear effects were not considered in this study. Ilmenite, pyroxene or olivine, and plagioclase mineral phases correspond to Hp (intensity = 225–255), Mp (intensity = 130–225), and Lp (intensity = 64–130), respectively, allowing the construction of mineral phase distribution maps based on radiograph intensity (Supplementary Fig. 2f, n).

Segmentation
For radiographs with a resolution of 1 μm, particles with equivalent diameters > 20 μm (effective particles) are sufficiently resolved to preserve relatively accurate 3D morphology and internal structures without significant distortion102. However, small particles and lunar dust complicate boundary identification due to insufficient resolution, making conventional threshold-based segmentation methods ineffective. To address this limitation, particle boundary extraction was performed using pixel classification-based machine learning via the open-source software ilastik103. Specifically, pixel classification was trained on 200 radiographs to robustly delineate particle boundaries, including internal pores and fractures, as well as to clearly differentiate effective particles from surrounding lunar dust and small particles (Supplementary Fig. 1b, d).

Due to the heterogeneous particle size distribution of lunar soils, extracting individual 3D particle shapes required additional segmentation. Traditional Chamfer-Conservative segmentation methods based on uniform boundary parameters proved ineffective for lunar soil because larger particles were prone to over-segmentation, whereas smaller particles frequently remained under-segmented104. To address this issue, a two-step “over-segmentation correction method” was implemented. Initially, an aggressive segmentation (Skeleton-Aggressive method) was conducted, capable of adequately separating smaller particles but inevitably causing over-segmentation of larger particles. Subsequently, segmentation lines were extracted and analyzed using Avizo. Segmentation lines representing accurately segmented smaller particles typically appeared spatially discrete (Supplementary Fig. 1b), whereas lines resulting from over-segmentation of large particles were interconnected. Therefore, to correct over-segmentation, lines corresponding to excessively segmented regions were identified and removed based on a threshold volume criterion. This filtering approach effectively retained accurate segmentation lines while eliminating erroneous segmentation lines. Following segmentation, effective particles for subsequent classification and morphological analysis. Particles smaller than 20 μm in equivalent diameter were retained solely for the purpose of quantifying the overall particle size distribution of the sample. The derived results show close agreement with bulk soil fraction contents independently obtained from extensive 2D imaging analyses of CE5 and CE6 soils24,25 (Supplementary Fig. 11).

To quantitatively evaluate the accuracy of this segmentation procedure, a representative breccia particle from the CE6 lunar soil was physically isolated, rescanned individually, and compared to the same particle previously auto-segmented within the bulk soil CT scanning. The comparison demonstrated that key 3D morphological measurements obtained from separate micro-CT scans of the manually selected particle (Supplementary Table 1).

Classification
Lunar regolith particles were initially categorized according to their mineralogical and textural features. Single-phase or homogeneous particles were classified based on intensity into three mineral-density groups: high-density (Hm, density > 4.72 g·cm−3), intermediate-density (Mm, density = 3.21–4.72 g·cm−3), and low-density (Lm, density < 3.21 g·cm−3) monomineralic particles. Polymineral particles exhibiting heterogeneous internal textures were classified into basalt, breccia, and agglutinate based on characteristic textural patterns observed internally24,25.

Particle classification was conducted using the object classification module implemented in ilastik. A training dataset was manually generated by labeling particle types on 400 radiographs (200 images from the CE5 sample and 200 images from the CE6 sample), which were simultaneously trained to ensure consistent classification criteria across both lunar soils (Supplementary Fig. 1d). Subsequently, automated classification was performed, yielding preliminary particle-type classifications for every particles in each 2D slice. However, due to limited resolution, some small polymineral particles were challenging to classify accurately based solely on internal textures and were thus categorized as “unknown”. It should be noted that many breccia particles, which are internally composed of fine fragments, could only be identified in this study when they were relatively large, contributing substantially to the total breccia volume (Fig. 4e) but representing a minor fraction in terms of particle count. At the bulk soil fraction, breccias were often broken down into monomineralic or polymineral debris, and due to resolution limitations, the internal clastic structure of smaller breccia particles was often indistinguishable. As a result, a significant number of small breccia particles may have been classified as “unknown”. This category predominantly comprises polymineral particles and may also include small basalt and agglutinate fragments that could not be reliably identified. Based on these classification criteria, particles were categorized into seven types: basalt, agglutinate, breccia, Hm, Mm, Lm, and unknown.

Given the inherent textural heterogeneity within particles, inconsistent classifications across different slices within an individual 3D particle frequently occurred. To reconcile such inconsistencies arising from slice-by-slice classifications, a dedicated Python-based algorithm was developed to determine the statistically dominant particle type for each particle in 3D space (Supplementary Fig. 1c). Specifically, particle classification consistency was resolved based on “majority voting” strategy from 2D slice classifications, where each particle’s final 3D category was assigned according to the following criterion:

kj = max(ki) (1)

where ki represents the total number of 2D slices within a given particle classified as particle type i (i = 1, 2, 3, 4, 5, 6, 7), and j represents the final 3D classification category assigned to the particle. Following classification, morphological parameters were analyzed for each particle, and particles with S > 0.92 were classified as glass beads.

Morphological parameters
Most morphological parameters analyzed in this study were dimensionless quantities commonly used in image analysis to characterize particle morphology independently of particle size. Particle size was quantified using the equivalent diameter D, defined as the diameter of a sphere having the same volume as the particle (Supplementary Fig. 10b)105:

D = (6πV)^(1/3) (2)

where V represents the volume of the particle.

Axial ratios describe the proportional relationships among the three principal orthogonal dimensions, defined as P1, P2, and P3 (P1 ≥ P2 ≥ P3) (Supplementary Fig. 10a). The elongation index (EI), flatness index (FI), and aspect ratio (AR) were employed to quantify particle elongation and flatness, and are mathematically expressed as follows106:

EI = P2 / P1 (3)

FI = P3 / P2 (4)

AR = (EI + FI) / 2 (5)

Sphericity (S) was defined as the ratio between the surface area of an equivalent-volume sphere (Supplementary Fig. 10b) and the actual particle surface area (SA), quantifying the closeness of particle shape to a perfect sphere105:

S = (π^(1/3) * (6V)^(2/3)) / SA (6)

Convexity (Cx) quantified the degree to which the particle shape resembles its corresponding convex hull and was defined as the ratio between the particle volume (V) and the volume of its convex hull (Vch) (Supplementary Fig. 10c)33,105:

Cx = V / Vch (7)

Roundness (R) describes the relative curvature of a particle’s projected outline. Specifically, it quantifies the sharpness of particle edges and corners and is calculated based on the particle’s triangulated surface mesh projection107,108. The definition of roundness is the ratio of the average curvature of the corners of the particle to the curvature of the maximum inscribed circle (Supplementary Fig. 10d)33.

R = (ΣNi=1 (Ai * ki)) / (ΣNi=1 (Ai) * km) (8)

where Ai is the area of the i-th triangular element, which is part of the corners on the particle, and ki is the mean curvature of the i-th triangular element, and km is the curvature of the maximum inscribed sphere.

Overall regularity (OR), provides a composite description of the aforementioned morphological parameters and is defined as follows:

OR = (AR + S + Cx + R) / 4 (9)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a highly valuable and large-scale analysis of lunar regolith particles collected by the Chang’e-5 (CE5) and Chang’e-6 (CE6) missions, utilizing micro-CT scanning and machine learning-based classification. The study provides rare and rich primary data that significantly contributes to our understanding of key lunar surface processes including volcanism, meteoritic impacts, and space weathering.\nHowever, there are several critical points that require clarification and appropriate revision as follows:\n1. On the Saturation of Space Weathering and the Concept of “Dynamic Equilibrium”:\nThe authors propose a central model in which particle morphology reaches a \"dynamic equilibrium\" through long-term space weathering. However, this model appears to rest primarily on the observed statistical similarity in morphometric distributions between CE5 and CE6 bulk soil particles.\nYet, there are notable differences in the proportions of breccia, agglutinate, and fine particles between these two sites, which suggests that the regolith as a whole has not reached equilibrium. The authors limit the model to the bulk soil size fraction (20-200um), but considering the fact that larger particles supply the bulk soil through fragmentation, and bulk particles themselves continue to break down into finer particles, all of these particle populations coexist and interact. Therefore, it is questionable to assert that equilibrium applies exclusively to one size fraction without considering the whole regolith system. Moreover, if such a size-specific model is to be proposed, a clear rationale for the selection and definition of the bulk soil size range must be provided.\nLastly, the authors suggest that morphological saturation has occurred by ~2 Ga, but this claim lacks direct comparison with older Apollo regolith particles. Without examining whether Apollo particles also exhibit similar morphometrics, it is difficult to confidently conclude that the saturation has been reached. Why don’t the authors interpret the observed morphological similarity between CE5 and CE6 simply due to their similar ages?\n2. On regolith particle evolution processes:\nIn paragraph [20], the authors note that impact gardening, thermal cycling, and solar wind irradiation contribute to fragmentation and abrasion. However, the role of agglomeration processes is not discussed. For example, breccia particles may be formed by the strong compression due to meteoroid impact, and agglutinate particles are formed by the gathering of partly melted fragments. Also, fragmentation and abrasion can have opposing effects on particle morphology. Such processes must be considered to accurately evaluate regolith evolution. The current interpretation risks oversimplifying the particle evolution process.\n3. On the Comparison with Lunar Regolith Simulant (LRS):\nIn paragraph [14], the authors observe that the LRS particles exhibit lower aspect ratios and higher sphericity, convexity, and roundness compared to lunar basalt particles, and state that this resembles the morphology of asteroid surface particles.\nHowever, since LRS particles are produced by mechanical crushing and have not undergone long-term abrasion, it is not surprising that they resemble the asteroid surface particles which have very short surface residence times.\n4. On the Interpretation of Agglutinate Porosity Differences:\nThe discussion in paragraphs [18] and [19] are largely speculative. Paragraph [18] explains the lower internal porosity observed in CE6 agglutinates from the different micrometeorite bombardment condition in nearside and farside. This can be proved by comparing with the data of Apollo samples. In paragraph [19], the connection between agglutinate structure, volatile contents, and gas escape efficiency is mentioned, but the argument lacks cohesion and empirical support. These sections would benefit from a clearer presentation and more cautious language regarding the proposed mechanisms.", "reviewer_2": "References do not appear to be in correct order (ie reference 14 is cited in line 39 which is followed by reference 51 in line 41). I would suggest reordering the reference list so that they are increasing consecutively as they are being used in the text for clarity.\nAdditional lunar sourcebook chapters I feel would be relevant here are Lunar Rocks (Taylor et al. 1991) and The Lunar Regolith (McKay et al. 1991).\nRecommended references for agglutinate formation: Solar wind and micrometeoroid alteration of the lunar regolith (Housley et al. 1974); Chemical Variability and Origin of Agglutinitic Glass (Basu and McKay 1985); Space Weathering from Mercury to the asteroid Belt (Hapke 2001).\nLine 43: Saying that agglutinates are vesicular in nature and exhibit internal pore structures sounds like they are two different things. I would just say that “…agglutinates exhibit vesicles…” for conciseness.\nRecommended reference for agglutinate vesicle volatile contents and mobility: Nanoscale reservoirs store solar wind derived water on the lunar surface (Kling et al. 2025).\nLine 46-48: I would include a citation here. What are these subdivisions based off of? Are they somehow related to other space weathering indices? Why is 20-200μm considered “bulk soil”? Why is “lunar dust” <20μm?\nLine 53: Define “digital twin model”. Or get rid of that and just say “enabling predictive simulations…”.\nLine 56-57: Convert g to kg.\nLine 74: I would clarify that micro-CT is applied to individual particles from bulk samples.\nLine 79-81: “Second, the subsequent classification of particles by type is essential to support more refined interpretations of regolith formation and space weathering processes.” You introduce this as a technical challenge to micro-CT but this doesn’t sound like a technical challenge, this is more like a gap in our knowledge of grain morphology.\nLine 82: Recommended reference for the effects of space weathering on regolith grain morphology: The nature and origin of rims on lunar soil grains (Keller and McKay 1997); The microstructure of lunar micrometeoroid impact craters (Noble et al. 2016).\nLine 78-79: This sounds like it is the motivating paragraph for the study, but I am confused about some of the points. I would clarify somewhere how lava composition influences grain morphology with regards to comminution and space weathering.\nThere is a lot of work out there that discusses how mineralogy on the lunar surface influences regolith fractionation (i.e. size fraction). In addition, there is a lot of work that has been done that focuses on how different minerals react to space weathering processes, such as irradiation, melting and vaporization from impacts, and agglutination. So if you are trying to tie the importance of understanding grain morphology with respect to collection site, space weathering, and composition, I would highlight more evidence that suggests these things are related. Maybe don’t include the micro-CT content in the beginning, since that is a tool and unrelated to the question you are trying to answer. Also, it would be good to succinctly state your hypothesis here.\nLine 90 – 103: This reads like an abstract. You include a lot of results and discussion that should be included in the body, rather than introduction. I would briefly introduce what was done here. This might also be a good place to include the technical limitations of micro-CT.\nFig. 1: Panels d, e, and f should be moved closer to where they are referenced in the article. It doesn’t make sense to have them in the same figure as the one that shows the geological context of the Chang’e landing sites. Coordinates on panel a would be useful when comparing to Apollo and Luna landing sites.\nLine 119: I would include citations from orbiter data confirming basalt makes up the mare such as Clementine, Kaguya, and Chandrayaan M3.\nLine 121: You reference Fig. 2c before 2b. Fig. 2a isn’t referenced until the methods section. You should reorder your figure panels so they align better with their reference order in the text.\nLine 124: I would write “Mineral phase densities in basalts…” because it’s not clear what densities you are referring to as you are defining hd, md, and ld.\nLine 131: Supplementary video 2 and 4 are cited before supplementary video 1 and 3.\nI see that there are four data points on Supplementary Figure 2e, which were confirmed using EDX. Have you done these chemical/intensity confirmations with any additional particles? It would be interesting to see how the intensity varies for each mineral phase.\nLine 137: Weathering refers to what? Space weathering? If you mean the products of continuous impacts, I would say that instead of “weathering” as that term encapsulates irradiation, micrometeoroid impact, etc..\nLine 149-154: Maybe I am missing it, but why do you define high and low TiO2 and Al2O3 at 6% and 11%, respectively. Is that value relative to only CE samples or all lunar samples?\nLine 158: What is the difference between internal pore structure and a vesicle? How does this information relate to the flow distance of the volcanic source?\nLine 187-189: Citation\nLine 199-200: Pore structures in agglutinates are usually denoted as “vesicles”.\nLine 204: How does Ir relate to internal fragments when vesicles are typically associated with agglutinitic melt, not inclusions.\nLine 213-224: Since vesiculation is associated with agglutinitic melt, could the inverse relationship be related to the melt content of the agglutinate? You also infer from the slight decrease of Ir in agglutinates from CE5 relative to CE6, that the CE5 soil underwent additional processing (i.e. secondary agglutinate formation). That implies that we should see higher levels of maturity for the CE5 collection area. Firstly, I would like to see the error for the two Ir values (and maybe include that on Figure 3a and 3b). Secondly, if additional processing happened on the CE5 collection site, then that should be reflected in other maturity indices. Is there any work that measured Is/FeO or Optical Maturity for each collection site that you can refer to?\nLine 255-256: Why does sphericity of glass beads inhibit your ability to acquire morphometric data?\nLine 267-268: I would spend more time thinking about this interpretation. This saturation effect is likely associated with the smaller size fraction you are using for this study. Studies that looked into size fraction as it relates to space weathering are McKay et al. (1974): Grain size and the evolution of lunar soils; and Morris (1976): Surface exposure indices of lunar soils: A comparative FMR study. In these studies, researchers looked at different size fractions and found that a correlation exists between surface exposure and grain size for regolith with mean grain size <1cm, however that correlation does not exist for mean grain sizes <1mm. At such small size fractions the regolith undergoes a mixing and agglutinate production dominate regime rather than a comminution dominant regime, which could be why the morphologies for each CE landing site looks similar. A better morphological indication for space weathering at smaller size fractions (and larger for that matter) would be agglutinate abundance and the prevalence of micrometeoroid impact melt, vapor, and zap pits.\nLine 281-295: This paragraph compares the results of the CE samples to two Apollo expedition sites. However, I do not think it is appropriate to compare the CE samples (which were collected from single locations) to the totality of the Apollo samples (which were collected from multiple locations during each mission, with each one having different exposure histories and geological context). It is important to note that the degree of space weathering for a single soil sample does not necessarily reflect the amount of space weathering that occurred over the timescale since mare formation. If an impact that occurred in the mare 100 Ma overturned regolith, then that regolith would only have been space weathered for 100 Ma. Could you be more specific with the Apollo samples you are referring to and do a more robust comparison of those particular samples to that of the CE samples with respect to recent geologic context, surface exposure age, maturity index, etc.?\nLine 357-370: Reference that discusses the latitudinal and longitudinal influence on micrometeoroid impact: Sim et al. 2017: Asymmetric space weathering on lunar crater walls. There are also plenty of other studies that model the flux of micrometeoroids on the lunar surface.\nLine 379: The water before boiling analogy is not very descriptive.\nLine 380-382: I’m not sure how abrasion and fragmentation leads to the loss of “internal” fragments.\nLine 387-388: How does gardening, thermal cycling, and solar wind irradiation lead to particle abrasion and fragmentation?\nLine 397-348: As mentioned before, this interpretation may be flawed due to the small size fraction that was selected."}, "author_response": "Point-by-point response to the issues raised by the referees\n(Manuscript No: NCOMMS-25-47464)\nResponses in blue.\nRevisions in red.\nIn the revised manuscript and supplementary information, all revisions are highlighted.\nReviewer #1\nThis manuscript presents a highly valuable and large-scale analysis of lunar regolith particles\ncollected by the Chang’e-5 (CE5) and Chang’e-6 (CE6) missions, utilizing micro-CT scanning\nand machine learning-based classification. The study provides rare and rich primary data that\nsignificantly contributes to our understanding of key lunar surface processes including\nvolcanism, meteoritic impacts, and space weathering.\nHowever, there are several critical points that require clarification and appropriate revision as\nfollows:\nResponse: We sincerely appreciate the reviewer’s positive and constructive evaluation of our\nmanuscript. We have carefully revised the manuscript to address the reviewer’s concerns and\nbelieve that the changes have improved both the clarity and the robustness of our study.\n1. (1) On the Saturation of Space Weathering and the Concept of “Dynamic Equilibrium”:\nThe authors propose a central model in which particle morphology reaches a “dynamic\nequilibrium” through long-term space weathering. However, this model appears to rest\nprimarily on the observed statistical similarity in morphometric distributions between CE5 and\nCE6 bulk soil particles.Yet, there are notable differences in the proportions of breccia, agglutinate,\nand fine particles between these two sites, which suggests that the regolith as a whole has not\nreached equilibrium.\nResponse: We sincerely thank the reviewer for this insight and apologize that our original\nexplanation of “dynamic equilibrium” was not sufficiently clear, which may have led to a\nmisunderstanding of our proposed mechanism. Additionally, in the initial version of the\nmanuscript, we used both “saturation” and “dynamic equilibrium” to describe the same\nmechanism, which may have introduced conceptual confusion. In the revised version, we\nconsistently use the term “saturation” to articulate our mechanism.\nIndeed, as the reviewer notes, the CE5 and CE6 soils contain notably different proportions\nof impact-related components such as breccia and agglutinates (Reply Fig. 1a). This difference\nmay reflect the slightly longer surface exposure age of the CE6 sampling site (~2.1 Ma; Zhang\net al., 2025b) compared with CE5 (~1.5 Ma; Zhang et al., 2025b), which may have allowed\nCE6 soils to undergo more extended histories of space weathering and meteoritic bombardment,\nthereby producing a higher abundance of impact products (McFadden et al., 2024). CE6 soils\nalso contain a larger fraction of fine particles (Reply Fig. 1b). These observations confirm that,\nin terms of impact products abundance and grain size, the regolith as a whole has not yet\nreached equilibrium.\nand grain sizes, and is not directly linked to the saturation in morphological modification. In\nother words, the timescale to achieve morphological equilibrium is shorter than that required\nfor impact products abundance or grain size equilibrium.\nFor example, McKay et al., (1983) modeled the temporal evolution of agglutinate\nabundance, showing that a balance is eventually reached because two opposing processes act\nsimultaneously: micrometeorite impacts continuously produce new agglutinates, while\nexternal additions such as ejecta or burial remove or dilute existing ones, respectively. Over\ntime, production slows as fewer fresh particles remain, while removal continues, until both\nprocesses balance at a constant agglutinate fraction. Similarly, Lindsay (1975) described\nequilibrium in grain size evolution as the long-term balance between destructive processes\n(comminution) and constructive processes (agglutination) driven by meteoroid impacts.\nIt is important to note that grain size types of equilibrium are strongly environment-\ndependent. As King (1977) demonstrated, local factors such as proximity to boulders or recent\ncraters significantly influence grain size characteristics. Thus, comparing CE5 and CE6 in\nterms of grain size cannot, by itself, determine whether equilibrium has been achieved at a\ngiven site. Agglutinate abundance, by contrast, can to some extent reflect regolith maturity,\nand equilibrium in this parameter is correspondingly more difficult to achieve (McKay et al.,\n1983).\nFrom a morphological perspective, however, impact events exert simultaneous\n“agglutination” effects (forming irregular agglutinates and breccias), “fragmentation” effects\n(breaking down particles through high-energy impacts), and broad “abrasion” effects\n(modifying particle morphologies by wearing down angular edges and other unstable surface\nfeatures). Through repeated contact, displacement, and abrasion among particles, unstable\nstructures are progressively worn away, and particles of a given type converge toward stable\nmorphologies (for a detailed discussion of this mechanism, see our response to Reviewer #1,\nComment 2).\nIn our study, we classify particles as either “simple particles” (basalts, Hm, Mm and Lm)\nor “complex particles” (agglutinates and breccias), based on the presence or absence of internal\naggregate structures. After extended gardening, both groups tend toward equilibrium\nmorphologies: simple particles, despite mineralogical differences, converge toward similar\nshapes due to their internal simplicity (Reply Fig. 1c), while complex particles achieve\nequilibrium at more irregular morphologies owing to their intricate internal structures (Reply\nFig. 1c). Importantly, the formation mechanisms driving this convergence are similar for both\nsimple and complex particles; therefore, even though CE5 and CE6 regolith contain different\nproportions of particle types, their internal structures are fundamentally alike (Reply Fig. 1d-\ng), which in turn explains why stable morphologies are comparable across both sites despite\ndifferences in regolith composition.\nWe have accordingly revised the manuscript to provide a clearer description of the\nmechanisms underlying saturation in morphological modification.\nsoil samples determined in this study. Owing to a longer history of meteoritic bombardment at\nthe CE6 landing site, the sample contains approximately ~9% more agglutinates and ~4% more\nbreccia than the CE5 regolith. b Cumulative volume distribution curves of CE5 and CE6\nregolith particles as a function of equivalent particle diameter, showing finer particle sizes in\nthe CE6 sample. c Boxplots comparing the overall regularity (OR) of six particle types in\nnearside and farside bulk soils. Black lines indicate medians; boxes represent the lower and\nupper quartiles; whiskers denote the full range of the distributions. The shaded region\nhighlights the concentration OR range for basalt, Hm, Mm, and Lm particles. Agglutinate and\nbreccia particles show considerably lower OR values compared to other particle types. In\ngeneral, the OR distributions for CE5 and CE6 are very similar. d-g External morphologies\nand internal cross-sections of impact-related components (agglutinates and breccia) from CE5\nand CE6. Despite differences in the sampling sites, the internal structures of their particles\nappear broadly similar, likely due to comparable formation mechanisms.\nRevision: In the revised manuscript, we have made several changes clarifying our explanation\nof saturation in morphological modification to address the reviewer’s concern.\nIn paragraph [15], our initial discussion inferred enhanced space weathering at the CE6 site\nprimarily from its higher fine-particle abundance. As the reviewer rightly noted, this parameter\nis strongly affected by multiple factors and cannot, by itself, be used to constrain maturity.\nWe have therefore revised the text to evaluate regolith maturity using a combination of\nagglutinate abundance, Is/FeO, and surface exposure age, which together provide a more robust\nassessment.\nParagraph [15]:\n“Building on particle classification, we quantified the relative abundances of each particle type\n(Fig. 4e). In particular, agglutinate abundance, an established indicator of soil maturity59, was\nretrieved with high accuracy. This index correlates positively with the maturity index Is/FeO\n(refs. 69,70). The CE5 bulk soil yields an Is/FeO value of ~14, consistent with immature soils71,\nand an agglutinate abundance of ~8%. By contrast, CE6 soil yields an an Is/FeO value of ~56.5,\n3\ncorresponding to mature soils72, and a higher agglutinate abundance of ~17%. These values are\nmutually consistent, confirming agglutinate content as a reliable maturity proxy. The Is/FeO\nindex is closely linked to the abundance of submicroscopic metallic iron, produced on grain\nrims and within agglutinates by micrometeorite vaporization and solar wind\nsputtering/redeposition, and therefore reflects the cumulative intensity of these space\nweathering processes. Another critical component of space weathering is regolith “gardening”\ninduced by meteorite impact, typically manifest in terms of surface soil exposure age73.\nAccording to regolith evolution models, CE5 surface soils have an exposure age of ~1.5 Ma,\nwhereas CE6 soils have been exposed since ~2.1 Ma (ref. 77). The slightly longer exposure\nduration of CE6 regolith suggests a greater degree of surface soil gardening, and together with\nthese maturity indices, consistently demonstrates that CE6 soils have experienced a higher\noverall degree of space weathering.”\nConsidering the substantial revision of paragraph [15], we now treat the fine-particle content\nin CE5 and CE6 soils as part of the validation of our segmentation method, rather than as a\ndirect indicator of regolith maturity. To clarify this point, we have added the following\nstatement in lines 588-592:\n“Particles smaller than 20 µm in equivalent diameter were retained solely for the purpose of\nquantifying the overall particle size distribution of the sample. The derived results show close\nagreement with bulk soil fraction contents independently obtained from extensive 2D imaging\nanalyses of CE5 and CE6 soils24,25 (Supplementary Fig. 11).”\nIn addition, we have updated Supplementary Fig. 11 accordingly to reflect this modification.\nSupplementary Fig. 11 | Cumulative volume distribution versus equivalent particle\ndiameter. Samples being compared include CE5 and CE6 lunar regolith sample determined by\nour study and previous work24–25. The particle size distributions of CE5 and CE6 samples\n4\nmeasured in this study show excellent agreement with physical measurements from previous\nstudies within the bulk soil fraction (20–200 μm).\nWe have also updated Figure 4e to highlight the differences in agglutinate abundance between\nCE5 and CE6. This figure was originally presented as part of Figure 1 in the previous version,\nbut relocating it here improves the logical flow of the manuscript.\nFig. 4 | Morphometrics of diverse particle types in CE5 and CE6 soils. a Representative\n3D morphologies of each of the seven lunar regolith particle types—basalt, breccia, agglutinate,\nHm, Mm, Lm, and glass beads—identified via our machine learning approach of lunar bulk\nsoil. b Morphometrics statistical distributions for rock particles (basalt, breccia, and agglutinate)\nin bulk soil and LRS (IGG-01). c Morphometrics statistical distributions for monomineralic\nparticles (Hm, Mm, and Lm) in bulk soil. Size-dependent effects on morphometrics were\n5\nspecifically investigated for Mm and Lm particles by subdividing them into large (solid lines)\nand small (dashed lines) size groups. Comparisons between nearside and farside samples reveal\nhighly similar mean values, standard deviations, and distribution ranges across all particle types.\nd Boxplots comparing the overall regularity (OR) of six particle types in nearside and farside\nbulk soils. Black lines indicate medians; boxes represent the lower and upper quartiles;\nwhiskers denote the full range of the distributions. The shaded region highlights the\nconcentration OR range for “simple particles.” Agglutinate and breccia particles show\nconsiderably lower OR values compared to other particle types. In general, the OR distributions\nfor CE5 and CE6 are very similar, with slightly more regular particle shapes observed in the\nCE6 regolith. e Volume fractions of each particle type in the CE5 and CE6 regolith bulk soil\nsamples determined in this study. Owing to higher degree of space weathering at the CE6\nlanding site, it contains approximately ~9% more agglutinates and ~4% more breccia than the\nCE5 regolith.\nIn paragraph [20], we have substantially expanded our explanation of saturation in\nmorphological modification offering a more detailed description of the processes involved. In\naddition, we have revised lines 465-469 to clearly separate the increase in fine-particle\nabundance—as a result of ongoing space weathering—from the distinct concept of\nmorphological equilibrium, thereby avoiding potential conflation of the two.\nParagraph [20]:\n“Particle reshaping processes at the lunar surface include impact gardening (mainly abrasion\ncaused by interactions among large numbers of particles within a given region78), impact\nfragmentation (grain breakage due to direct energy deposition during impacts90), impact\nagglutination (where impact-induced melts or semi-plastic phases fuse surrounding grains into\nagglutinates and breccias), impact sputtering (deposits from impact events appear randomly on\nthe surface of minerals, forming sputtering deposit layer that modify particle morphology46),\nimpact abrasion (localized surface wear induced by micrometeoroid impacts20,91 or secondary\nimpacts that generate linear scratches92), thermal fatigue (fracturing driven by thermal stresses\nleading to ultimate grain disintegration90), and solar wind irradiation93 (high-energy ion\nimplantation generates crystalline defects, making grains more susceptible to subsequent space\nweathering induced reshaping94; in addition, sputtering by solar wind may also erode particle\nsurfaces67). Impact agglutination is distinctive in that it can transform “simple particles” into\n“complex particles” and operates cumulatively, producing highly irregular aggregates that\nstrongly influence the overall particle morphology of the regolith. By contrast, other reshaping\nprocesses generally do not alter particle type (although “complex particles” may shed minor\nfragments of “simple particles”). Both “simple” and “complex” particles are subsequently\nmodified by additional reshaping processes. Among these mechanisms, only impact gardening\nand impact fragmentation significantly modify regolith within a given depth, exerting the\ngreatest influence on particle morphology. For impact fragmentation, previous studies have\nshown a correlation between surface exposure and mean grain size in regolith samples with\ngrain sizes <1 cm (refs. 70,95); however, this correlation does not persist for mean grain sizes\n<1 mm. Moreover, Weibull statistics in rock mechanic model outputs demonstrate that smaller\ngrains require higher stresses for complete failure96,97. Thus, for lunar regolith with mean grain\nsizes <1 mm, impact fragmentation likely contributes little to reshaping, with the deposited\nenergy instead redistributed into broader-scale impact gardening. Consequently, impact\ngardening is considered the dominant driver of morphological modification in surface soils\n(Fig. 5). The bulk soil fraction of lunar regolith was the primary target of the micro-CT scans\nof this study, as it represents the dominant fraction. Despite notable differences in age,\ngeological setting, lava origin, micrometeorite bombardment intensity, and solar wind\nirradiation between the CE5 and CE6 landing sites24,25,31,53,98, the morphometrics of\ncorresponding particle types remain remarkably consistent across both sites. This convergence\nindicates that long-term surface particle reshaping processes, dominated by impact gardening,\nhas likely driven all particle types toward stable morphologies, such that further space\nweathering processes exert limited influence on particle morphometrics.”\nLines 465-469:\n“During space weathering, regolith particles undergo repeated reshaping that ultimately leads\nto progressive soil comminution. Larger particles are continually broken down into the bulk\nsoil fraction, while the bulk soil fraction itself steadily generates lunar dust. This process results\nin the gradual enrichment of lunar dust within the regolith (Fig. 5).”\n(2) The authors limit the model to the bulk soil size fraction (20-200um), but considering the\nfact that larger particles supply the bulk soil through fragmentation, and bulk particles\nthemselves continue to break down into finer particles, all of these particle populations coexist\nand interact. Therefore, it is questionable to assert that equilibrium applies exclusively to one\nsize fraction without considering the whole regolith system.\nResponse: We thank the reviewer for this important and constructive comment. As noted, the\nbulk soil fraction (equivalent diameter 20–200 μm) was the primary target of our micro-CT\nscans because it constitutes the dominant component of the regolith (>60 vol%) and contains\nthe largest number of identifiable particles with clearly internal textures. Consequently,\nmorphometric statistics derived from this fraction are robust and form the backbone of our\nmain conclusions. However, as the reviewer rightly noted, larger particles continuously supply\nthe bulk soil fraction through fragmentation, and bulk particles themselves further disintegrate\ninto finer dust; all of these particle populations coexist and interact dynamically. Therefore, it\nis indeed questionable to claim that equilibrium applies exclusively to one size fraction without\nconsidering the regolith system as a whole.\nBecause lunar dust particles are too small to be reliably characterized by conventional\nmicro-CT scans, we instead sought to evaluate whether a saturation in morphological\nmodification might also exist in the large particle population (equivalent diameter >200 μm).\nIt should be emphasized, however, that the CE5 and CE6 materials available to us were surface\nscooped samples with total masses of only ~2 g each. Consequently, the number of large\nparticles within them is naturally very limited, making it difficult to establish statistical\nregularities comparable to those obtained for the bulk soil fraction. To help mitigate this issue,\nwe adopted a randomized selection strategy. For both CE5 and CE6, we randomly picked ~200\nparticles each within the 500–700 μm size range (Reply Fig. 2a).\nTo enable a meaningful comparison with Apollo regolith at the same resolution (see our\nreply to Comment 1(4)), all particles were scanned at a voxel size of 25.8 μm. Basalt was\nchosen as the focus of analysis for two reasons: (i) basalt is a principal component of mare soils\n7\nand therefore highly representative of lunar regolith; and (ii) basalt exhibits clear and\nrecognizable textures, which allows unambiguous identification in micro-CT radiographs from\nboth CE5/CE6 and Apollo 17 (73002) drive-tube sample radiographs. Careful inspection of the\nCE5 and CE6 radiographs yielded 22 basalt particles from CE5 and 27 from CE6. For Apollo\n17 (73002), owing to the particular characteristics of the drive-tube sample, a sufficiently large\nset could be obtained, and we analyzed 110 basalt particles (see reply to Comment 1(4)).\nIn total, we analyzed 159 basalt particles across the three datasets, deriving distributions\nof AR, S, Cx, and R (Reply Fig. 2b–e). The results show that the three large basalt populations\nhave highly similar mean values and standard deviations, with differences in the mean values\nwithin ~10%. The weaker agreement in morphometric trends compared to the bulk soil fraction\nlikely reflects the limited number of large basalts available in the CE5 and CE6 powdered\nsamples, which diminishes statistical robustness. Nevertheless, our use of randomized particle\nselection helps ensure consistency in average values. Overall, the morphometric results suggest\nthat even for larger particles, a saturation state in morphology may persist, although more\nextensive datasets will be needed to rigorously confirm this conclusion in future work.\nWe have added the above discussion to paragraph [21] and included Supplementary Fig.\n9 to present the large particle experiment and its morphometric results.\nReply Figure 2. a Photograph of CE6 large particles (500–700 μm) arranged in an orderly\nmanner on yellow adhesive tape prior to micro-CT scanning. b-e Morphometric statistical\ndistributions for large basalt particles in the CE5, CE6, and Apollo 17 (73002) samples.\nRevision: We have revised lines 460–465 to include additional results from our additional\nmicro-CT scanning tests on larger particles. These analyses were conducted to assess whether\nthe concept of saturation in morphological modification may also apply beyond the bulk soil\nsize fraction.\nLines 460-465:\n“We selected 22 basalt particles (500–700 µm) from CE5, 27 particles of the same size range\nfrom CE6, and 110 particles from the upper 3 cm of Apollo 17 sample 73002 radiographs88.\nCE5 and CE6 basalts were analyzed using micro-CT scans at a voxel size of 25.8 µm, the same\nresolution as applied to the 73002 radiographs. Morphometric results show that even for larger\nparticles in more strongly weathered soils, the saturation state may persist (Supplementary\nFig. 9), although further validation with larger datasets will be required.”\nadditional experiments and further illustrates the potential persistence of saturation trends in\nlarger particle populations.\nSupplementary Fig. 9:\nSupplementary Fig. 9 | Morphometrics statistical distributions for large basalt particles\nin CE5, CE6, and Apollo 17 (73002) samples. To match the 25.8 μm resolution of the 73002\nradiographs, large basalt particles with equivalent diameters of 500–700 μm were selected from\nall three samples. Comparisons across the three datasets reveal highly similar mean values and\nstandard deviations, with differences in the means <10%. The weaker agreement in\nmorphometric trends compared to the bulk soil fraction may result from the limited number of\nlarge basalt particles in the CE5 and CE6 powdered samples, potentially diminishing statistical\nrobustness. However, the use of randomized particle selection helps ensure consistency in\naverage values.\n(3) Moreover, if such a size-specific model is to be proposed, a clear rationale for the selection\nand definition of the bulk soil size range must be provided.\nResponse: Thanks for highlighting this important issue. We acknowledge that in the original\nversion we did not provide a sufficiently clear rationale for subdividing lunar regolith into\n“lunar dust,” “bulk soil,” and “large particles.”\nThe definition of “lunar dust” follows the commonly adopted convention of particles\nsmaller than 20 μm (Sun et al., 2017; Wallace et al., 2009). Although this fraction is an essential\ncomponent of the regolith, the extremely small size makes it difficult to capture morphological\ninformation at scale with current micro-CT techniques. In this study, therefore, we treat this\ngroup primarily as the end-product of space weathering.\nFor the “large particles” fraction, we defined the lower threshold at 200 μm based on the\nrelatively low abundance of such particles in the CE5 and CE6 soils. As shown in Reply Figure\n3, particles larger than 200 μm account for <15% of the total regolith volume. Within the bulk\nsoil micro-CT framework adopted here, this fraction yields only a limited number of\nmeasurable particles, which constrains the statistical robustness of their morphometric\ndistributions. Nonetheless, this does not imply that large particles are excluded from the\nsaturation in morphological modification of space weathering, as we have further elaborated in\nour response to Comment 1(2).\nBy contrast, the 20–200 μm fraction constitutes the dominant component of the CE5 and\nCE6 soils, accounting for >60% of the total regolith volume. This size range therefore\nrepresents the principal carrier of surface-evolutionary signatures and provides the most\nstatistically robust basis for morphometric analysis. Accordingly, we refer to this interval as\nthe “bulk soil fraction” and focus on it as the primary foundation for our model.\nfunction of equivalent particle diameter. Yellow, red, and green shaded regions indicate the\nparticle size ranges corresponding to “lunar dust,” “bulk soil,” and “large particles,”\nrespectively.\nRevision: We have revised lines 48-53 as follows\n“Based on increasing degrees of space weathering, lunar regolith can be conceptually\nsubdivided into large particles (equivalent diameter >200 μm), bulk soil (20–200 μm), and\nlunar dust (<20 μm), with the size range of the latter defined according to established\ncriteria22,23. Among these, the “bulk soil” fraction constitutes the dominant component,\naccounting for >60% of the total regolith volume, thereby serving as the primary carrier of\nsurface-evolutionary signatures on the Moon24,25.”\n(4) Lastly, the authors suggest that morphological saturation has occurred by ~2 Ga, but this\nclaim lacks direct comparison with older Apollo regolith particles. Without examining whether\nApollo particles also exhibit similar morphometrics, it is difficult to confidently conclude that\nthe saturation has been reached. Why don’t the authors interpret the observed morphological\nsimilarity between CE5 and CE6 simply due to their similar ages?\nResponse: Thanks for raising this constructive and important point. We fully acknowledge that\nthis was a limitation of the previous version of our manuscript, as we were unable to directly\naccess Apollo soils for micro-CT scanning test under the same experimental conditions, and\ntherefore could not provide solid first-hand evidence for morphological saturation. However,\nadditionally inspired by Reviewer #2’s Comment 29, we realized that the formation age of the\nChang’e basalts (CE5 ~2.0 Ga, CE6 ~2.8 Ga) is not directly linked to the extent of space\nweathering experienced by the regolith. For example, if an impact event occurred in the mare\nundergone 100 million years of space weathering, irrespective of the basalt crystallization age\n10\n(Morris, 1978). Consequently, in the revised manuscript we describe the degree of space\nweathering using a multi-proxy suite of maturity indicators, including agglutinate abundance,\nIs/FeO values, and surface exposure ages of the regolith (see revision to Comment 1(1)). The\ndominant process influencing particle morphology is impact gardening, and as McFadden et al.\n(2024) emphasize, where its effect on the modification of the surface regolith is best captured\nby surface exposure ages. We therefore characterize morphological saturation primarily in\nterms of surface exposure age.\nTo address the reviewer’s concern regarding the absence of Apollo soils in our dataset, we\nadopted a compromise approach. The Apollo missions returned 382 kg of material from six\nsites, comprising ~2,200 individual samples. Although nearly all of these samples have been\nexamined, a few remained unopened until recently (Eckley et al., 2025). Among these was the\nApollo 17 double drive tube 73001/2, which sampled regolith to ~70 cm depth in the Light\nMantle landslide deposit at the base of the South Massif in Taurus-Littrow Valley. The upper\nsection (73002) was opened and scanned by micro-CT in 2019, with experimental data\nsubsequently released (Eckley et al., 2025). Previous work indicates that 73002 sampled\nseveral basalt flows, and its surface maturity is well constrained: the uppermost regolith\nexhibits an Is/FeO value of ~74 (Morris et al., 2022) and a surface exposure age of ~3.0 Ma\n(McFadden et al., 2024). In comparison, CE5 soils show an Is/FeO value of ~14 and an\nexposure age of ~1.5 Ma (Qian et al., 2024; Zhang et al., 2025b), while CE6 soils exhibit an\nIs/FeO of ~56.5 and an exposure age of ~2.1 Ma (Zhang et al., 2025b; Li et al., 2025). Thus,\nthe Apollo 17 sample is more mature than both CE5 and CE6, and its derivation from a\nlandslide deposit ensures mineralogical and lithological complexity. If morphological\nsaturation is still observed in 73002 surface regolith particles, it would imply that such\nsaturation can be achieved by the relatively short exposure age of CE5 (~1.5 Ma).\nIt should be noted, however, that the Apollo 73002 CT radiographs were acquired at a\nresolution of 25.8 μm (Eckley et al., 2025; Reply Figure 4a). This restricts morphological\ncharacterization to particles larger than ~500 μm, precluding direct comparison with the bulk\nsoil fraction (20–200 μm) analyzed in this study. To enable meaningful comparison, we\ntherefore selected basalt particles in the 500–700 μm range from both CE5 and CE6 soils, using\nbasalt as the target material (see also response to Comment 1(2)).\nFor CE5 and CE6, which are shallow scoop samples with limited large particle populations,\nwe manually selected ~200 particles each, yielding 22 basalt grains from CE5 and 27 from\nCE6. For 73002, we restricted our analysis to the uppermost ~3 cm of regolith, to both match\nthe near-surface context of CE5 and CE6 and to ensure strong space weathering effects. Basalt\ngrains were readily identified in the radiographs (Reply Figure 4b), manually extracted, and\nreconstructed in 3D for morphometric analysis (Reply Figure 4c–d). The larger sample size\navailable from Apollo 73002 allowed us to extract 110 basalt particles, ensuring robust\nstatistics.\nOur morphometric results (Reply Figure 2b–e) reveal highly similar distributions of AR,\nS, Cx, and R across Apollo 17 large basalts and those from CE5 and CE6. The mean values\ndiffer by <10%, with comparable standard deviations. This result suggests that morphological\nsaturation is already established by the CE5 surface exposure age of ~1.5 Ma. Nevertheless,\nwe emphasize that further validation with larger CE5 and CE6 datasets is required to test this\nconclusion in future work.\nlight-purple shaded area indicating the region selected for large basalt particle (data source:\nhttps://data.mendeley.com/datasets/5y725j47wf/1). b-d Procedure for extracting large basalt\nparticles from the 73002 radiographs.\nRevision: This section has been revised to include the new supplementary analyses described\nin our response to Comment 1(2). Specifically, we have:\n(i) Added text in the main manuscript (paragraph [21]) clarifying the rationale for using Apollo\n17 (73002) large basalts as a comparative dataset for large particle morphologies.\n(ii) Incorporated new results in Supplementary Figure 9, which illustrate the morphometric\ncomparison of large basalt particles between CE5, CE6, and Apollo 17.\nWe have also revised lines 26-29 of the Abstract to clarify the timescale required to reach the\nmorphological saturation state:\n“Despite their contrasting volcanic and impacting histories, the soil particles for these youngest\nnearside/farside samples exhibit strikingly similar morphometric distributions, suggesting that\nspace weathering reached saturation in shaping surficial soil particle morphology in ~1.5\nmillion years or less.”\n2. On regolith particle evolution processes:\nIn paragraph [20], the authors note that impact gardening, thermal cycling, and solar wind\nirradiation contribute to fragmentation and abrasion. However, the role of agglomeration\nprocesses is not discussed. For example, breccia particles may be formed by the strong\n12\ncompression due to meteoroid impact, and agglutinate particles are formed by the gathering of\npartly melted fragments. Also, fragmentation and abrasion can have opposing effects on\nparticle morphology. Such processes must be considered to accurately evaluate regolith\nevolution. The current interpretation risks oversimplifying the particle evolution process.\nResponse: Very insightful comment. We fully agree that our initial version of Figure 5 and the\nassociated discussion in paragraph [20] oversimplified the mechanisms underlying\nmorphological equilibrium, as we did not adequately address the role of impact agglutination,\nnor did we provide sufficient discussion of the contrasting effects of fragmentation and\nabrasion. As the reviewer correctly notes, agglutination plays a critical role in the formation of\nbreccias and agglutinates, while fragmentation and abrasion may exert opposing influences on\nparticle morphology. The omission of these aspects in our initial submission indeed made our\nconceptual model less comprehensive.\nIn the revised manuscript, we have restructured our discussion of morphological saturation\nmechanisms to provide a more complete representation of lunar surface processes. Specifically,\nparticle reshaping mechanisms now include the following.\n Impact gardening (mainly abrasion caused by repeated interactions among numerous\ngrains within the regolith)\n Impact fragmentation (grain breakage via direct energy deposition during impacts)\n Impact agglutination (fusion of surrounding grains into breccias or agglutinates due to\nimpact-induced melts or semi-plastic phases)\n Impact sputtering (deposits from impact events appearing randomly on the surface of\nminerals, forming a sputtering deposit layer that modifies particle morphology)\n Impact abrasion (localized surface wear caused by micrometeoroid strikes or secondary\nimpacts that generate linear scratches)\n Thermal fatigue (fracturing and disintegration from diurnal thermal stresses)\n Solar wind irradiation (implantation of high-energy ions producing lattice defects, which\nincrease susceptibility to subsequent weathering-induced reshaping; in addition, sputtering\nby the solar wind which may also erode particle surfaces)\nAmong these processes, impact agglutination is particularly distinctive as it transforms\n“simple particles” into “complex particles” in a cumulative manner, producing irregular\naggregates that strongly affect overall regolith morphology. Other processes generally reshape\nparticles without altering their fundamental type, although “complex particles” may shed small\nfragments that revert to “simple particles”. Both particle types are subsequently subject to\nfurther modification by the full suite of processes.\nRegarding the relative influence of fragmentation and abrasion, previous studies\ndemonstrate that surface exposure age correlates with the grain size of lunar regolith in the >1\nmm fraction (Morris et al., 1976; McKay et al., 1974), but this relationship weakens below the\ncentimetre scale. Furthermore, Weibull statistics show that smaller grains require\ndisproportionately higher stresses to fail completely (Weibull, 1951; McDowell et al., 2000).\nThis result implies that for lunar regolith dominated by <1 mm particles, impact fragmentation\nis limited, and the majority of impact energy is redistributed into broad-scale impact gardening,\nwhich thus emerges as the primary driver of morphological modification.\nDespite marked differences in age, geological setting, basaltic source regions,\nmicrometeorite flux, and solar wind exposure between CE5 and CE6 sites, the morphometrics\nof corresponding particle types remain remarkably consistent. This convergence strongly\nsuggests that long-term particle reshaping, governed primarily by impact gardening with\ncontributions from agglutination, drives both “simple” and “complex” particles toward stable\nmorphologies, such that additional space weathering exerts only minor influence on particle\nmorphometrics (Reply Figure 5).\nReply Figure 5. Prolonged space weathering (exposure age >1.5 Ma) drives particles in the\nsurface regolith bulk soil fraction to exhibit saturation in morphological modification, as shown\nhere. Solid arrows denote dominant processes exerting strong influence on particle morphology,\nwhereas dashed arrows indicate weaker effects. Although this figure illustrates the saturation\neffect primarily within the bulk soil fraction, we also provide evidence that similar saturation\noccurs for larger particle fractions (see response to Comment 1(2)). During space weathering\nprocessing, the proportion of lunar dust steadily increases until a grain size steady-state is\nachieved.\nRevision: We have refined paragraph [20] to incorporate the above mechanisms, providing a\nmore complete framework consistent with the lunar surface environment. In addition, Figure 5\nhas been revised to reflect the interplay among agglutination, fragmentation, and abrasion,\nensuring a balanced depiction of the particle evolution processes.\nParagraph [20]:\n“Particle reshaping processes at the lunar surface include impact gardening (mainly abrasion\ncaused by interactions among large numbers of particles within a given region78), impact\nfragmentation (grain breakage due to direct energy deposition during impacts90), impact\nagglutination (where impact-induced melts or semi-plastic phases fuse surrounding grains into\nagglutinates and breccias), impact sputtering (deposits from impact events appear randomly on\nthe surface of minerals, forming sputtering deposit layer that modify particle morphology46),\nimpact abrasion (localized surface wear induced by micrometeoroid impacts20,91 or secondary\n14\nimpacts that generate linear scratches92), thermal fatigue (fracturing driven by thermal stresses\nleading to ultimate grain disintegration90), and solar wind irradiation93 (high-energy ion\nimplantation generates crystalline defects, making grains more susceptible to subsequent space\nweathering induced reshaping94; in addition, sputtering by solar wind may also erode particle\nsurfaces67). Impact agglutination is distinctive in that it can transform “simple particles” into\n“complex particles” and operates cumulatively, producing highly irregular aggregates that\nstrongly influence the overall particle morphology of the regolith. By contrast, other reshaping\nprocesses generally do not alter particle type (although “complex particles” may shed minor\nfragments of “simple particles”). Both “simple” and “complex” particles are subsequently\nmodified by additional reshaping processes. Among these mechanisms, only impact gardening\nand impact fragmentation significantly modify regolith within a given depth, exerting the\ngreatest influence on particle morphology. For impact fragmentation, previous studies have\nshown a correlation between surface exposure and mean grain size in regolith samples with\ngrain sizes <1 cm (refs. 70,95); however, this correlation does not persist for mean grain sizes\n<1 mm. Moreover, Weibull statistics in rock mechanic model outputs demonstrate that smaller\ngrains require higher stresses for complete failure96,97. Thus, for lunar regolith with mean grain\nsizes <1 mm, impact fragmentation likely contributes little to reshaping, with the deposited\nenergy instead redistributed into broader-scale impact gardening. Consequently, impact\ngardening is considered the dominant driver of morphological modification in surface soils\n(Fig. 5). The bulk soil fraction of lunar regolith was the primary target of the micro-CT scans\nof this study, as it represents the dominant fraction. Despite notable differences in age,\ngeological setting, lava origin, micrometeorite bombardment intensity, and solar wind\nirradiation between the CE5 and CE6 landing sites24,25,31,53,98, the morphometrics of\ncorresponding particle types remain remarkably consistent across both sites. This convergence\nindicates that long-term surface particle reshaping processes, dominated by impact gardening,\nhas likely driven all particle types toward stable morphologies, such that further space\nweathering processes exert limited influence on particle morphometrics.”\nFigure 5:\n15\n3. On the Comparison with Lunar Regolith Simulant (LRS):\nIn paragraph [14], the authors observe that the LRS particles exhibit lower aspect ratios and\nhigher sphericity, convexity, and roundness compared to lunar basalt particles, and state that\nthis resembles the morphology of asteroid surface particles. However, since LRS particles are\nproduced by mechanical crushing and have not undergone long-term abrasion, it is not\nsurprising that they resemble the asteroid surface particles which have very short surface\nresidence times.\nResponse: We appreciate the reviewer’s insightful comment regarding our comparison\nbetween LRS and asteroid surface particles. We fully agree that the observed similarity is not\nunexpected, as LRS is produced by mechanical crushing and therefore retains the morphology\nof fresh fragments that have not been subjected to long-term abrasion. Likewise, asteroid\nsurface particles typically exhibit short residence times and limited regolith gardening, which\nleads to comparable morphometric characteristics. We have revised the text to clarify that the\nresemblance arises primarily from the lack of prolonged surface processing, rather than\n16\nsuggesting a unique evolutionary process. This adjustment provides a more accurate context\nfor interpreting the morphometric comparison and better aligns with the reviewer’s observation.\nRevision: In paragraph [14], we have revised the description of the comparison between LRS\nand asteroid particles. Specifically, we replaced the phrasing “Interestingly, such a lower aspect\nratio resembles…” with “Such characteristics are consistent with…,” and added an explanation\nthat the similarity arises from the lack of prolonged abrasion and comminution in both\nmechanically crushed simulants and asteroid regolith. This revision clarifies that the\nresemblance is expected and avoids overstating the interpretation.\nLines 293-297:\n“Such characteristics are consistent with the angular-to-subrounded morphologies observed on\nasteroid regolith surfaces, where particles typically experience short surface residence times\nand limited abrasion67,68. This similarity likely reflects the fact that both mechanically crushed\nsimulants and asteroid regolith are dominated by relatively “fresh” fragments that have not\nundergone prolonged regolith gardening.”\nWe also noted that the LRS morphometrics is not directly relevant to the core mechanisms\ndiscussed in our study. Placing the LRS data at the top of Figure 4b may be misleading;\ntherefore, we have adjusted the layout of Figure 4b accordingly. In addition, since our analysis\nof OR also does not involve LRS morphometrics, we have removed the LRS data from Figure\n4d as well. Detailed revisions are provided in the revision to Comment 1(1).\n4. On the Interpretation of Agglutinate Porosity Differences:\nThe discussion in paragraphs [18] and [19] are largely speculative. Paragraph [18] explains the\nlower internal porosity observed in CE6 agglutinates from the different micrometeorite\nbombardment condition in nearside and farside. This can be proved by comparing with the data\nof Apollo samples. In paragraph [19], the connection between agglutinate structure, volatile\ncontents, and gas escape efficiency is mentioned, but the argument lacks cohesion and empirical support. These sections would benefit from a clearer presentation and more cautious language regarding the proposed mechanisms.\nResponse: Thanks for this insightful comment. In paragraph [18], we noted that previous\nresearch has shown CE5 soils experienced relatively lower temperatures as induced by\nmicrometeorite impacts compared to Apollo soils (Nie et al., 2024; Guo et al., 2022). In\ncontrast, silicon isotope modeling suggests that for CE6 soils, the impact-induced glass\nformation temperatures—primarily controlled by micrometeorite impact velocities—are likely\ncomparable to those documented for Apollo soils (Zhang et al., 2025a). We hypothesized that\nthe higher glass formation temperatures associated with more intense micrometeorite\nbombardment at the CE6 landing site may have contributed to the lower agglutinate void ratios\nobserved in CE6 relative to CE5.\nThe reviewer’s suggestion to verify this hypothesis using Apollo data is particularly\nvaluable. Since our CE5–CE6 comparison already uses Apollo soils as a benchmark, Apollo\nsamples provide an excellent reference dataset for agglutinate porosity. We selected an Apollo\n17 sample (73002) for this comparison because this site is incorporated in the thermal modeling\nof Nie et al. (2024). Following the same analytical procedures described in response to\nFig. 6a–b) and calculated their void ratios using the same methodology as for CE5 and CE6.\nThe resulting average void ratio for Apollo 17 agglutinates (e = 0.10) is nearly identical to\nA17\nthat of CE6 agglutinates and significantly lower than CE5 (Reply Fig. 6c). Combined with the\nevidence for similar glass formation temperatures between CE6 and Apollo soils, this result\nprovides additional support for our hypothesis.\nIt is important to note that, additionally inspired by Reviewer #2’s Comment 26, we revised\nthe method for calculating the average Ir and void ratios of bulk soil agglutinates. In the original\nversion, these values were computed as weighted averages across all particles:\nk\nn\nri\nIr  i1 (1)\naverage k\nn\nti\ni1\nk\nV\nvi\ne  i1 (2)\naverage k\nV\npi\ni1\nwhere n and n are the counts of regular and total vesicles in the i-th agglutinate, V and V\nri ti vi pi\nare vesicle and particle volumes, and k is the total number of agglutinates analyzed. This\napproach suffers, however, from two shortcomings: (i) it is biased toward large particles,\nand (ii) it does not allow reliable estimation of sample variance.\nIn the revised approach, we calculate Ir and void ratios for each bulk soil agglutinate\nindividually and treat these values as statistical samples to compute the mean and standard\ndeviation. As shown in Reply Fig. 6c, this refinement causes a slight rightward shift in the data,\nindicating that the previous version method slightly underestimated the vitrification of\nagglutinates in the bulk soil. However, the overall trends remain unchanged: CE6 bulk soil\nagglutinates still exhibit lower Ir values (Ir = 0.46) than CE5 (Ir = 0.49), and the error\nCE6 CE5\nbars confirm the statistical robustness of this result.\nFinally, regarding paragraphs [18–19], we agree with the reviewer that the original\ndiscussion linking agglutinate structure, volatile contents, and gas escape efficiency lacked\ncoherence and empirical support. We have substantially revised this section to present a\nmechanism-based, but non-exclusive, interpretation.\nIn silicate melts, lower viscosity—typically at higher temperatures—facilitates bubble\nascent, coalescence, and the development of permeable vesicle networks, allowing for efficient\nopen-system degassing once pathways connect to the atmospheric vacuum (Harris et al., 2008;\nGonnermann & Manga, 2013). In micrometeorite-formed agglutinates, transient melt films can\nenable gases to vent through connected vesicles before cooling; when degassing is efficient,\nthe resulting internal porosity can be significantly reduced compared to less-degassed clasts\n(Rust et al., 2004).\nWe now interpret the lower porosity of CE6 agglutinates as consistent with (rather than\nproving) more efficient syn-formation gas escape under open-system conditions. At the same\ntime, we explicitly acknowledge alternative explanations, including viscosity contrasts during\n18\nformation, variations in melt fraction, vesicle connectivity, and post-formation vesicle collapse.\nWe also clarify that we do not directly measure volatile contents in this study; our inference is\ntextural and mechanistic, supported by established degassing literature. These clarifications\nhave been incorporated into the revised manuscript, with appropriately cautious language.\nReply Figure 6. a Radiograph of the upper 3 cm of Apollo 17 (73002) regolith (Eckley et al.,\n2025), with agglutinate particles highlighted by white boxes (data source:\nhttps://data.mendeley.com/datasets/5y725j47wf/1). b Extracted agglutinate particles from\nApollo 17 (73002). c 42 Apollo 17 (73002) agglutinates were added as a horizontal solid line\nto Figure 3a with the average void ratio data, showing values consistent with CE6 agglutinates\nbut significantly lower than CE5 agglutinates.\nRevision: Lines 231–232 were revised to reflect the updated Ir values:\n“For agglutinates in bulk soils, Ir values are higher in CE5 (Ir = 0.49) than in CE6 (Ir = 0.46).”\nFigure 4a was updated to include Apollo 17 data.\nLines 384–388 were revised to clarify the mechanism:\n“The comparatively lower internal porosity observed in larger CE6 agglutinates is consistent\nwith more efficient syn-formation gas escape under open-system conditions, where transient\nhigh temperatures reduce melt viscosity and promote rapid bubble ascent and vesicle\ncoalescence, thereby facilitating volatile venting before cooling83-85 (Fig. 5).”\ncautious:\n“Based on radiographs of Apollo 17 sample 73002 (ref. 88), we further extracted the 3D\nmorphologies of 42 agglutinates, which yield an average void ratio of 0.10, comparable to that\nof CE6 larger agglutinates (Fig. 3a), providing additional support for this hypothesis. However,\nwe emphasize that final porosity also depends on melt viscosity, cooling and welding history,\nvesicle connectivity, and possible post-formation collapse, so this interpretation should be\nregarded as plausible rather than unique.”\n[1] Eckley, S. A., Zeigler, R. A., Ketcham, R. A., Edey, D., Hanna, R. D., Gross, J., ... &\nANGSA Science Team. (2025). Utilization of X‐ray computed tomography during the\npreliminary examination of unopened Apollo drive tube samples 73001 and 73002. Journal\nof Geophysical Research: Planets, 130(8), e2024JE008583.\n[2] Gonnermann, H. M., Manga, M., & Fagents, S. A. (2013). Dynamics of magma ascent in\nthe volcanic conduit. Modeling volcanic processes: The physics and mathematics of\nvolcanism, 55-84.\n[3] Guo, Z., Li, C., Li, Y., Wen, Y., Tai, K., Li, X., ... & Ouyang, Z. (2022). Nanophase iron\nparticles derived from fayalitic olivine decomposition in Chang'E‐5 lunar soil:\nImplications for thermal effects during impacts. 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Permeability of vesicular silicic magma: inertial\nand hysteresis effects. Earth and Planetary Science Letters, 228(1-2), 93-107.\n[18] Sun, H., Yi, M., Shen, Z., Zhang, X., & Ma, S. (2017). Developing a new controllable lunar\ndust simulant: BHLD20. Planetary and Space Science, 141, 17-24.\n[19] Wallace, W. T., Taylor, L. A., Liu, Y., Cooper, B. L., McKAY, D. S., Chen, B., & Jeevarajan,\nA. S. (2009). Lunar dust and lunar simulant activation and monitoring. Meteoritics &\nplanetary science, 44(7), 961-970.\n[20] Weibull, W. (1951). A statistical distribution function of wide applicability. Journal of\napplied mechanics.\n[21] Zhang, H. Y., Yu, H. M., Tang, H. L., Lin, Y. C., Xiao, Z., Yang, L., ... & Huang, F. (2025).\nSpace weathering on the lunar nearside and farside constrained from Si isotopes. Nature\nCommunications, 16(1), 4248.\n[22] Zhang, M., Fa, W., & Jia, B. (2025). Provenance and evolution of lunar regolith at the\nChang’e-6 sampling site. Nature Astronomy, 1-11.\n1. References do not appear to be in correct order (ie reference 14 is cited in line 39 which is\nfollowed by reference 51 in line 41). I would suggest reordering the reference list so that they\nare increasing consecutively as they are being used in the text for clarity.\nResponse: We thank the reviewer for carefully checking the reference sequence and pointing\nout this inconsistency. We apologize for this oversight on our part. We fully agree that the\nreferences should be presented in consecutive order according to their first appearance in the\ntext, and we appreciate this valuable suggestion for improving the clarity of the manuscript.\nRevision: We have carefully reordered the reference list to ensure that all citations now appear\nin strictly increasing numerical order as they are cited in the manuscript.\n2. Additional lunar sourcebook chapters I feel would be relevant here are Lunar Rocks (Taylor\net al. 1991) and The Lunar Regolith (McKay et al. 1991).\nResponse: We thank the reviewer for this insightful suggestion. We acknowledge that our\nprevious citation of the Lunar Sourcebook was too general. Upon careful verification, we\nconfirm that both 2 citations in the manuscript specifically refer to the chapter The Lunar\nRegolith (McKay et al., 1991). We apologize for the earlier lack of precision and have now\ncorrected the reference accordingly.\nRevision: In the bibliography, we have revised reference 59 to:\n“McKay, D. S., Heiken, G., Basu, A., Blanford, G., Simon, S., Reedy, R. & Papike, J. The\nlunar regolith. Lunar Sourcebook 567, 285–356 (1991).”\n3. Recommended references for agglutinate formation: Solar wind and micrometeoroid\nalteration of the lunar regolith (Housley et al. 1974); Chemical Variability and Origin of\nAgglutinitic Glass (Basu and McKay 1985); Space Weathering from Mercury to the asteroid\nBelt (Hapke 2001).\nResponse: Good suggestion. In response, we have expanded the review of agglutinate\nformation at the beginning of paragraph [10] to provide a more comprehensive overview.\nRevision: Lines 194–201 have been revised as follows to incorporate a concise review of\nagglutinate formation:\n“Agglutinates are special regolith particles consisting of smaller lunar soil constituents (mineral\nparticles, glasses, and older agglutinates) bonded together by vesicular, flow-banded glass,\nformed during micrometeorite impacts59. They originate through the combined action of solar\nwind irradiation and micrometeorite melting, which reduce Fe in mineral surfaces and generate\nvesiculated glassy matrices incorporating submicroscopic metallic iron60. The chemical\nvariability of these agglutinate glasses reflects the total melting of microtargets and\nincorporation of the finest soil fraction during impact events61, while subsequent vapor\ndeposition and space weathering processes further enrich agglutinates with nanophase iron that\ncontrols their optical and magnetic properties62.”\n4. Line 43: Saying that agglutinates are vesicular in nature and exhibit internal pore structures\nsounds like they are two different things. I would just say that “…agglutinates exhibit\nvesicles…” for conciseness.\nhave revised the sentence accordingly for conciseness and clarity. This revision better reflects\nthe reviewer’s suggestion while preserving the intended meaning of the statement.\nRevision: We have modified the sentence in lines 43–46 to read as follows:\n“Agglutinates are irregularly shaped and contain vesicles that not only modulate volatile escape\nand encode the thermomechanical conditions of micrometeorite bombardment17, but also serve\nas nanoscale reservoirs capable of storing and mobilizing solar wind-derived hydrogen and\nwater18.”\n5. Recommended reference for agglutinate vesicle volatile contents and mobility: Nanoscale\nreservoirs store solar wind derived water on the lunar surface (Kling et al. 2025).\nResponse: Valuable recommendation, thanks. In response, we have expanded our discussion\nto include a review of agglutinate vesicle volatile contents and their mobility.\nRevision: Lines 43–46 have been revised as follows to incorporate this aspect:\n“Agglutinates are irregularly shaped and contain vesicles that not only modulate volatile escape\nand encode the thermomechanical conditions of micrometeorite bombardment17, but also serve\nas nanoscale reservoirs capable of storing and mobilizing solar wind-derived hydrogen and\nwater18.”\n6. Line 46-48: I would include a citation here. What are these subdivisions based off of? Are\nthey somehow related to other space weathering indices? Why is 20-200μm considered “bulk\nsoil”? Why is “lunar dust” <20μm?\nResponse: We sincerely thank the reviewer for pointing out this important issue. We\nacknowledge that in the original version we did not clearly explain the basis for subdividing\nlunar regolith into “lunar dust,” “bulk soil,” and “large particles.” The definition of “lunar dust”\nfollows the commonly adopted convention that classifies particles’ equivalent diameter smaller\nthan 20 μm (Sun et al., 2017; Wallace et al., 2009). Although this fraction is an essential\ncomponent of the regolith, its extremely small size makes large-scale morphological analysis\ndifficult. In this study, therefore, we treat this group primarily as the end-product of space\nweathering.\nFor the “large particles” fraction, we establish a threshold based on the relatively low\nvolume percentage of particles’ equivalent diameter >200 μm in the CE5 and CE6 soils. As\nshown in Reply Figure 1, such particles account for <15% of the total regolith volume. Within\nthe bulk soil micro-CT framework adopted in this study, this fraction yields only a limited\nnumber of measurable particles, making robust statistical analysis of their morphology\nchallenging. Nonetheless, this does not imply that large particles are excluded from the\nsaturation in morphological modification of space weathering, as we further discuss in response\nto Comment #28-29.\nBy contrast, the 20–200 μm fraction constitutes the dominant component of the CE5 and\nCE6 soils, accounting for >60% of the total volume. This size range therefore represents the\nmain carrier of surface-evolutionary signatures and is the primary focus of our analysis, which\nwe refer to as the “bulk soil fraction.”\nfunction of equivalent particle diameter. Yellow, red, and green shaded regions indicate the\nparticle size ranges corresponding to “lunar dust,” “bulk soil,” and “large particles,”\nrespectively.\nRevision: We have revised lines 48-53 as follows\n“Based on increasing degrees of space weathering, lunar regolith can be conceptually\nsubdivided into large particles (equivalent diameter >200 μm), bulk soil (20–200 μm), and\nlunar dust (<20 μm), with the size range of the latter defined according to established\ncriteria22,23. Among these, the “bulk soil” fraction constitutes the dominant component,\naccounting for >60% of the total regolith volume, thereby serving as the primary carrier of\nsurface-evolutionary signatures on the Moon24,25.”\n7. Line 53: Define “digital twin model”. Or get rid of that and just say “enabling predictive\nsimulations…”.\nResponse: We fully agree with the reviewer’s insightful comment. Compared with “digital\ntwin model,” the term “high-fidelity predictive simulations” is indeed more broadly applicable.\nWe have therefore revised the sentence accordingly.\nRevision: We have modified the text in lines 55–57 to read as follows:\n“Moreover, accurate morphological data serve as a key input for building high-fidelity\npredictive simulations of the Moon’s surface, enabling predictive simulations of regolith\nbehavior under various exploration scenarios27.”\n8. Line 56-57: Convert g to kg.\nResponse: We thank the reviewer for this comment. Due to the technical limitations of the\nCE5 mission, the total amount of returned lunar soil was relatively small, and the precise weight\nwas indeed 1,731 g, as officially reported (Li et al., 2022).\n25\n9. Line 74: I would clarify that micro-CT is applied to individual particles from bulk samples.\nResponse: Valuable comment. We agree that it is indeed necessary to clarify this point, as\ndoing so highlights the novelty of our study.\nRevision: We have revised lines 75-77 as follows\n“As a non-destructive technique, micro-CT scanning allows for high-resolution reconstruction\nof both external particle 3D morphology and internal structures of each individual particle\nwithin the bulk soil.”\n10. Line 79-81: “Second, the subsequent classification of particles by type is essential to\nsupport more refined interpretations of regolith formation and space weathering processes.”\nYou introduce this as a technical challenge to micro-CT but this doesn’t sound like a technical\nchallenge, this is more like a gap in our knowledge of grain morphology.\nResponse: Thanks for this helpful clarification. We agree that the original wording framed this\npoint as a micro-CT “technical challenge,” whereas it is better described as a knowledge gap\nin grain morphology interpretation. Accordingly, we have revised the sentence to explicitly\nreflect this and to specify the actual technical requirement on our side—namely, that the\nclassification algorithm must operate on accurately segmented particles while preserving their\ninternal textures to enable refined categorization and interpretation. Inspired by Comment #13,\nwe have also relocated this statement to paragraph [6], where we outline the workflow.\nRevision: We have updated lines 105–107 (moved to paragraph [6]) to read:\n“Second, the classification algorithm must operate on accurately segmented particles while\nretaining their internal textures, thereby enabling refined categorization and more refined\ninterpretations of regolith formation and space weathering processes.”\n11. Line 82: Recommended reference for the effects of space weathering on regolith grain\nmorphology: The nature and origin of rims on lunar soil grains (Keller and McKay 1997);\nThe microstructure of lunar micrometeoroid impact craters (Noble et al. 2016).\nResponse: Thanks for this valuable recommendation. We have incorporated both references\nand briefly summarize their key contributions to grain-scale morphological transformations\nunder space weathering. Keller & McKay (1997) used TEM to show that most lunar soil\nparticles develop nanometer-scale surface rims (typically ~60–200 nm) that fall into four end-\nmember types: amorphous (solar-wind irradiation damage with preferential cation sputtering),\ninclusion-rich (impact-vapor–deposited, containing abundant npFe0), multiple (irradiation\nlayer overlain by vapor-deposited material), and vesicular (abundant <50 nm vesicles from\nevolved implanted gases following pulse heating). These rims, and especially npFe0,\nsystematically modify surface texture and optical/magnetic properties, directly linking\nmicrostructure to macroscopic “maturity” signals.\nNoble et al. (2016) characterized natural micrometeoroid microcraters on lunar grains and\ndocumented melt linings (~0.5–2 µm), recrystallized layers, extensive shock-damaged zones\nextending up to tens of micrometers, near-surface vesiculation, annealing of solar-flare tracks,\nand pervasive npFe0 (~3–7 nm) in melt/recrystallized layers, demonstrating how localized\nmelting, vapor deposition, and shock collectively reshape grain surfaces and rims at the\nnanoscale.\nwind implantation, localized melting, and vaporization, which further modify particle\nmorphology. We appreciate the reviewer’s guidance, which strengthens the context and basis\nfor our statements.\nRevision: We have revised lines 89–91 to read\n“At the microscale, different mineral phases also exhibit varying responses to space weathering\nprocesses such as solar wind implantation, localized melting, and vaporization45,46, further\nmodifying particle morphology47,48.”\n12. Line 78-79: This sounds like it is the motivating paragraph for the study, but I am confused\nabout some of the points. I would clarify somewhere how lava composition influences grain\nmorphology with regards to comminution and space weathering. There is a lot of work out there that discusses how mineralogy on the lunar surface influences regolith fractionation (i.e.\nsize fraction). In addition, there is a lot of work that has been done that focuses on how different\nminerals react to space weathering processes, such as irradiation, melting and vaporization\nfrom impacts, and agglutination. So if you are trying to tie the importance of understanding\ngrain morphology with respect to collection site, space weathering, and composition, I would\nhighlight more evidence that suggests these things are related. Maybe don’t include the micro-\nCT content in the beginning, since that is a tool and unrelated to the question you are trying to\nanswer. Also, it would be good to succinctly state your hypothesis here.\nResponse: We thank the reviewer for this constructive guidance. In the revised Introduction,\nwe clarify the connections between mineralogy, space weathering, impact processes, and\nresulting particle shape, and we support these links with additional references. Variations in\nprimary lava composition yield distinct mineralogical assemblages in lunar regolith, which in\nturn govern particle-scale mechanical responses to impact processes. For example, plagioclase\nis more prone to brittle fracture than pyroxene, rendering plagioclase-rich basalts and breccias\nmore susceptible to crack propagation and localized abrasion during meteoroid bombardment\n(Hörz et al., 1984; Taylor et al., 2001). At the microscale, individual mineral phases respond\ndifferently to space weathering processes such as solar-wind implantation, localized melting,\nand vaporization, further altering particle morphology (Denevi et al., 2023; Cao et al., 2025).\nIn addition, spatial variations in impact energy across the lunar surface drive differences in\nlithologic fragmentation and regolith gardening, both of which significantly influence particle\nshape (Papike et al., 1982). Taken together, we emphasize that regolith particle morphology\nencodes an integrated signature of local geological context and cumulative space weathering\nmodification.\nFollowing the reviewer’s suggestion, we have relocated the mention of micro-CT (a\nmethodological tool) to paragraph [6], which focuses on the analytical workflow. To further\nmaintain a coherent narrative, we also now state our working hypothesis succinctly at the end\nof paragraph [6], allowing this paragraph to focus solely on framing the scientific motivation\nand evidential basis of the study.\nRevision: We have revised lines 84-94 as follows\n“Variations in primary lava composition give rise to distinct mineralogical assemblages in lunar\nregolith, which in turn affect particle-scale mechanical responses to space weathering. For\ninstance, plagioclase is more susceptible to brittle fracture than pyroxene, rendering\n27\nplagioclase-rich basalts and breccias more prone to crack formation and localized abrasion,\nthereby influencing particle morphology during meteoroid bombardment15,44. At the microscale,\ndifferent mineral phases also exhibit varying responses to space weathering processes such as\nsolar wind implantation, localized melting, and vaporization45,46, further modifying particle\nmorphology47,48. Additionally, differences in impact energy across the lunar surface produce\nvarying degrees of lithologic fragmentation and regolith gardening effects, both of which\nsignificantly affect particle morphology49. Thus, the morphology of lunar regolith particles\nrepresents an integrated signature of sampling site geological and space weathering processes.”\n13. Line 90-103: This reads like an abstract. You include a lot of results and discussion that\nshould be included in the body, rather than introduction. I would briefly introduce what was\ndone here. This might also be a good place to include the technical limitations of micro-CT.\nResponse: Thanks for this valuable suggestion. We agree that the original paragraph [6] read\ntoo much like an abstract by including results and interpretations. In the revised version, we\nnow (i) briefly state what was done, and (ii) explicitly note the key technical limitations of the\nmicro-CT method relevant to our dataset. Result-oriented statements have been removed from\nthis paragraph and are instead treated in the Results/Discussion sections. Also, methodological\ndetails are now moved to the Methods.\nRevision: We have revised paragraph [6]:\n“In this study, we apply micro-CT scans to bulk soil from the CE5 (CE5C0400YJFM01001)\nand CE6 (CE6C0200YJFM008) samples under identical settings, and use machine learning\napproaches to segment and classify individual particles in the scans. This standardized, non-\ndestructive workflow enables high-precision quantification of 3D morphometrics and internal\nstructures across multiple particle types. Several intrinsic limitations of micro-CT data\nprocessing are addressed in our study design. First, segmentation accuracy must be maintained\nacross a wide range of particle size to ensure the fidelity of measured morphometrics. Second,\nthe classification algorithm must operate on accurately segmented particles while retaining\ntheir internal textures, thereby enabling refined categorization and more refined interpretations\nof regolith formation and space weathering processes. These challenges are explicitly\naccounted for in our analysis. Based on this comprehensive dataset, we develop a model for\nthe formation and evolutionary processes of the surficial mare regolith, revealing that space\nweathering drives morphology saturation at the particle scale across diverse particle types. Our\nfindings provide new insights into the temporal trajectory of lunar surface modification and the\nconvergent morphological pathways induced by prolonged space weathering.”\n14. Fig. 1: Panels d, e, and f should be moved closer to where they are referenced in the article.\nIt doesn’t make sense to have them in the same figure as the one that shows the geological\ncontext of the Chang’e landing sites. Coordinates on panel a would be useful when comparing\nto Apollo and Luna landing sites.\nResponse: Thanks for this thoughtful suggestion. After careful consideration, we have retained\nonly panels a-c in Figure 1. We nevertheless decided to keep the Apollo and Luna landing sites\nannotated in panel a, for two reasons: (i) throughout the manuscript we compare CE5 and CE6\nresults with Apollo soils in sections such as “Mineralogical differences between CE5 and CE6\nbasalts”, “Distinct internal vesicles of agglutinates in CE5 and CE6 soils”, and “Morphological\n28\nhomogenization among similar lunar regolith particle types”; therefore, showing the Apollo\nlanding sites in the first contextual figure provides important reference points for the rest of the\npaper; (ii) CE5 landed in northern Oceanus Procellarum and CE6 in the farside SPA basin,\nboth geographically distant from Apollo and Luna sites. Highlighting these relative positions\nin Figure 1 underscores the geological distinctiveness of the CE soils and allows readers to\nappreciate this context from the get-go.\nIn addition, we have removed the original panel d (as it was not discussed in the main text\nexcept in Methods). We relocated panel e to the beginning of Figure 4 (now Figure 4a), where\nit aligns with the section “Morphological homogenization among similar lunar regolith particle\ntypes” and illustrates the 3D morphologies of the classified bulk soil particle types. The original\npanel f has been moved to the end of Figure 4 (now Figure 4e), replacing the previous Figure\n4d. This adjustment better illustrates: (i) that basalt and monomineralic particles dominate the\nCE soils, and (ii) following the reviewer’s Comment #28, that agglutinate abundance, rather\nthan fine particle content, is the appropriate parameter for explaining space weathering. We\nelaborate further on this point in detail in our response to Comment #28.\nRevision: Figure 1: revised to retain only panels a-c\nFig. 1 | Mare regolith samples from the lunar near and far sides. a Global distribution of\nlunar mare regions, color-coded by Ti abundance to distinguish different mare units (vht, very\nhigh-Ti basalt; ht, high-Ti basalt; mt, medium-Ti basalt; lt, low Ti basalt; vlt, very low-Ti basalt),\nwith major lunar endogenic lithologies50 (ma, magnesian anorthositic suite; kr, KREEP suite;\npy, pyroclastic deposit; fn, ferroan noritic suite; fa, ferroan anorthositic suite). Landing sites\nare color-coded by mission: Apollo (green), Luna (yellow), and Chang’e (red). In b, c, CE5\nand CE6 landing sites are shown on an orthographic albedo map imaged orbitally by Chang’e-\n2.\nFigure 4: revised by adding the former Figure 1e as Figure 4a and replacing the former Figure\n4d with the former Figure 1f.\n3D morphologies of each of the seven lunar regolith particle types—basalt, breccia, agglutinate,\nHm, Mm, Lm, and glass beads—identified via our machine learning approach of lunar bulk\nsoil. b Morphometrics statistical distributions for rock particles (basalt, breccia, and agglutinate)\nin bulk soil and LRS (IGG-01). c Morphometrics statistical distributions for monomineralic\nparticles (Hm, Mm, and Lm) in bulk soil. Size-dependent effects on morphometrics were\n30\nspecifically investigated for Mm and Lm particles by subdividing them into large (solid lines)\nand small (dashed lines) size groups. Comparisons between nearside and farside samples reveal\nhighly similar mean values, standard deviations, and distribution ranges across all particle types.\nd Boxplots comparing the overall regularity (OR) of six particle types in nearside and farside\nbulk soils. Black lines indicate medians; boxes represent the lower and upper quartiles;\nwhiskers denote the full range of the distributions. The shaded region highlights the\nconcentration OR range for “simple particles.” Agglutinate and breccia particles show\nconsiderably lower OR values compared to other particle types. In general, the OR distributions\nfor CE5 and CE6 are very similar, with slightly more regular particle shapes observed in the\nCE6 regolith. e Volume fractions of each particle type in the CE5 and CE6 regolith bulk soil\nsamples determined in this study. Owing to higher degree of space weathering at the CE6\nlanding site, it contains approximately ~9% more agglutinates and ~4% more breccia than the\nCE5 regolith.\nLines 262-271: modified so that Figure 4a is explicitly referenced in the correct order, ensuring\nconsistency between figure order and text citations.\n“Using machine learning segmentation and classification methods, we obtain morphometrics—\naspect ratio, sphericity, convexity, and roundness—with definitions provided in the Methods\nand Supplementary Fig. 10, for no fewer than 100 individual intact particles from each particle\ncategory from both CE5 and CE6 bulk soil CT scans, ensuring statistically representative\ndatasets for each particle type. The analyzed particle types include basalts, agglutinates,\nbreccias, high-density (Hm), medium-density (Mm), and low-density (Lm) monomineralic\nparticles, as well as glass beads (Fig. 4a). The Hm, Mm, and Lm groups are readily\ndistinguishable in radiographs based on their X-ray attenuation intensities, where Hm is\nprimarily composed of the opaque Fe-oxide mineral ilmenite, Mm mainly represented by\npyroxene, olivine, or anisotropic glass, and Lm consists predominantly of plagioclase\ngrains38,65.”\n15. Line 119: I would include citations from orbiter data confirming basalt makes up the mare\nsuch as Clementine, Kaguya, and Chandrayaan M3.\nResponse: Valuable suggestion. Upon further review of the literature, we have incorporated\nadditional citations from orbital datasets. In particular, Varatharajan et al. (2014) used Moon\nMineralogy Mapper (M3) data from Chandrayaan-1 to assess the mineralogical composition of\nmare basalts and to define mare regions based on their basaltic characteristics, confirming that\nthe lunar mare are predominantly composed of basalts.\nRevision: We have revised lines 123-125 to read as follows\n“Based on returned mare soils24,30 and lunar orbiter data51, basalts represent the dominant\nparticle type within lunar mare soils and can be used to reconstruct the thermochemical\nvolcanic history of the Moon7,52.”\n16. Line 121: You reference Fig. 2c before 2b. Fig. 2a isn’t referenced until the methods section.\nYou should reorder your figure panels so they align better with their reference order in the text.\nResponse: Thanks for the attention to detail. In accordance, we have reordered the panels of\nFigure 2 a–c to match the order in which they first appear in the main text (also a requirement\nof the journal). We have also ensured that Figure 2a is explicitly referenced in the main text,\nrather than only in the Methods section.\nRevision: Figure 2: the sequence of panels has been adjusted to align with the order of first\nmention in the manuscript.\n“Among basaltic textures, subophitic and poikilitic types (Fig. 2a, b) are readily distinguished\ndue to their relatively large crystal sizes and dominate the basaltic population, and can also be\naccurately classified using machine learning algorithms24,25,38,40. Basalt fragments, which are\nalso prevalent (Fig. 2b), exhibit distinct striped textures that facilitate their recognition. Mineral\nphase densities in basalts can be also effectively distinguished through CT radiographs (Fig.\n2b, c), enabling clear separation into high-density (Hp), medium-density (Mp), and low-density\n(Lp) phases based on grayscale intensity.”\n17. Line 124: I would write “Mineral phase densities in basalts…” because it’s not clear what\ndensities you are referring to as you are defining hd, md, and ld.\nResponse: Thanks for pointing out this ambiguity. The original phrasing indeed lacked clarity,\nas “densities” could be misinterpreted. What we intended to describe were the densities of\ndistinct mineral phases within basalts, specifically high-density (Hp, primarily ilmenite),\nmedium-density (Mp, mainly olivine or pyroxene), and low-density (Lp, predominantly\nplagioclase) phases identified from CT radiographs. Following the reviewer’s suggestion, we\nhave revised the sentence to explicitly state “Mineral phase densities in basalts…,” which\nremoves potential confusion and makes the meaning more precise.\nRevision: We have revised the wording in lines 128-131 to read:\n“Mineral phase densities in basalts can be also effectively distinguished through CT\nradiographs (Fig. 2b, c), enabling clear separation into high-density (Hp), medium-density\n(Mp), and low-density (Lp) phases based on grayscale intensity.”\n18. Line 131: Supplementary video 2 and 4 are cited before supplementary video 1 and 3.\nof the Supplementary Videos in the Supplementary Information to ensure consistency with\ntheir first appearance in the manuscript (also a requirement of the journal).\nRevision: We have corrected the sequence of the Supplementary Videos in the Supplementary\nInformation and updated the corresponding citations in the manuscript accordingly.\nSupplementary Information (Line 187-194):\nSupplementary Video 1 | 3D morphological assembly of segmented particles from sample\nCE5C004. After removal of incomplete edge particles.\nSupplementary Video 2 | 3D morphological assembly of segmented particles from sample\nCE6C002. After removal of incomplete edge particles.\nSupplementary Video 3 | Original radiographs of a bulk soil CT scanning region from\nSupplementary Video 1.\nSupplementary Video 4 | Original radiographs of a bulk soil CT scanning region from\nSupplementary Video 2.\nManuscript (Line 135-138):\n“Based on analyses of 5,605 basalt particles from bulk soils (considering only intact particles,\nwith edge-truncated particles excluded; Supplementary Video 1; Supplementary Video 2),\nthe volume fractions of Hp, Mp, and Lp in CE5 basalts were determined as 6.80%, 53.84%,\nand 39.36%, respectively, and 7.16%, 61.52%, and 31.32% for CE6 (Fig. 2d).”\nManuscript (Line 511-513):\n“Each detailed scan involved a full 360° rotation (~260 min per scan), producing ~1,000 2D\nradiographs per region, covering a cylindrical volume of ~1 mm in diameter and 1 mm in height\n(Supplementary Video 3; Supplementary Video 4).”\n19. I see that there are four data points on Supplementary Figure 2e, which were confirmed\nusing EDX. Have you done these chemical/intensity confirmations with any additional\nparticles? It would be interesting to see how the intensity varies for each mineral phase.\nResponse: We sincerely thank the reviewer for recognizing the value of our radiograph\nintensity-density calibration approach. Using this method, we confirmed that the Hp\ncorresponds to ilmenite, the Mp to olivine or pyroxene, and the Lp to plagioclase. These three\nphases represent the principal mineralogical components of lunar mare basalts (Tian et al.,\n2021; Che et al., 2025). In the original version, we placed one calibration point on each of these\nmajor phases to establish the chemical/intensity correlation.\nWe fully agree with the reviewer’s suggestion and therefore carried out an additional test\non another representative particle, specifically a breccia particle from the CE6 regolith (Reply\nFig. 2a). The selection of this particle was motivated by two reasons: (i) the distinctive clastic\nstructure of breccia makes it straightforward to locate bulk soil CT scanning radiographs\nconsistent with the corresponding thin section (Reply Fig. 2b); (ii) the breccia contains diverse\nmineral fragments, allowing multiple calibration points to be set within a single particle. To\nensure reliability, we selected five relatively large fragments as data points (Reply Fig. 2b).\nAs shown in Reply Fig. 2c–h, EDS analyses confirmed their mineral phases:\nParticle #1 is identified as a basaltic clast containing pyroxene and plagioclase.\nParticle #3 contains pyroxene.\nParticle #4 is also pyroxene-rich.\nParticle #5 similarly consists of pyroxene.\nIn total, we added six new calibration points, and reconstructed the linear calibration\nrelationship between grayscale intensity and mineral phase density (Reply Fig. 3). The overall\nlinear trend remains consistent, confirming that the calibration is robust. Importantly, the\ninclusion of the CE5 basalt and CE6 breccia data further demonstrates the applicability and\naccuracy of this method across different particle types.\nReply Figure 2. a Precise mineral–phase determination of the breccia thin section performed\nusing the EDS method. b Reconstruction of the mineral-phase distribution of 5 large fragments\nin breccia from the bulk soil CT scanning region based on intensity ranges. The reconstructed\ndistribution of large fragments show close agreement with mineral phases identified by EDS\nmapping in a. c–h Chemical zoning in plagioclase (c) and pyroxene (d-h) within 5 large\nfragments in breccia. The corresponding grayscale intensity values at these positions from the\nbulk soil CT scanning regions are labeled in white.\nReply Figure 3. Linear calibration relationship established between grayscale intensity (from\nradiographs) and mineral phase density determined from chemical composition, updated with\nadditional data points.\nimproving the calibration basis from one representative particle to two (CE5 basalt and CE6\nbreccia). Accordingly, we updated Supplementary Fig. 2 and Supplementary Table 2. With the\nexpanded dataset of ten calibration points, we also added the estimated densities of each\nmineral phase to Supplementary Table 2.\nLines 535-539:\n“Two representative particles were utilized for the intensity–density calibration (Fig. 2a,\nSupplementary Fig. 2e, m). These selected particles were cut, and mineral phases within the\npolished thin section were identified using a Thermo Scientific Quattro S field-emission\nscanning electron microscope (SEM) equipped with a Thermo Scientific UltraDry energy\ndispersive X-ray spectroscopy (EDS) detector (Bruker XFlash6|30).”\nLines 545-556:\n“Mineral distributions on thin sections were obtained (Supplementary Fig. 2e, m). Bulk soil\nCT scanning radiographs containing these two particles were spatially aligned to precisely\nmatch their corresponding SEM planes. Subsequently, four mineral phases of interest—\nplagioclase (Supplementary Fig. 2a, g), pyroxene (Supplementary Fig. 2b, h-l), olivine\n(Supplementary Fig. 2c) and ilmenite (Supplementary Fig. 2d)—were identified in the SEM\nimages. Representative regions within each mineral phase were also selected for detailed\nelemental analyses using EDS, providing precise compositional data for atomic percent\n(Supplementary Table 2). Corresponding intensity values of these mineral regions were then\nextracted from aligned radiographs (Supplementary Fig. 2f, n). For density estimation, mineral\ndensities were assigned based on literature values corresponding to mineral phases with\nelemental compositions closely matching those measured by EDS analysis65 (Supplementary\nTable 2), enabling accurate linear calibration of the intensity–density relationship for the lunar\nsamples (Supplementary Fig. 2o).”\nSupplementary Fig. 2:\na–d Chemical zoning in plagioclase (a), pyroxene (b), olivine (c), and ilmenite (d) within a\nCE5 subophitic–textured basalt. The corresponding grayscale intensity values at these positions\nfrom the bulk soil CT scanning regions are labeled in white. e Precise mineral–phase\ndetermination of the CE5 basalt thin section performed using the EDS method. f\nReconstruction of the mineral–phase distribution within the basalt particle from the bulk soil\nCT scanning region based on intensity. The reconstructed distribution shows close agreement\nwith mineral phases identified by EDS mapping in e. g–l Chemical zoning in plagioclase (g)\nand pyroxene (h-l) within 5 large fragments in CE6 breccia. m Precise mineral-phase\ndetermination of the CE6 breccia thin section performed using the EDS method. n\nReconstruction of the mineral-phase distribution of 5 large fragments in breccia from the bulk\nsoil CT scanning region based on intensity ranges. The reconstructed distribution of large\nfragments show also close agreement with mineral phases identified by EDS mapping in m. o\nLinear calibration relationship established between grayscale intensity (from radiographs) and\nmineral phase density estimated from chemical composition. A noticeable deviation from\nlinearity occurs, however, when intensity exceeds 225. In this study, ilmenite is defined as the\nhigh–density phase (Hp, intensity = 225–255), olivine and pyroxene as the medium–density\n36\nphase (Mp, intensity = 130–225), and plagioclase as the low–density phase (Lp, intensity = 64–\n130).\nSupplementary Table 2:\nSupplementary Table 2 | Atomic percent (%) and density estimation (g·cm-3) of different\nmineral phases obtained during analysis of representative particles from CE5 and CE6.\nPoints No. 1-4 correspond to measurement spots on the CE5 basalt, and Points No. 5-10\ncorrespond to those on the CE6 breccia. Elements with only trace amounts are indicated with\n“–”.\nPhase No. O Si Ca Fe Mg Al Na Ti Mn Density\nPlagioclase 1 64.84 15.47 6.05 0.12 – 12.78 0.74 – – 2.69\nOlivine 2 60.71 13.66 0.17 11.04 14.29 – – – 0.13 3.80\nPyroxene 3 63.82 16.96 6.42 3.61 6.27 1.85 – 0.80 – 3.35\nIlmenite 4 65.27 – – 16.85 0.37 – – 17.35 0.16 4.72\nPlagioclase 5 64.29 16.43 5.70 0.28 0.19 12.10 1.01 – – 2.69\nPyroxene 6 63.55 17.56 4.90 4.82 6.95 1.31 – 0.59 0.10 3.35\nPyroxene 7 64.03 16.58 4.13 7.18 2.28 3.97 0.26 1.25 0.11 3.55\nPyroxene 8 63.84 18.33 1.98 5.82 9.34 0.32 – 0.15 0.11 3.35\nPyroxene 9 64.18 17.89 4.04 8.94 3.76 0.71 – 0.36 0.12 3.55\nPyroxene 10 63.58 17.58 5.17 4.13 7.42 1.32 – 0.48 0.08 3.35\n20. Line 137: Weathering refers to what? Space weathering? If you mean the products of\ncontinuous impacts, I would say that instead of “weathering” as that term encapsulates\nirradiation, micrometeoroid impact, etc..\nResponse: Thanks for this helpful comment. The reviewer is correct that our original wording\nwas ambiguous. In this context, we specifically meant the products of continuous meteoritic\nimpacts, rather than the broader term “weathering”, which also includes solar wind irradiation\nand other processes. We have therefore revised the sentence to clarify the intended meaning.\nRevision: We have revised the sentence in lines 142–145 to read as follows\n37\n“As products of continuous meteoritic impacting of a likely diverse range of cooled lava flows,\nbasalt particles in bulk soils collectively preserve the original mineralogical signatures of lavas,\nenabling reliable classification of local basalt types.”\n21. Line 149-154: Maybe I am missing it, but why do you define high and low TiO and Al O\n2 2 3\nat 6% and 11%, respectively. Is that value relative to only CE samples or all lunar samples?\nResponse: We thank the reviewer for this valuable comment and apologize for the lack of\nclarity in our original wording. We did not include the appropriate reference for the\nclassification scheme, which may have caused confusion. As defined by Neal & Taylor (1992),\nTiO content provides the primary discriminator for mare basalts (i.e., <1 wt% = very low-Ti\n2\nor VLT; 1-6 wt% = low-Ti; >6 wt% = high-Ti), while Al O content serves as a secondary\n2 3\ndiscriminator (i.e., ≤11 wt% = low-Al; >11 wt% = high-Al). This classification scheme has\nbeen widely adopted for lunar basalts, including Apollo, Luna, and Chang’e series soils (e.g.,\nYin et al., 2025), and is therefore broadly applicable. We have now clarified this point in the\nrevised manuscript.\nRevision: We have revised lines 156-159 to include the citation for the mare basalt\nclassification criteria as follows:\n“Compared to CE6 lunar soil, CE5 exhibits considerably elevated Al concentrations, consistent\nwith previous findings31, thus categorizing CE5 mare basalts as medium-Ti and high-Al types,\nwhereas CE6 basalts correspond to the medium-Ti and low-Al basalts, based on mare basalt\nclassification criteria54.”\n22. Line 158: What is the difference between internal pore structure and a vesicle? How does\nthis information relate to the flow distance of the volcanic source?\nResponse: We thank the reviewer for raising this important point. In our manuscript, the term\npore structure in basalts refers specifically to internal closed cavities, which is consistent with\nthe definition of vesicles. Following Comment #24, we have revised the terminology for\nagglutinates by replacing “pores” with “vesicles” to avoid confusion, whereas for basalts we\nintentionally retain the term “pore structures” in order to distinguish them from vesicles in\nagglutinates, consistent with the usage adopted by Kiefer et al. (2012).\nAccording to Head et al. (2020), mare basalt protolith types include: solidified\nnonvesicular coherent mare basalt; inflated flows: surface topography, vesicularity, and\nmesoporosity-macroporosity; inflated flows: second boiling, vertical bubble migration and\nextrusion of magmatic foam; foam flows and autoregolith formation, foam flows with coherent\nsurfaces, pyroclastic layers, emplacement of anomalous xenolithic volcanic glass beads, and\nvolcanic pit crater floor surfaces. Among these, Apollo 12 basalt sample 12038 (Neal et al.,\n1994) is a well-documented example of a solidified non-vesicular coherent mare basalt (Head\net al., 2020). XCT analysis of basalts from this sample (Reply Figure 4a) further confirms its\nrelatively dense interior, with a void ratio of only 3.8% (Gawronska et al., 2022). In comparison,\nthe CE5 and CE6 basalts analyzed here exhibit even lower porosities (<2%), supporting their\nclassification as solidified non-vesicular coherent mare basalts.\nThe formation of this basalt type is attributed to lava that predominantly degassed near or\nat the vent during Hawaiian-style activity, producing far-traveled (hundreds of kilometers),\nflat, and smooth-surfaced flows with low vesicularity that cooled into solidified basalts several tens\nof meters thick. Such distal flows, often associated with sinuous rilles, are expected to extend\nfar from the eruption vent and form regolith protoliths consistent with the standard regolith\nevolution model (Head et al., 2020). Our results therefore indicate that the CE5 and CE6 basalts\nlikely represent distal flows emplaced at considerable distance from their volcanic source vents.\nIn contrast, Apollo 15 basalt sample 15556 exhibits a void ratio of 13.6% (Gawronska et\nal., 2022), representative of a vesicular basalt likely produced by inflated flows, highlighting\nthe clear distinction between these basalt types (Reply Figure 4b).\nReply Figure 4. a XCT slice of Apollo 12 (12038) basalt, showing a relatively dense internal\nstructure with a void ratio of 3.8%. b XCT slice of Apollo 15 (15556) basalt, exhibiting a more\nporous texture with a void ratio of 13.6% (Gawronska et al., 2022).\nRevision: We have revised lines 165-169 to include citations supporting our classification of\nCE5 and CE6 basalts as solidified nonvesicular coherent mare basalts, and we now briefly\ndescribe the formation mechanism of this basalt type as being linked to lavas that had flowed\na considerable distance from their eruption vents.\n“Such exceptionally low porosity values are characteristic of solidified, non-vesicular coherent\nmare basalts11,56,57, likely formed as a result of CE5 and CE6 lavas having erupted and been\nemplaced at considerable distances from the source vents as flat, smooth-surfaced flows with\nlow vesicularity, which subsequently cooled into dense basaltic units11.”\n23. Line 187-189: Citation\nResponse: Good catch. The definition of agglutinates in our manuscript is based on McKay et\nal. (1991), which is now included.\nRevision: We have added the citation in lines 194-196 to read as follows:\n“Agglutinates are special regolith particles consisting of smaller lunar soil constituents (mineral\nparticles, glasses, and older agglutinates) bonded together by vesicular, flow-banded glass,\nformed during micrometeorite impacts59.”\n24. Line 199-200: Pore structures in agglutinates are usually denoted as “vesicles”.\nResponse: Good point. We agree with this terminology, as in many previous studies the\ninternal pore structures of agglutinates are referred to as “vesicles” (e.g., Rubin, 1997; Xian et\nal., 2023). This terminology is now adopted throughout the manuscript for consistency and\nclarity.\nmanuscript, including in subtitles, main text, figures, and captions.\n25. Line 204: How does Ir relate to internal fragments when vesicles are typically associated\nwith agglutinitic melt, not inclusions.\nResponse: Many thanks for highlighting this oversight. In the original manuscript, we only\nmentioned that the internal regular void fraction (Ir) is related to the glassy matrix content of\nagglutinates, but we did not explicitly clarify its relationship with internal fragment content.\nAs noted by McKay et al. (1990), the solid body agglutinate is primarily composed of two\ncomponents: internal fragments (including lithic, mineral, and glass fragments) and the glassy\nmatrix. Since the volumetric fraction of internal fragments is complementary to that of the\nglassy matrix, Ir—being related to the abundance of the glassy matrix—also shows a\ncorresponding relationship with fragment content. We have now made this point explicit in the\nrevised text.\nRevision: We have revised lines 216-217 to clarify that the fragment content is complementary\nto that of the glassy matrix, thereby elucidating the potential relationship between Ir and\nfragment content.\n“As agglutinates are composed of both fragments and a glassy matrix63, the glassy matrix and\nfragment (Fr) contents are inherently complementary.”\n26. Line 213-224: Since vesiculation is associated with agglutinitic melt, could the inverse relationship be related to the melt content of the agglutinate? You also infer from the slight decrease of Ir in agglutinates from CE5 relative to CE6, that the CE5 soil underwent additional processing (i.e. secondary agglutinate formation). That implies that we should see higher levels of maturity for the CE5 collection area. Firstly, I would like to see the error for the two Ir values (and maybe include that on Figure 3a and 3b). Secondly, if additional processing happened on the CE5 collection site, then that should be reflected in other maturity indices. Is there any work that measured Is/FeO or Optical Maturity for each collection site that you can refer to?\nResponse: Thanks for these constructive and insightful comments. Regarding the first point, we fully agree that investigating the relationship between vesicle abundance (represented by void ratio) and melt content is indeed meaningful, given that vesiculation is closely associated with agglutinitic melts. As we explained in our response to Comment #25, melt content is also related to fragment content (Fr). Therefore, here we focus on examining the relationship between the void ratio of larger agglutinates and their Fr. Based on statistical measurements from 43 CE5 and 43 CE6 larger agglutinates, we constructed the correlation shown in Reply Figure 5a. The results indicate that no clear relationship exists between void ratio and Fr. This may be because, although vesiculation is associated with agglutinitic melt, void ratio only partially reflects melt content. Vesicles can also form in inter-fragment void spaces or at the junctions between glass matrix and fragments during agglutinate formation, which decouples void ratio from melt content. Consequently, no inverse relationship between void ratio and Fr is observed.\nTo the reviewer’s second point, we appreciate the opportunity to clarify a potential misunderstanding. The observed difference in Ir values of bulk soil agglutinates between CE5 and CE6 is attributed to differences in the initial internal structures of larger agglutinates, rather than to additional processing experienced by the CE5 soils. Specifically, the larger CE5 agglutinates contain higher vesicularity, whereas CE6 agglutinates exhibit smaller vesicles and lower overall void ratio. These structural differences translate into subtle but statistically significant variations in the Ir values of their derived bulk soil agglutinates during subsequent space weathering. They do not imply that CE5 regolith underwent additional secondary agglutinate formation. The interpretation of the higher Ir in CE5 bulk soil agglutinates is explained in detail in our response to Comment #32.\nIn response to the reviewer’s suggestion, as other readers may also benefit from further clarification, we have added error bars for the CE5 and CE6 bulk soil agglutinate Ir and void ratio mean values in Figure 3a. (For Figure 3b, which shows individual data points, error bars are unnecessary.) To support this addition, we also revised our method for calculating the mean and standard deviation of Ir and void ratios. In the original version, these parameters were calculated as weighted averages across all particles:\nk\nn\nri\nIr  i1 (1)\naverage k\nn\nti\ni1\nk\nV\nvi\ne  i1 (2)\naverage k\nV\npi\ni1\nwhere n and n are the counts of regular and total vesicles in the i-th agglutinate, V and V\nri ti vi pi\nare vesicle and particle volumes, and k is the total number of agglutinates analyzed. This\napproach suffers from two shortcomings: (i) it is biased toward large particles, and (ii) it does not allow reliable estimation of sample variance.\nIn the revised approach, we calculate Ir and void ratio for each agglutinate individually, then treat these samples as statistical values by computing mean and standard deviation. As shown in Reply Figure 5b, this refinement causes a slight rightward shift in the data, suggesting that the previous version method slightly underestimated the vitrification of bulk soil agglutinates. However, the overall trends remain unchanged: CE6 bulk soil agglutinates still exhibit lower Ir values (Ir = 0.46) than CE5 (Ir = 0.49), and the error bars confirm the statistical robustness of this difference.\nFinally, we agree with the reviewer that integrating other maturity indices provides a more comprehensive picture of regolith evolution. Therefore, we now include agglutinate abundance, Is/FeO value, and surface exposure age as complementary maturity indicators for both CE5 and CE6, as detailed in our responses to Comments #28–29.\nRevision: We revised lines 235-236 to clarify that the difference in bulk soil agglutinate Ir values arises from internal structural differences in larger agglutinates, rather than secondary processing:\n“this trend may reflect inherent structural differences in the initial agglutinates of the two regions”\nFigure 4a was updated to include error bars.\n27. Line 255-256: Why does sphericity of glass beads inhibit your ability to acquire morphometric data?\nResponse: Thanks for asking, as other readers may have wondered the same thing. We realize that our original wording may have caused a misunderstanding, giving the impression that the sphericity of glass beads inhibits our ability to acquire morphometric data. What we intended to convey is that glass beads exhibit highly regular, nearly spherical shapes, with morphometrics that closely approximate those of ideal spheres. As a result, including them in the statistical analysis of particle morphometrics would add little interpretive value, since their shape characteristics can be directly inferred. Therefore, we chose to exclude glass beads from the morphometric statistics.\nRevision: We have revised lines 272-274 to clarify this point, adding that the morphometrics of glass beads closely approximate those of ideal spheres, thereby removing the ambiguity.\n“Glass beads were omitted, given their highly regular, nearly spherical shapes, with morphometrics closely approximating those of ideal spheres (Fig. 4a).”\n28. Line 267-268: I would spend more time thinking about this interpretation. This saturation effect is likely associated with the smaller size fraction you are using for this study. Studies that looked into size fraction as it relates to space weathering are McKay et al. (1974): Grain size and the evolution of lunar soils; and Morris (1976): Surface exposure indices of lunar soils: A comparative FMR study. In these studies, researchers looked at different size fractions and found that a correlation exists between surface exposure and grain size for regolith with mean grain size <1cm, however that correlation does not exist for mean grain sizes <1mm. At such small size fractions the regolith undergoes a mixing and agglutinate production dominate regime rather than a comminution dominant regime, which could be why the morphologies for each CE landing site looks similar. A better morphological indication for space weathering at smaller size fractions (and larger for that matter) would be agglutinate abundance and the prevalence of micrometeoroid impact melt, vapor, and zap pits.\nResponse: Very valuable comment. We agree that in our initial discussion we overstated the role of fragmentation in shaping particle morphologies within the bulk soil fraction, and we now recognize that the dominant regime in this size range is better described by a combination of mixing and agglutinate production.\nIn the revised manuscript, we have reorganized our discussion of particle reshaping mechanisms to capture the full spectrum of processes acting on lunar regolith. Specifically, include the following.\n Impact gardening (including mixing; primarily abrasion caused by repeated interactions among numerous grains within the regolith matrix)\n Impact fragmentation (grain breakage via direct energy deposition during impacts);\n Impact agglutination (fusion of surrounding grains into breccias or agglutinates due to impact-induced melts or semi-plastic phases)\n Impact sputtering (deposits from impact events appearing randomly on the surface of minerals, forming a sputtering deposit layer that modifies particle morphology)\n Impact abrasion (localized surface wear caused by micrometeoroid strikes or secondary impacts that generate linear scratches)\n Thermal fatigue (fracturing and disintegration from diurnal thermal stresses)\n Solar wind irradiation (implantation of high-energy ions producing lattice defects, which increase susceptibility to subsequent weathering-induced reshaping; in addition, sputtering by the solar wind which may also erode particle surfaces)\nAmong these, impact agglutination is particularly distinctive as it transforms “simple particles” into “complex particles” in a cumulative fashion, generating irregular aggregates that strongly influence bulk regolith morphology. By contrast, other processes reshape particles without fundamentally altering their type, although “complex particles” may shed small fragments that revert to “simple particles.” Both particle classes are then subject to additional modification by the full suite of processes.\nRegarding the relative influence of fragmentation and abrasion, previous studies demonstrate that surface exposure age correlates with the grain size of lunar regolith in the >1 mm fraction (Morris, 1976; McKay et al., 1974), but this correlation weakens at <1 mm. Furthermore, Weibull fracture statistics demonstrate that smaller grains require higher stresses to fail completely (Weibull, 1951; McDowell et al., 2000). Thus, for the <1 mm grains that dominate the lunar regolith, impact fragmentation likely contributes only minimally to reshaping, and most of the impact energy is redistributed into broad-scale impact gardening, which thus emerges as the dominant driver of morphological modification (Reply Figure 6).\nDespite clear differences in age, geological setting, basalt provenance, micrometeoroid flux, and solar wind irradiation between the CE5 and CE6 sampling sites, the morphometrics of corresponding particle types remain remarkably consistent. This convergence strongly suggests that long-term particle reshaping, governed primarily by impact gardening with contributions from agglutination, has driven both simple and complex particles toward stable morphologies, such that additional weathering exerts only a minor influence on their morphometrics.\nFinally, we also fully agree with the reviewer that a more robust assessment of regolith maturity requires integrating multiple indicators. Accordingly, we now describe space weathering intensity using a multi-proxy suite of maturity parameters, including agglutinate abundance, Is/FeO values, and regolith surface exposure ages. For example, CE5 soils contain ~8% agglutinates, exhibit an Is/FeO value of ~14, and an exposure age of ~1.5 Ma (Qian et al., 2024; Zhang et al., 2025), whereas CE6 soils contain ~17% agglutinates, exhibit an Is/FeO value of ~56.5, and an exposure age of ~2.1 Ma (Zhang et al., 2025; Li et al., 2025). These parameters collectively indicate that CE6 experienced stronger space weathering than CE5. The consistent morphometrics observed across both soils therefore imply that even the less weathered CE5 regolith had already reached morphological saturation by its exposure age of ~1.5 Ma. This conclusion is further supported by our comparison with Apollo 17 sample 73002 (see Response to Comment #29), which represents an even more mature regolith yet exhibits similarly stable morphologies.\nRevision: We have refined paragraph [20] to incorporate the full set of reshaping mechanisms (impact gardening, fragmentation, agglutination, sputtering, abrasion, thermal fatigue, and solar wind irradiation), ensuring a more complete framework consistent with the lunar surface environment. Figure 5 has been revised accordingly to illustrate the interplay among agglutination, fragmentation, and abrasion.\nParagraph [20]:\n“Particle reshaping processes at the lunar surface include impact gardening (mainly abrasion caused by interactions among large numbers of particles within a given region78), impact fragmentation (grain breakage due to direct energy deposition during impacts90), impact agglutination (where impact-induced melts or semi-plastic phases fuse surrounding grains into agglutinates and breccias), impact sputtering (deposits from impact events appear randomly on the surface of minerals, forming sputtering deposit layer that modify particle morphology46), impact abrasion (localized surface wear induced by micrometeoroid impacts20,91 or secondary impacts that generate linear scratches92), thermal fatigue (fracturing driven by thermal stresses leading to ultimate grain disintegration90), and solar wind irradiation93 (high-energy ion implantation generates crystalline defects, making grains more susceptible to subsequent space weathering induced reshaping94; in addition, sputtering by solar wind may also erode particle surfaces67). Impact agglutination is distinctive in that it can transform “simple particles” into “complex particles” and operates cumulatively, producing highly irregular aggregates that strongly influence the overall particle morphology of the regolith. By contrast, other reshaping processes generally do not alter particle type (although “complex particles” may shed minor fragments of “simple particles”). Both “simple” and “complex” particles are subsequently modified by additional reshaping processes. Among these mechanisms, only impact gardening and impact fragmentation significantly modify regolith within a given depth, exerting the greatest influence on particle morphology. For impact fragmentation, previous studies have shown a correlation between surface exposure and mean grain size in regolith samples with grain sizes <1 cm (refs. 70,95); however, this correlation does not persist for mean grain sizes <1 mm. Moreover, Weibull statistics in rock mechanic model outputs demonstrate that smaller grains require higher stresses for complete failure96,97. Thus, for lunar regolith with mean grain sizes <1 mm, impact fragmentation likely contributes little to reshaping, with the deposited energy instead redistributed into broader-scale impact gardening. Consequently, impact gardening is considered the dominant driver of morphological modification in surface soils (Fig. 5). The bulk soil fraction of lunar regolith was the primary target of the micro-CT scans of this study, as it represents the dominant fraction. Despite notable differences in age, geological setting, lava origin, micrometeorite bombardment intensity, and solar wind irradiation between the CE5 and CE6 landing sites24,25,31,53,98, the morphometrics of corresponding particle types remain remarkably consistent across both sites. This convergence indicates that long-term surface particle reshaping processes, dominated by impact gardening, has likely driven all particle types toward stable morphologies, such that further space weathering processes exert limited influence on particle morphometrics.”\nFigure 5:\nParagraph [15]:\n“Building on particle classification, we quantified the relative abundances of each particle type (Fig. 4e). In particular, agglutinate abundance, an established indicator of soil maturity59, was retrieved with high accuracy. This index correlates positively with the maturity index Is/FeO (refs. 69,70). The CE5 bulk soil yields an Is/FeO value of ~14, consistent with immature soils71, and an agglutinate abundance of ~8%. By contrast, CE6 soil yields an an Is/FeO value of ~56.5, corresponding to mature soils72, and a higher agglutinate abundance of ~17%. These values are mutually consistent, confirming agglutinate content as a reliable maturity proxy. The Is/FeO index is closely linked to the abundance of submicroscopic metallic iron, produced on grain rims and within agglutinates by micrometeorite vaporization and solar wind sputtering/redeposition, and therefore reflects the cumulative intensity of these space weathering processes. Another critical component of space weathering is regolith “gardening” induced by meteorite impact, typically manifest in terms of surface soil exposure age73. According to regolith evolution models, CE5 surface soils have an exposure age of ~1.5 Ma, whereas CE6 soils have been exposed since ~2.1 Ma (ref. 77). The slightly longer exposure duration of CE6 regolith suggests a greater degree of surface soil gardening, and together with these maturity indices, consistently demonstrates that CE6 soils have experienced a higher overall degree of space weathering.”\n29. Line 281-295: This paragraph compares the results of the CE samples to two Apollo expedition sites. However, I do not think it is appropriate to compare the CE samples (which were collected from single locations) to the totality of the Apollo samples (which were collected from multiple locations during each mission, with each one having different exposure histories and geological context). It is important to note that the degree of space weathering for a single soil sample does not necessarily reflect the amount of space weathering that occurred over the timescale since mare formation. If an impact that occurred in the mare 100 Ma overturned regolith, then that regolith would only have been space weathered for 100 Ma. Could you be more specific with the Apollo samples you are referring to and do a more robust comparison of those particular samples to that of the CE samples with respect to recent geologic context, surface exposure age, maturity index, etc.?\nResponse: Thanks for this valuable comment. We acknowledge that paragraph [15] in the original manuscript contained several shortcomings. First, our initial references to Apollo 11 and Apollo 17 soils did not specify sampling locations, which could have led to misunderstanding. Second, as Reviewer #1 correctly notes, the use of fine particle content as an indicator of space weathering intensity is problematic. Inspired by this (see Response to Comment 1(1)), we revisited the literature and found that local factors, such as proximity to boulders or recent craters, exert a strong influence on grain size characteristics (King, 1977). Thus, fine particle content cannot reliably serve as a proxy for space weathering intensity. In response, we have revised paragraph [15] to focus instead on a comprehensive description of space weathering intensity in CE5 and CE6 soils (see Revision to Comment #28).\nAnd as this reviewer (#2) further suggests, validation of the morphological saturation effect requires more specific comparison with Apollo soils. To this end, we have incorporated data from the Apollo 17 double-drive tube sample 73001/2, specifically the upper 20 cm section (73002). This sample, recovered from the Light Mantle landslide deposit at the base of the South Massif in Taurus–Littrow Valley, was opened and scanned by micro-CT in 2019, with data subsequently released (Eckley et al., 2025). Previous work indicates that 73002 sampled multiple basalt flows, and its maturity is well constrained: the uppermost regolith exhibits an Is/FeO value of ~74 (Morris et al., 2022) and a surface exposure age of ~3.0 Ma (McFadden et al., 2024). By comparison, CE5 soils have an Is/FeO value of ~14 and an exposure age of ~1.5 Ma (Qian et al., 2024; Zhang et al., 2025), while CE6 soils show an Is/FeO of ~56.5 and an exposure age of ~2.1 Ma (Zhang et al., 2025; Li et al., 2025). Thus, the Apollo 17 sample is more mature than both CE5 and CE6, and its derivation from a landslide deposit ensures mineralogical and lithological complexity. If morphological saturation is also observed in 73002, this would imply that such saturation can be achieved by the relatively short exposure age of CE5 (~1.5 Ma).\nIt should be noted, however, that the Apollo 73002 CT radiographs were acquired at a resolution of 25.8 μm (Eckley et al., 2025; Reply Fig. 7a), which limits morphological characterization to particles larger than ~500 μm. This precludes direct comparison with the bulk soil fraction (20–200 μm) analyzed in this study. To enable robust comparison, we therefore selected basalt particles in the 500–700 μm range from both CE5 and CE6 soils, using basalt as the common material for analysis (see Response to Reviewer#1 Comment 1(2)).\nFor CE5 and CE6, which are shallow scoop samples with relatively few large particles, we manually selected ~200 particles for each, from which we obtained 22 basalts from CE5 and 27 from CE6. For Apollo 73002, we restricted our analysis to the uppermost ~3 cm of regolith (to align with the near-surface context of CE5 and CE6 and to ensure comparable space weathering intensity). Basalt grains were readily identified from radiographs (Reply Fig. 7b), manually extracted, and reconstructed in 3D (Reply Fig. 7c–d). The larger sample size of Apollo 73002 allowed us to extract 110 basalt grains, ensuring statistical robustness.\nOur morphometric results (Reply Fig. 7e–h) show highly similar distributions of AR, S, Cx, and R across basalts from CE5, CE6, and Apollo 17 (73002). The mean values differ by <10%, with comparable standard deviations. These results indicate that morphological saturation is likely already established by the CE5 surface exposure age of ~1.5 Ma, although larger CE5 and CE6 datasets will be necessary to further test this conclusion in the future.\nReply Figure 7. a Radiograph through central sections of 73002 (Eckley et al., 2025), with the light-purple shaded area indicating the region selected for large basalt particle (data source:\nhttps://data.mendeley.com/datasets/5y725j47wf/1). b-d Procedure for extracting large basalt\nparticles from the 73002 radiographs. e-h Morphometric statistical distributions for large basalt\nparticles in the CE5, CE6, and Apollo 17 (73002) samples.\nRevision: We have fully revised paragraph [15] to remove the earlier comparison based on fine particle content and instead provide a more comprehensive description of CE5 and CE6 space weathering intensity (see Revision to Comment #28). In addition, we have expanded paragraph [21] to include new analyses of Apollo 17 sample 73002, based on additional micro-49\nCT scanning of larger particles. These results provide more specific geologic context and a more rigorous comparison with CE5 and CE6, thereby strengthening our evaluation of morphological saturation.\nParagraph [21]:\n“Moreover, the data imply that space weathering-induced morphological modification may\nhave reached a saturation state in the youngest, least weathered CE5 regolith46, with the onset\nof saturation thus occurring before ~1.5 Ma. To further test this striking hypothesis, we\nexamined Apollo 17 sample 73002, collected from the upper ~20 cm of the Light Mantle\nlandslide deposit at the base of the South Massif in the Taurus-Littrow Valley, a site at the mare-\nhighland boundary containing basalts of multiple ages88. The <1 mm fraction of this soil\nexhibits an Is/FeO value of ~74 (ref. 101) and a surface exposure age of ~3.0 Ma (ref. 73),\nindicating a higher degree of space weathering than either CE5 or CE6 soils. To verify whether\nthe same saturation trend holds for larger particles, we focused on mare basalts, the dominant\nregolith component. We selected 22 basalt particles (500–700 µm) from CE5, 27 particles of\nthe same size range from CE6, and 110 particles from the upper 3 cm of Apollo 17 sample\n73002 radiographs88. CE5 and CE6 basalts were analyzed using micro-CT scans at a voxel size\nof 25.8 µm, the same resolution as applied to the 73002 radiographs. Morphometric results\nshow that even for larger particles in more strongly weathered soils, the saturation state may\npersist (Supplementary Fig. 9), although further validation with larger datasets will be required.\nDuring space weathering, regolith particles undergo repeated reshaping that ultimately leads to\nprogressive soil comminution. Larger particles are continually broken down into the bulk soil\nfraction, while the bulk soil fraction itself steadily generates lunar dust. This process results in\nthe gradual enrichment of lunar dust within the regolith (Fig. 5).”\nIn addition, we have added Supplementary Fig. 9 to present the results of these experiments\nand further illustrate the potential persistence of saturation trends in larger particle populations.\nSupplementary Fig. 9:\nSupplementary Fig. 9 | Morphometrics statistical distributions for large basalt particles\nin CE5, CE6, and Apollo 17 (73002) samples. To match the 25.8 μm resolution of the 73002\nradiographs, large basalt particles with equivalent diameters of 500–700 μm were selected from\nall three samples. Comparisons across the three datasets reveal highly similar mean values and\nstandard deviations, with differences in the means <10%. The weaker agreement in\nmorphometric trends compared to the bulk soil fraction may result from the limited number of\nlarge basalt particles in the CE5 and CE6 powdered samples, potentially diminishing statistical\nrobustness. However, the use of randomized particle selection helps ensure consistency in\naverage values.\n50\n30. Line 357-370: Reference that discusses the latitudinal and longitudinal influence on\nmicrometeoroid impact: Sim et al. 2017: Asymmetric space weathering on lunar crater walls.\nThere are also plenty of other studies that model the flux of micrometeoroids on the lunar\nsurface.\nResponse: We thank the reviewer for this valuable comment. We acknowledge that our\nmanuscript did not originally address the latitudinal and longitudinal influence on\nmicrometeoroid flux. After carefully reviewing Sim et al. (2017), we note that micrometeoroid\nflux is indeed latitude-dependent. This observation is consistent with the lunar surface flux\nmodel of Jeong et al. (2015), which shows reduced flux at higher latitudes but remains\nsymmetric with respect to latitude. Since the CE5 and CE6 sampling sites are located at\n43.06°N and 41.625°S in opposite hemispheres, respectively, their latitudes are nearly identical\n(as designed by orbital mission planning requirements), suggesting comparable fluxes under\nthe similar longitudinal conditions.\nIn theory, the nearside may experience partial shielding by the Earth, potentially resulting\nin a higher flux on the farside. However, Cremonese et al. (2013) quantified the “Earth\nshielding effect” and found it to be on the order of 10-3, which is negligible. Their model also\nindicates no significant longitudinal asymmetry in micrometeoroid flux. Taken together, these\nresults suggest that the CE5 and CE6 sites should experience broadly similar micrometeoroid\nfluxes. We would like to emphasize that this inference is based on existing theoretical\nframeworks and still requires more solid evidence from lunar regolith samples to be fully\nvalidated by future work.\nRevision: We have revised lines 380-384 to clarify our reasoning that CE5 and CE6 regions\nare expected to experience comparable micrometeoroid fluxes. We now explicitly state that the\ndistinct internal structures observed between CE5 and CE6 agglutinates likely reflect regionally\ndistinct micrometeoroid bombardment intensities.\n“Based on existing theoretical models, given the absence of hemispheric asymmetry in\nmicrometeorite flux at the lunar surface81 and the nearly identical absolute latitudes of the CE5\nand CE6 sampling sites, the micrometeorite fluxes at the two regions are expected to be highly\nsimilar81,82. Therefore, the notably distinct internal structures observed between CE5 and CE6\nagglutinates likely reflect regionally different micrometeorite bombardment intensities.”\n31. Line 379: The water before boiling analogy is not very descriptive.\nResponse: We thank the reviewer for this helpful comment. We agree that the analogy of\n“water before boiling” was abstract and not sufficiently descriptive. We have therefore adopted\na more precise expression, directly stating that CE6 agglutinates contain smaller vesicles and\nexhibit lower porosity.\nRevision: We have revised lines 407-409 to read as follows:\n“Accordingly, CE5 agglutinates tend to exhibit honeycomb-like structures with elevated\nporosity (Supplementary Fig. 8a, b), whereas those from CE6 contain smaller vesicles and\nlower porosity (Supplementary Fig. 8d, e).”\n32. Line 380-382: I’m not sure how abrasion and fragmentation leads to the loss of “internal”\nfragments.\nnot provide sufficient explanation on this point, and that the description in Supplementary Fig. 8\nmay have been too abstract. For agglutinates, we observe distinct internal structural differences\nbetween CE5 and CE6 soils: CE5 agglutinates generally contain larger vesicles and exhibit\nhigher overall porosity (Reply Fig. 8a), whereas CE6 agglutinates contain smaller vesicles and\nshow lower overall porosity (Reply Fig. 8b). These structural differences may influence how\nagglutinates respond to space weathering processes (mainly “gardening” processes) that drive\nthe continual modification of fine particles through both fragmentation and abrasion (further\nelaborated in our response to Comment #28).\nAs shown in Fig. 3a, for bulk soil agglutinates, Ir values are slightly higher in CE5 (Ir =\n0.49) than in CE6 (Ir = 0.46). This difference indicates that CE5 agglutinates in the 20–200 μm\nsize range contain marginally fewer internal fragments compared to their CE6 counterparts.\nWe interpret this as a possible consequence of long-term gardening: in CE5 agglutinates, the\nhigher porosity and larger vesicles mean that fragments are bound by relatively fewer glassy\nmatrix connections. Under prolonged gardening, shear stresses acting on such particles can\ncause breakage, leading to preferential loss of fragment-bearing portions and leaving behind\nglass-dominated structures with lower fragment content (Reply Fig. 8c).\nIn contrast, CE6 agglutinates contain more densely packed fragments embedded within\nglass, and are therefore modified predominantly through abrasion (Reply Fig. 8b). This allows\nthem to better preserve their internal fragment structures, resulting in slightly lower Ir values\n(Reply Fig. 8d). We emphasize that this difference is very subtle at the bulk soil scale: porosity\ndifferences between CE5 and CE6 agglutinates are negligible, with only marginal variation\nreflected in Ir values. Such slight differences may reflect saturation of gardening-induced\nmodification at the particle scale. However, the potential saturation of gardening effects on the\ninternal structures of agglutinates is beyond the scope of this study and should be addressed in\nfuture work.\nrepresentative larger agglutinates selected from the CE5 and CE6 samples. c-d Possible internal\nstructural evolution of agglutinates from CE5 and CE6 following gardening-induced\nmodification.\nRevision: We have added text in lines 409-414 and lines 416-419 to clarify the underlying\nreason why abrasion and fragmentation in CE5 larger agglutinates may lead to the preferential\nloss of internal fragments.\nLines 409-414:\n“In the case of CE5, space weathering processes (mainly “gardening” processes), including\nsuccessive abrasion and fragmentation, act on agglutinates with higher porosity and larger\nvesicles, where internal fragments are bound by relatively fewer glassy matrix connections.\nUnder prolonged regolith gardening, shear stresses acting on such structures can promote\nparticle fragmentation, leading to the preferential loss of fragment-bearing regions\n(Supplementary Fig. 8b-c).”\nLines 416-419:\n“In contrast, CE6 agglutinates, which contain densely packed fragments embedded in glass,\nare shaped predominantly by abrasion (Supplementary Fig. 8e-f), thereby preserving their\ninternal fragment structures and resulting in slightly lower Ir values.”\nIn addition, we have expanded Supplementary Fig. 8 to include new panels (c and f), which\nprovide diagrams illustrating the possible internal structural evolution of agglutinates from\nCE5 and CE6 under gardening-induced modification. These diagrams more clearly\ndemonstrate the relatively higher fragment content preserved in CE6 bulk soil agglutinates.\nSupplementary Fig. 8 | CT scanning radiographs and inferred space weathering trends\n53\nof representative selected larger agglutinates from CE5 and CE6. a-b CE5 agglutinates\nexhibit more developed internal porosity and tend to fracture along fragile, thin glassy matrices\nduring space weathering. d-e CE6 agglutinates have lower void ratios and display stronger\nresistance to structural fragmentation, suggesting a space weathering mode dominated by\nsurface abrasion rather than internal fracturing. c, f Possible internal structural evolution of\nagglutinates from CE5 and CE6 following gardening-induced modification.\n33. Line 387-388: How does gardening, thermal cycling, and solar wind irradiation lead to\nparticle abrasion and fragmentation?\nResponse: Thanks for raising this important issue. We acknowledge that in the original version\nour explanation of how gardening, thermal cycling, and solar wind irradiation contribute to\nparticle abrasion and fragmentation was not sufficiently clear, which may have led to confusion.\nFirst, impact gardening is one of the dominant mechanisms reshaping lunar regolith\nparticles in our model (see Response to Comment #28-29). Within a given region, large\nnumbers of particles repeatedly make contact and undergo displacement, which results\npredominantly in mechanical abrasion. (Tsuchiyama et al., 2022). For particles with structural\nweaknesses—such as agglutinates containing abundant vesicles—gardening-induced\ncompression and stress concentration may also cause partial breakage.\nSecond, long-term thermal cycling, driven by daily non-uniform solar heating, induces\ninternal thermal stresses within particles. Over time, these stresses accumulate and may lead to\nthermal fatigue, generating cracks that ultimately cause the particles to fracture (Ruesch et al.,\n2020).\nFinally, solar wind irradiation exerts its influence primarily in an indirect manner. The\nimplantation of high-energy ions generates crystalline defects that weaken lattice stability,\nthereby making grains more susceptible to subsequent reshaping by other space weathering\nprocesses (Maurette et al., 1975). In addition, sputtering by the solar wind which may also erode\nparticle surfaces (Tsuchiyama et al., 2022).\nTogether, these processes act in combination, progressively modifying particle\nmorphologies through abrasion and fragmentation.\nRevision: We have revised lines 420-430 to provide a clearer and more concise explanation of\nhow impact gardening, thermal cycling, and solar wind irradiation contribute to particle\nabrasion and fragmentation.\n“Particle reshaping processes at the lunar surface include impact gardening (mainly abrasion\ncaused by interactions among large numbers of particles within a given region78), impact\nfragmentation (grain breakage due to direct energy deposition during impacts90), impact\nagglutination (where impact-induced melts or semi-plastic phases fuse surrounding grains into\nagglutinates and breccias), impact sputtering (deposits from impact events appear randomly on\nthe surface of minerals, forming sputtering deposit layer that modify particle morphology46),\nimpact abrasion (localized surface wear induced by micrometeoroid impacts20,91 or secondary\nimpacts that generate linear scratches92), thermal fatigue (fracturing driven by thermal stresses\nleading to ultimate grain disintegration90), and solar wind irradiation93 (high-energy ion\nimplantation generates crystalline defects, making grains more susceptible to subsequent space\nweathering induced reshaping94; in addition, sputtering by solar wind may also erode particle\nsurfaces67).”\n54\n34. Line 397-348: As mentioned before, this interpretation may be flawed due to the small\nResponse: Thanks again for this important comment. As outlined in our responses to\nComments #28 and #29, we have carefully reconsidered and reconstructed the interpretation to\naddress the shortcomings in the original version. We again appreciate the reviewer’s attention\nand constructive feedback, which we hope has greatly improved the clarity and rigor of our\nstudy.\n[1] Cao, Z., Wang, X., Chen, Y., Li, C., Zhao, S., Li, Y., ... & Liu, J. (2025). Nature of space-\nweathered rims on Chang'e-5 lunar soil grains. Earth and Planetary Science Letters, 658,\n119327.\n[2] Che, X., Long, T., Nemchin, A., Xie, S., Qiao, L., Li, Z., ... & Liu, D. (2025). Isotopic and\ncompositional constraints on the source of basalt collected from the lunar farside. Science,\n387(6740), 1306-1310.\n[3] Cremonese, G., Borin, P., Lucchetti, A., Marzari, F., & Bruno, M. (2013). Micrometeoroids\nflux on the Moon. 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Rethinking lunar mare basalt regolith formation: New\nconcepts of lava flow protolith and evolution of regolith thickness and internal structure.\nGeophysical Research Letters, 47(20), e2020GL088334.\n[8] Hörz, F., Cintala, M. J., See, T. H., Cardenas, F. & Thompson, T. D. Grain size evolution\nand fractionation trends in an experimental regolith. J. Geophys. Res. Solid Earth 89,\nC183–C196 (1984).\n[9] Jeong, M., Kim, S. S., Garrick-Bethell, I., Park, S. M., Sim, C. K., Jin, H., ... & Choi, Y. J.\n(2015). Multi-band polarimetry of the lunar surface. I. Global properties. The\nAstrophysical Journal Supplement Series, 221(1), 16.\n[10] Keller, L. P., & McKay, D. S. (1997). The nature and origin of rims on lunar soil grains.\nGeochimica et Cosmochimica Acta, 61(11), 2331-2341.\n[11] Kiefer, W. S., Macke, R. J., Britt, D. T., Irving, A. J., & Consolmagno, G. J. (2012). The\ndensity and porosity of lunar rocks. Geophysical Research Letters, 39(7).\n[12] King, E. A. (1977). 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Asymmetric\nspace weathering on lunar crater walls. Geophysical Research Letters, 44(22), 11-27.\n[31] Sun, H., Yi, M., Shen, Z., Zhang, X., & Ma, S. (2017). Developing a new controllable lunar\n57\ndust simulant: BHLD20. Planetary and Space Science, 141, 17-24.\n[32] Taylor, L. A., Pieters, C., Keller, L. P., Morris, R. V., McKAY, D. S., Patchen, A., &\nWentworth, S. (2001). The effects of space weathering on Apollo 17 mare soils:\nPetrographie and chemical characterization. Meteoritics & Planetary Science, 36(2), 285-\n299.\n[33] Tian, H. C., Wang, H., Chen, Y., Yang, W., Zhou, Q., Zhang, C., ... & Wu, F. Y. (2021).\nNon-KREEP origin for Chang’e-5 basalts in the Procellarum KREEP Terrane. Nature,\n600(7887), 59-63.\n[34] Tsuchiyama, A., Sakurama, T., Nakano, T., Uesugi, K., Ohtake, M., Matsushima, T., ... &\nGalimov, E. M. (2022). Three-dimensional shape distribution of lunar regolith particles\ncollected by the Apollo and Luna programs. Earth, Planets and Space, 74(1), 172.\n[35] Varatharajan, I., Srivastava, N., & Murty, S. V. (2014). Mineralogy of young lunar mare\nbasalts: Assessment of temporal and spatial heterogeneity using M3 data from\nChandrayaan-1. Icarus, 236, 56-71.\n[36] Wallace, W. T., Taylor, L. A., Liu, Y., Cooper, B. L., McKAY, D. S., Chen, B., & Jeevarajan,\nA. S. (2009). Lunar dust and lunar simulant activation and monitoring. Meteoritics &\nplanetary science, 44(7), 961-970.\n[37] Weibull, W. (1951). A statistical distribution function of wide applicability. Journal of\napplied mechanics.\n[38] Xian, H., Zhu, J., Yang, Y., Li, S., Lin, X., Xi, J., ... & Xu, Y. G. (2023). Ubiquitous and\nprogressively increasing ferric iron content on the lunar surfaces revealed by the Chang’e-\n5 sample. Nature Astronomy, 7(3), 280-286.\n[39] Yin, C., Chen, J., Fu, X., Cao, H., Lu, X., Liu, Y., ... & Ling, Z. (2025). Petrogenesis of\nChang’e-6 Basalts and Implication for the Young Volcanism on the Lunar Farside. The\nAstrophysical Journal Letters, 981(1), L2.\n[40] Zhang, M., Fa, W., & Jia, B. (2025). Provenance and evolution of lunar regolith at the\nChang’e-6 sampling site. Nature Astronomy, 1-11.\n58"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The response letter provided very courteous and detailed explanations. In particular,\n•A comparison with the Apollo samples has been added effectively.\n•Analysis results for larger particles have been included.\n•The term \"dynamic equilibrium\" has been removed.\n•The evolutionary model of the lunar regolith has been improved.\n•The connections with the referenced literature have become clearer.\n•Overall, the accuracy of the descriptions has greatly increased.\nAs a result, the reviewer concluded that the manuscript is now suitable for publication in Nature Communication.", "reviewer_2": "We thank the authors for addressing our comments. From this, I see an immense improvement. This is demonstrated by a more robust literature review, which has helped guide the authors towards better interpretations of the data. However, there are still some outstanding clarifications and suggestions that I feel should be addressed before publication.\nParagraph 2:\nAccording to the authors citations, the authors subdivisions for regolith particle sizes are based on simulant studies. How do these studies tie to natural regolith samples, other than their development to broadly mimic regolith composition? For example, the authors said that these divisions are based on different degrees of space weathering. Could the authors explain how these studies made this determination? I feel like a more robust description of this is warranted because to many researchers, anything <1 mm would be considered “dust” and described as “bulk soil”. If these classifications are the authors preference for the purposes of this work, I would make that clear in the text.\nIn addition, the observation of 20-200 um size fraction being more abundant than the “large” and “lunar dust” size fractions is in agreement with McKay et al 1974. It would be good to cite that here and elaborate why this phenomenon occurs when one would expect comminution to make smaller size fractions the most dominant.\nParagraph 5:\nI would note here that regolith composition influences agglutinate abundance due to some mineral phases being more prone to melting. I recommend reviewing Adams and Charette, 1975: Chemical Fractionation of the Lunar Regolith by Impact Melting; Rhodes et al., 1975: Chemistry of Agglutinate Fractions in Lunar Soils.\nFigure 2 appears to be blurry.\nSupplementary Figure 2o: Nice work adding the extra data points. For stylistic purposes, each point is very similar in color, with Pyroxene and Olivene points being very close to the shaded region of Mp. Is it possible to change the color scheme of the figure so that the points for each mineral type are more easily distinguishable?\nParagraph 10:\nLine 196-198: Solar wind contributes to npFe production embedded within agglutinates after exposure to kinetic energies associated from micrometeorite impacts. However, the glassy matrix is caused by impact melt, and solar wind is not a contributing factor. I would rephrase this to omit solar wind as an originating component to agglutinate formation and clarify that H from solar wind implantation into surface exposed regolith reduces iron in impact melt to form metallic iron nanoparticles (npFe) within agglutinitic glass. In addition, I would also elaborate on the proposed mechanisms that form vesicles within agglutinates.\nLine 200-201: The phrasing of this sentence seems awkward. I recommend splitting these into two sentences, and introduce the influence of npFe when it is first introduced in the previous sentence.\nI would also clarify what additional space weathering processes produce npFe, other than vapor deposition.\nParagraph 11:\nHas the differences of vesicle morphology between glassy interiors and fragment interfaces been observed elsewhere using other methods?\nBasu and McKay, 1985 conducted EDX analyses of the interfaces between agglutinitic melt and incorporated fragments and found no evidence of chemical gradation, suggesting that the two components don’t chemically influence each other. The authors results suggest that the interfaces influence vesicle morphology. Could the authors explain why this phenomenon occurs?\nAdditionally, do interfaces include older incorporated agglutinates? How can these agglutinate fragments be differentiated from lithic fragments in the authors CT data? How is this reconciled in the authors vesicle ratio calculations?\nI am also curious how the authors define abundance of internal fragments. Is it a volumetric abundance? Numerical abundance? This is an important distinction as a particular Ir ratio may indicate one large fragment or multiple small fragments.\nParagraph 12:\nI recommend removing the introduction of differences in micrometeorite bombardment and volatile mobilization as those are more relevant to the discussion.\nThe authors mention that the greater Ir values for bulk soils may be due to the inherent morphologies of fragmented larger agglutinates. Could it also be that bulk soil agglutinates form from smaller volumes of melt relative to the larger size fraction, and therefore would incorporate fewer fragments?\nParagraph 15:\nLine 303-304: Agglutinate abundance is only a valid proxy for maturity for Is/FeO < 60. Agglutinate abundance saturates in more mature soils (Morris, 1976).\nLine 306: I would say bombardment rather than vaporization, because npFe is found in melt AND vapor.\nLine 307-310: Could the authors elaborate as to how regolith gardening manifests itself as exposure age? How is this age determined? The authors reference a paper that uses SEP tracks to identify exposure ages, however, for a given soil there ages can span millions of years while the average may only be only a fraction of the maximum age. The maximum age, therefore, may be a more appropriate metric to gauge the overall exposure age of a soil, since that is closer to the full duration that regolith had been exposed to space weathering processes. Are the ages the authors used for the Chang’e 5/6 samples the maximum age?\nParagraph 18\nPlease define “syn-formation”\nElaborate on transient temperatures. Do the authors mean diurnal heating? Would the authors suggested transient temperature realistically influence the temperatures which very quickly melt and quench regolith?\nI am confused as to why the authors compare their results to 73002. Why would we expect this Apollo sample to be similar/different? The Apollo missions were largely equatorial and the Chang’e missions landed at much higher latitudes. If the transient temperature is micrometeorite flux or diurnal heating, which are latitudinally dependent, why is 73002 samples similar to the Chang’e 6 samples?\nAt the end of this paragraph the authors mention potential contributions to vesicle formation in agglutinates. The authors mention melt viscosity and cooling/welding history. These can be further elaborated on considering that melt compositions tend towards that of mafic compositions due to low melting temperatures of basaltic components and that the cooling history of agglutinitic melt is extremely fast (quenched).\nParagraph 19\nLine 414-415: The authors mention that abrasion preferentially removes the fragments within agglutinates. How would this affect the Ir value if the vesicles are inherent to melt, not the fragments.\nDo CE6 agglutinates generally contain more fragments? Why might this be the case?\nParagraph 20\nLine 437-438: Generally, when correlating surface exposure proxies, be specific as to which ones. In this context, what exactly was grain size fraction compared to.\nParagraph 21\nLine 452-454: I would specify that this saturation affects the bulk size fraction, as opposed to the entirety of the regolith (i.e. all size fractions).\n73002 was found to be largely composed of highlands material with small amounts of basaltic material found throughout, mostly concentrated near the top 3 cm (Neuman et al., 2025: Revealing the Moon's Taurus‐Littrow Landslide via Integrated Analysis of Pristine Apollo 17 Soil Core 73001/2). I would clarify that 73002 is compositionally distinct from pure mare regolith and elaborate on this when making comparisons to Chang'e samples.\nIn Morris et al., 2022, they found that the in situ reworking zone of 73002 had an upper development age of ~17 Ma. The 3.0 Ma age found in SEP track densities was the average of exposure ages from individual particles in the core, and not reflective of the overall time regolith had been subject to regolith gardening. Therefore, it would be more appropriate to compare the maximum exposure ages of a soil to evaluate the saturation timescales of bulk soil morphology.", "reviewer_3": "Point-by-point response to the issues raised by the referees\n(Manuscript No: NCOMMS-25-47464)\nResponses in blue.\nRevisions in red.\nIn the revised manuscript and supplementary information, all revisions are highlighted.\nReviewer #1\nThis manuscript presents a highly valuable and large-scale analysis of lunar regolith particles\ncollected by the Chang’e-5 (CE5) and Chang’e-6 (CE6) missions, utilizing micro-CT scanning\nand machine learning-based classification. The study provides rare and rich primary data that\nsignificantly contributes to our understanding of key lunar surface processes including\nvolcanism, meteoritic impacts, and space weathering.\nHowever, there are several critical points that require clarification and appropriate revision as\nfollows:\nReviewer #2\n1. References do not appear to be in correct order (ie reference 14 is cited in line 39 which is\nfollowed by reference 51 in line 41). I would suggest reordering the reference list so that they\nare increasing consecutively as they are being used in the text for clarity."}, "author_response": "Point-by-point response to the issues raised by the referees\n(Manuscript No: NCOMMS-25-47464A)\nResponses in blue.\nRevisions in red.\nIn the revised manuscript and supplementary information, all revisions are highlighted.\nReviewer #1\nThe response letter provided very courteous and detailed explanations. In particular,\n•A comparison with the Apollo samples has been added effectively.\n•Analysis results for larger particles have been included.\n•The term \"dynamic equilibrium\" has been removed.\n•The evolutionary model of the lunar regolith has been improved.\n•The connections with the referenced literature have become clearer.\n•Overall, the accuracy of the descriptions has greatly increased.\nAs a result, the reviewer concluded that the manuscript is now suitable for publication in Nature Communication.\nResponse: We appreciate the reviewer’s accurate summary of the scope of our extensive first\nround of revisions. Thank you again for taking the time to review our paper. Your feedback\nhas enabled us to make significant improvements to it.\nWe thank the authors for addressing our comments. From this, I see an immense improvement.\nThis is demonstrated by a more robust literature review, which has helped guide the authors\ntowards better interpretations of the data. However, there are still some outstanding\nclarifications and suggestions that I feel should be addressed before publication.\nResponse: We thank you for the positive and encouraging feedback. We greatly appreciate\nyour recognition of the substantial improvements made in the revised manuscript. We also\nthank you for providing additional constructive suggestions and requests for clarification. We\nhave carefully addressed each remaining point in detail below to further enhance the clarity,\nrigor, and overall quality of the manuscript.\n1. Paragraph 2: According to the authors citations, the authors subdivisions for regolith\nparticle sizes are based on simulant studies. How do these studies tie to natural regolith samples,\nother than their development to broadly mimic regolith composition? For example, the authors\nsaid that these divisions are based on different degrees of space weathering. Could the authors\nexplain how these studies made this determination? I feel like a more robust description of this\nis warranted because to many researchers, anything <1 mm would be considered “dust” and\ndescribed as “bulk soil”. If these classifications are the authors preference for the purposes of\nthis work, I would make that clear in the text. In addition, the observation of 20-200 um size\nfraction being more abundant than the “large” and “lunar dust” size fractions is in agreement\nwith McKay et al 1974. It would be good to cite that here and elaborate why this phenomenon\noccurs when one would expect comminution to make smaller size fractions the most dominant.\nResponse: Thanks for this very insightful and technically precise comment. We fully agree\nthat our previous justification for subdividing regolith particle sizes—based partly on simulant\nstudies—was not sufficiently rigorous and may not directly reflect the behavior of natural lunar\nsoils. After carefully re-evaluating our classification scheme, we acknowledge that (i) simulant-\nbased criteria cannot serve as primary evidence for subdivision, and (ii) our earlier statement\nthat the divisions correspond to “different degrees of space weathering” was not well supported\nby the literature.\nOur focus on lunar regolith fraction of particle size <1 mm arises from their well-established\nimportance in governing the space weathering properties of lunar soils (McKay et al., 1974).\nParameters such as their mean grain size and Is/FeO values are known to correlate with space\nweathering. This size fraction is also central to our study because it can be comprehensively\nimaged using micro-CT method. Due to the spatial resolution limit of our scans (~1 μm),\nparticles smaller than ~20 μm cannot be reliably reconstructed; we therefore categorize <20\nμm grains as “lunar dust.” After segmentation and particle size analysis, we found that grains\nbetween 20–200 μm overwhelmingly dominate the <1 mm fraction in both CE5 and CE6.\nConsequently, we define this interval as the “bulk soil” fraction, with particles >200 μm\nclassified as “large particles.”\nobservations are indeed consistent with Apollo measurements. Specifically, McKay et al.\ndemonstrated that the modal abundance peak of lunar soils typically lies within the ~90–150\nμm size range. They attribute this observation to a key physical mechanism: fine dust (<20 μm)\nis progressively depleted in lunar soils, because the finest particles are preferentially fused into\nagglutinates during micrometeorite melting events. These agglutinates are subsequently\nincorporated into intermediate size fractions, leading to a natural enrichment of grains between\n~20–200 μm. This constructional process acts alongside comminution and continual recycling\nof coarser lithic fragments, producing a stable modal peak in the intermediate size range. This\nexplanation clarifies why, even though fragmentation tends to increase the abundance of finer\ngrains, the intermediate (20–200 μm) fraction remains the most dominant size population in\nlunar regolith.\nWe appreciate the reviewer’s guidance, which has significantly strengthened the scientific\njustification and clarity of this section. We now explicitly state that our size subdivisions reflect\n(i) the physical capabilities and resolution constraints of current analytical techniques and (ii)\nthe natural modal distribution of lunar soils recognized since the Apollo era, rather than being\nbased on simulant studies.\nRevision: We have revised Paragraph 2 to incorporate the reviewer’s suggestions. The newly\nadded text clarifies the rationale for our size subdivisions, removes unsupported statements\nabout “degrees of space weathering,” integrates the Apollo sample findings of McKay et al.\n(1974), and explicitly states that the chosen size ranges reflect natural regolith characteristics\nand limitations in current analytical techniques rather than simulant-based definitions. We note\nthat, because this paragraph appears in the Introduction—before detailed descriptions of the\nmicro-CT scan methodology or specific space weathering parameters are introduced—we\nintentionally adopted concise and conceptually framed language to maintain readability and\nlogical flow at this stage of the manuscript.\nLine 48-58:\n“On the Moon, the fraction of lunar regolith with particle sizes <1 mm are particularly relevant\nto surface space weathering22 and warrant detailed investigation. Due to limitations in current\nanalytical techniques as encountered in this study, it remains challenging to obtain reliable\nmorphometric data for particle sizes <20 μm (ref. 23), which are typically classified as “lunar\ndust” (see Methods). Particles within the 20–200 μm size range, comprising over 60% of the\n<1 mm fraction, are defined as the “bulk soil” fraction24,25, while the remaining particles\n(>200 μm) are categorized as “large particles”. Notably, this dominance of the bulk soil fraction\nis also observed in Apollo samples and can be attributed to the progressive consumption of the\nlunar dust fraction during agglutinate formation and its subsequent redistribution within the\nbulk soil fraction22. As such, the bulk soil fraction serves as the primary reservoir of surface\nevolution signatures on the Moon.”\n2. Paragraph 5: I would note here that regolith composition influences agglutinate abundance\ndue to some mineral phases being more prone to melting. I recommend reviewing Adams and\nCharette, 1975: Chemical Fractionation of the Lunar Regolith by Impact Melting; Rhodes et\nal., 1975: Chemistry of Agglutinate Fractions in Lunar Soils.\nResponse: We sincerely thank the reviewer for this constructive suggestion. We fully agree\nthat regolith composition plays a critical role in governing agglutinate abundance, owing to the\nmarkedly different melting behaviors of individual mineral phases under micrometeoroid\nbombardment. As recommended, we have carefully reviewed Adams & Charette (1975) and\nRhodes et al., (1975), both of which clearly demonstrate that low-melting mafic minerals (e.g.,\nilmenite, pyroxene, olivine) are preferentially assimilated into impact melts, whereas high-\nmelting plagioclase remains largely crystalline during the agglutination process. These\nmineralogical controls directly influence the efficiency of agglutinate formation and therefore\nmodulate the abundance of agglutinates in mare versus highland soils.\nRevision: We have revised Paragraph 5 to incorporate the reviewer’s insightful suggestion\nregarding the mineralogical control on agglutinate abundance. The updated text (Lines 98–105)\nnow reads:\n“Importantly, the mineralogical composition of local regolith is a key factor influencing\nagglutinate abundance. During rapid, non-equilibrium impact melting, low-melting-\ntemperature mafic minerals such as ilmenite, pyroxene, and olivine are more readily\nincorporated into impact melts than high-melting-temperature plagioclase50,51. As a result, soils\nwith higher plagioclase contents may produce fewer agglutinates50. The regolith maturity also\nexerts a strong control on agglutinate abundance, with more mature soils typically containing\nhigher proportions of agglutinates22. Together, these factors influence agglutinate abundance\nand, consequently, the overall particle-scale morphological characteristics of regolith.”\n3. Figure 2 appears to be blurry.\nResponse: Thank you for noting the issue with Figure 2. The reduced clarity appears to have\nresulted from the Nature Communications submission system downsampling images during\nthe Word-to-PDF conversion process. We have carefully re-checked all original figures in the\nWord file and confirmed that the source images are of high resolution.\nRevision: Figure 2 has been updated using the original high-resolution files.\nFigure 2:\n4\n4. Supplementary Figure 2o: Nice work adding the extra data points. For stylistic purposes,\neach point is very similar in color, with Pyroxene and Olivene points being very close to the\nshaded region of Mp. Is it possible to change the color scheme of the figure so that the points\nfor each mineral type are more easily distinguishable?\nResponse: We sincerely thank the reviewer for the positive feedback and for recognizing the\nvalue of our additional data points. We fully agree that improving the visual distinction among\nmineral phases will enhance the clarity of the figure. Accordingly, we have revised the color\nscheme in Supplementary Figure 2o so that the data points for each mineral type are now more\nclearly and easily distinguishable from one another, as well as from the shaded Mp region.\nRevision: We have updated the color palette in Supplementary Figure 2o to increase contrast\nbetween mineral types. Pyroxene, olivine, plagioclase, and ilmenite now use distinct colors.\nSupplementary Figure 2o:\n5\n5. Paragraph 10: Line 196-198: Solar wind contributes to npFe production embedded within\nagglutinates after exposure to kinetic energies associated from micrometeorite impacts.\nHowever, the glassy matrix is caused by impact melt, and solar wind is not a contributing factor.\nI would rephrase this to omit solar wind as an originating component to agglutinate formation\nand clarify that H from solar wind implantation into surface exposed regolith reduces iron in\nimpact melt to form metallic iron nanoparticles (npFe) within agglutinitic glass. In addition, I\nwould also elaborate on the proposed mechanisms that form vesicles within agglutinates.\nResponse: We thank the reviewer for this very helpful clarification and agree that our previous\nwording could be interpreted as implying that the solar wind directly contributes to the\nformation of the glassy matrix. In the revised text, we now explicitly state that micrometeorite\nimpacts generate the impact melts that quench to form the glassy matrix of agglutinates, and\nsolar wind can reduce iron in the glassy matrix to produce metallic iron nanoparticles (npFe)\nwithin the agglutinitic glass. We have removed phrasing that might be read as treating the solar\nwind as an originating component of the glass itself.\nIn addition, we have expanded the description of vesicle formation in agglutinates. We now\nbriefly note that vesicles are generally thought to arise from two broad contributions: (i) the\nbehavior of solar wind implantation, which generates small vesicles along particle margins\n(McKay et al., 1991; Zhao et al., 2023; Kling et al., 2023), and (ii) the exsolution of volatiles,\nwhich produces internal larger vesicles (Housley, 1977; Huang et al., 2024; Li et al., 2025).\nRevision: We have revised lines 207-211 to read as follows, where we intentionally keep this\ndiscussion concise, so as not to overburden the reader here:\n“They form primarily through micrometeorite-induced melting, which generates vesiculated\nagglutinitic glass matrices. The vesicles within these matrices are widely interpreted as the\ncombined textural products of solar wind implantation, which generates small vesicles along\nparticle margins, and volatile gas exsolution during melt generation and quenching, which\nproduces larger internal vesicles17,59.”\n6. Line 200-201: The phrasing of this sentence seems awkward. I recommend splitting these\ninto two sentences, and introduce the influence of npFe when it is first introduced in the\nprevious sentence.\nResponse: Thanks for pointing this out and excellent suggestion. We agree that the original\nphrasing was awkward and could obscure the logical flow. In the revised version, we have\nreordered the earlier part of Paragraph 10 to introduce npFe and its influence on regolith\nproperties when it first appears.\nRevision: Lines 211–215 have been revised as follows:\n“Within glassy matrices, submicroscopic metallic iron nanoparticles (npFe) form through a\ncombination of micrometeorite impact60, solar wind (induced sputtering and reduction)61,\n6\ntransient thermal-spike (driven reduction)62, and externally triggered Fe2+ disproportionation63,\nall of which collectively modify the optical and magnetic properties of the soil.”\n7. I would also clarify what additional space weathering processes produce npFe, other than\nvapor deposition.\nResponse: Thanks for pointing out that our original wording placed too much emphasis on\nvapor deposition. We fully agree that npFe in lunar regolith, and particularly within\nagglutinates, is produced by a broader suite of space weathering processes. In the revised text,\nwe now clarify that npFe formation involves several complementary mechanisms, including:\n(i) Micrometeorite impacts at hypervelocity generate short-lived, high-temperature impact\nplumes in which target minerals are partially melted and vaporized; in this expanding vapor\ncloud, Fe-O bonds are broken, oxygen is preferentially lost, and the resulting Fe-bearing vapor\nor plasma subsequently condenses onto nearby grain surfaces and agglutinitic glass as ultra-\nthin coatings enriched in npFe (Wang et al., 2012; Denevi et al., 2023);\n(ii) Solar wind ions produce radiation-damaged amorphous rims and can both directly reduce\nFe2+ to npFe in near-surface layers and supply sputtered Fe that redeposits as npFe, some of\nwhich is subsequently incorporated into agglutinates during impact melting (Loeffler et al.,\n2009; Xiong et al., 2024);\n(iii) The short-lived thermal spikes and shock waves associated with micrometeorite impact or\nintense heating can instantaneously amorphize silicates and drive thermal reduction of Fe in\nimpact melts, generating npFe within agglutinitic glass independently of external reducing\nagents (Malina et al., 2015; Li et al., 2022);\n(iv) In addition, micrometeorite impact conditions can trigger charge disproportionation of Fe2+,\nas evidenced by the coexistence of npFe and Fe3+ in impact-modified amorphous mixtures from\nChang’e-5 soils, providing an internal redox pathway for npFe formation that does not require\nsolar wind hydrogen (Li et al., 2022; Shen et al., 2024).\nRevision: We have revised Paragraph 10 to explicitly state that npFe is produced by\nmicrometeorite impact-induced vaporation and deposition, solar wind-induced reduction and\nsputtering, impact-related thermal spikes and shock effects, and Fe²⁺ disproportionation. We\nalso have added corresponding explanatory sentences and citations to clarify these additional\nspace weathering pathways.\nLines 211-215:\n“Within glassy matrices, submicroscopic metallic iron nanoparticles (npFe) form through a\ncombination of micrometeorite impact60, solar wind (induced sputtering and reduction)61,\ntransient thermal-spike (driven reduction)62, and externally triggered Fe2+ disproportionation63,\nall of which collectively modify the optical and magnetic properties of the soil.”\n8. Paragraph 11: Has the differences of vesicle morphology between glassy interiors and\nfragment interfaces been observed elsewhere using other methods?\nevaluate whether such contrasts in vesicle morphology, between glass-dominated interiors\nand fragment-rich interfaces, have been documented using independent analytical techniques.\nIndeed, Hung et al. (2024) report SEM observations of an Apollo 11 agglutinate, and their\nback-scattered electron (BSE) image (shown below as Response Figure 1) shows a strikingly\ncomparable pattern: the clast in the upper-right portion of the section is dominated by glass\nwith few incorporated fragments, and exhibits predominantly regular, rounded vesicles,\nwhereas the clast in the lower-left contains abundant fragments and displays correspondingly\nirregular vesicle shapes. This independent SEM evidence aligns well with our interpretation.\nResponse Figure 1. BSE images of Apollo 11 sample lunar agglutinate (Huang et al., 2024).\nRevision: Accordingly, we have added this supporting evidence to Lines 231-232, where the\nBSE image of an Apollo 11 agglutinate exhibits glass-rich regions with predominantly regular\nvesicles and fragment-rich regions with irregular vesicles, consistent with the findings\nidentified in our micro-CT scan analyses.\nLines 231-232:\n“This phenomenon is also evident in SE imaging of Apollo 11 agglutinates64.”\n9. Basu and McKay, 1985 conducted EDX analyses of the interfaces between agglutinitic melt\nand incorporated fragments and found no evidence of chemical gradation, suggesting that the\ntwo components don’t chemically influence each other. The authors results suggest that the\ninterfaces influence vesicle morphology. Could the authors explain why this phenomenon\noccurs?\nResponse: Many thanks for asking for an explanation. We agree that our original explanation\ndid not sufficiently clarify why vesicles adjacent to fragments adopt more irregular shapes\ndespite the lack of chemical interaction at the glass-fragment interface reported by Basu &\nMcKay (1985). As those authors showed, agglutinitic melt and incorporated fragments exhibit\nno detectable chemical gradients across their interfaces, indicating that the solid fragments\nlargely retain their integrity during melting and do not chemically influence the surrounding\nglass.\nagglutinate formation, three phases coexist: gas (volatiles), melt (impact-generated glass),\nand solid fragments. When vesicles nucleate and expand within relatively homogeneous melt\nregions, surface tension dominates, promoting the development of spherical or near-spherical\nvesicles (blue region in Response Figure 2). In contrast, when vesicles form at glass-fragment\ninterfaces, the rigid fragments act as geometric and mechanical constrictions. Because these\nfragments do not appreciably dissolve or deform during the agglutinate formation event,\nexpanding gas bubbles must accommodate the pre-existing fragment boundaries (Duke et al.,\n1970). This results in asymmetric vesicle expansion and irregular bubble geometries (green\nregion in Response Figure 2). Once the melt quenches, these mechanically constrained\nvesicles are preserved as irregular vesicles.\nResponse Figure 2. Vesicles within agglutinates, illustrating that vesicles formed adjacent to\nfragments are more irregular due to mechanical constriction, while vesicles developed within\nglassy matrix are more regular. The yellow areas denote fragments, the green areas indicate\nirregular vesicles, and the blue areas represent regular vesicles. The blue dashed circles mark\nthe predicted boundaries of irregular vesicles in the absence of fragment-induced obstruction,\nand the white arrows show the compressive stresses exerted by surrounding fragments during\nthe formation of irregular vesicles.\nRevision: We have revised Lines 228-231 to explicitly incorporate this mechanistic\nexplanation:\n“This contrast likely arises because vesicles forming within relatively homogeneous glassy\nregions tend to adopt regular shapes controlled by surface tension, whereas vesicles near\nfragment-glass interfaces are mechanically constrained by surrounding fragments and thus\ndevelop irregular morphologies.”\n10. Additionally, do interfaces include older incorporated agglutinates? How can these\nagglutinate fragments be differentiated from lithic fragments in the authors CT data? How is\nthis reconciled in the authors vesicle ratio calculations?\nResponse: Thanks for raising this important point. In our original text, we did not explicitly\ndefine what we refer to as fragments within agglutinates. Here we clarify that “fragments”\ninclude all solid components incorporated during agglutinate formation: mineral grains, rock\nclasts, and older agglutinates. Thus, older incorporated agglutinates are indeed part of the\nfragment population, and the interfaces we describe necessarily include melt-lithic and melt-\nolder agglutinate boundaries.\nfragments is extremely challenging, for two main reasons:\n(i) Because agglutinates are mechanically weak (McKay et al., 1991), any older agglutinates\nincorporated into a later generation of melt are likely to have undergone substantial\nfragmentation during the impact event. Their characteristic vesicle networks and agglutinate\ntextures are often destroyed or highly disrupted, making them effectively indistinguishable\nfrom lithic fragments in CT data.\n(ii) Even in cases where an older agglutinate fragment was incorporated in a relatively intact\nform, its boundary with the newly formed agglutinitic glass would still be extremely difficult\nto identify in CT radiographs. Although previous studies have shown that agglutinitic glasses\nfrom the same sampling site may differ substantially in chemical composition (Basu & McKay,\n1985), such compositional variability does not necessarily translate into appreciable density\ncontrast, the primary physical parameter detected by micro-CT. As a result, older and newly\nformed agglutinitic glass typically occupy overlapping grayscale ranges in CT volumes, and\ntheir interface lacks the sharp density discontinuity that characterizes, for example, glass-lithic\nboundaries. Consequently, the outlines of older agglutinates cannot be reliably segmented in\nour CT data, making it impossible to distinguish them from lithic fragments.\nGiven these limitations, our fragment content measurements and the subsequent vesicle ratio\ncalculations inherently treat older incorporated agglutinates as part of the general fragment\npopulation. At present, the CT data do not allow us to isolate the vesicle structures belonging\nto older agglutinates and exclude them from our statistics. However, it is important to\nemphasize that this limitation is unlikely to affect our conclusions. In our study, we determined\nthat the CE5 lunar regolith powder sample contains ~8% agglutinates, whereas CE6 contains\n~17%. In principle, CE6 should therefore host a greater number of older incorporated\nagglutinates. If vesicles preserved within these older agglutinates significantly biased our\nvesicle ratio calculations, we would expect CE6 to exhibit higher void ratios in its larger\nagglutinates than CE5. Yet the opposite pattern is observed: CE6 has a lower mean void ratio\n(e = 0.11) compared to CE5 (e = 0.23). This inverse relationship indicates that older\nCE6 CE5\nagglutinates are not the primary factor influencing our vesicle ratio estimates.\nWe acknowledge this as a limitation of the current study and note that future work combining\nmicro-CT with targeted EDX or BSE imaging on extracted agglutinates could help better\ndiscriminate older agglutinates, refine interface classifications, and evaluate their influence on\nvesicle morphology metrics.\nRevision: We revised Lines 206–207 to clearly define the term fragments as follows:\n“Agglutinates are special regolith particles consisting of smaller lunar soil fragments (mineral\nparticles, rock clasts, and older agglutinates) bonded together by vesicular, flow-banded\nglass59.”\n10\n11. I am also curious how the authors define abundance of internal fragments. Is it a volumetric\nabundance? Numerical abundance? This is an important distinction as a particular Ir ratio may\nindicate one large fragment or multiple small fragments.\nResponse: Thank you for pointing out this omission! We apologize for not clearly defining\nthis parameter in the original manuscript. In our analysis, Fr represents the volumetric\nabundance of internal fragments within each agglutinate, rather than their numerical abundance.\nWe fully agree that this distinction is essential, as a given Ir value could indeed correspond to\neither a single large fragment or multiple small fragments, and only a volumetric definition can\nproperly reflect the relative contribution of fragments to the internal structure.\nRevision: We have revised lines 234–236 to explicitly clarify this definition as follows:\n“As agglutinates are composed of both fragments and a glassy matrix65, the glassy matrix and\nfragment contents (Fr) are inherently volumetrically complementary. This parameter thus may\nalso relate to the volumetric abundance of internal fragments.”\n12. Paragraph 12: I recommend removing the introduction of differences in micrometeorite\nbombardment and volatile mobilization as those are more relevant to the discussion.\nResponse: We fully agree with the reviewer’s suggestion. The introduction of differences in\nmicrometeorite bombardment and volatile mobilization is indeed more appropriate for the\nDiscussion rather than the Results section. Including such interpretations prematurely in\nParagraph 12 may distract from the presentation of observational results. We therefore\nappreciate the reviewer’s guidance on improving the structural clarity of the manuscript.\nRevision: We have removed the sentence in question from Paragraph 12: “likely reflecting\ndifferences in micrometeorite bombardment intensity between the two sites, and potentially\nalso indicating variations in volatile content (such as H₂O) between the two regions”. These\ninterpretations are now fully addressed in Paragraphs 18 and 19, where differences in\nmicrometeorite bombardment and volatile mobilization are discussed in detail within the\nappropriate context of the Discussion.\n13. The authors mention that the greater Ir values for bulk soils may be due to the inherent\nmorphologies of fragmented larger agglutinates. Could it also be that bulk soil agglutinates\nform from smaller volumes of melt relative to the larger size fraction, and therefore would\nincorporate fewer fragments?\nResponse: Thanks for raising this thoughtful point. We realize that our initial manuscript did\nnot explicitly define Fr, which may have contributed to a misunderstanding of our\ninterpretation. While it is true that bulk soil agglutinates generally derive from smaller melt\nvolumes compared with larger agglutinates, this does not necessarily imply that they\nincorporate a lower volumetric abundance of internal fragments. Moreover, this explanation\ndoes not account for our observed trend in which CE5 bulk soil agglutinates exhibit higher Ir\n11\nvalues than those in CE6, implying that CE6 bulk soil agglutinates contain a greater volumetric\nfraction of fragments. At present, we do not have sufficient evidence to support or refute the\nhypothesis that smaller melt volumes inherently limit fragment incorporation. To avoid\noverinterpretation, we therefore refrain from including this possibility in the current manuscript.\nWe nevertheless agree that this is a forward-looking idea, and we will explicitly consider it in\nfuture studies.\nRevision: As noted in the revision to Comment #11, we have now clarified the definition of Fr\nin Lines 234-236 stating explicitly that it represents the volumetric abundance of internal\nfragments, and addressing the potential ambiguity underlying this concern.\n14. Paragraph 15: Line 303-304: Agglutinate abundance is only a valid proxy for maturity\nfor Is/FeO < 60. Agglutinate abundance saturates in more mature soils (Morris, 1976).\nResponse: Thanks for this important clarification. You are absolutely correct that our original\nwording was not sufficiently rigorous. Agglutinate abundance is indeed only a reliable maturity\nindicator in soils with Is/FeO < 60, as Morris (1976) demonstrated that agglutinate abundance\nreaches saturation in more mature soils. We appreciate the reviewer’s careful attention to detail.\nRevision: We have revised Lines 316–318 accordingly to ensure greater accuracy and\nprecision in our description:\n“In particular, agglutinate abundance can serve as an indicator of soil maturity59. This\nparameter correlates positively with the maturity index Is/FeO, but only when Is/FeO <60,\nabove which agglutinate abundance is known to saturate in highly mature soils71,72.”\n15. Line 306: I would say bombardment rather than vaporization, because npFe is found in\nmelt AND vapor.\nResponse: Very helpful clarification. We fully agree that “bombardment” is more appropriate\nin this context, as npFe forms in both melt and vapor components generated during\nmicrometeorite impacts. We appreciate the reviewer’s attention to detail.\nRevision: We have revised Lines 322–325 by replacing “vaporization” with “bombardment”\nto ensure accurate representation of the npFe-forming processes:\n“Furthermore, the Is/FeO index is closely linked to the abundance of npFe, produced on grain\nrims and within agglutinates by micrometeorite bombardment and solar wind\nsputtering/redeposition, and therefore reflects the cumulative intensity of these space\nweathering processes.”\n16. Line 307-310: Could the authors elaborate as to how regolith gardening manifests itself as\nexposure age? How is this age determined? The authors reference a paper that uses SEP tracks\nto identify exposure ages, however, for a given soil there ages can span millions of years while\n12\nthe average may only be only a fraction of the maximum age. The maximum age, therefore,\nmay be a more appropriate metric to gauge the overall exposure age of a soil, since that is closer\nto the full duration that regolith had been exposed to space weathering processes. Are the ages\nthe authors used for the Chang’e 5/6 samples the maximum age?\nResponse: Thanks for this insightful and important comment. You are absolutely right that our\nprevious manuscript did not sufficiently clarify how regolith gardening is expressed in terms\nof exposure age, nor did we clearly justify the type of exposure age adopted.\nAs McFadden et al. (2024) demonstrate using their regolith mixing model (schematically\nillustrated in Response Figure 3 below), progressive reworking by meteorite induced\ngardening increases the depth and intensity of regolith overturning. Consequently, the\nmaximum surface exposure age serves as an indicator of the cumulative extent of gardening\nexperienced by the surface soil: older maximum exposure ages correspond to deeper reworking\nzones and stronger surface mixing.\nRegarding the determination of exposure age, we fully agree with the reviewer that solar\nenergetic particle (SEP) track analyses typically yield a broad distribution of track densities,\nreflecting the heterogeneous reworking history of a soil parcel (Cao et al., 2025). Thus, while\nthe average exposure age may represent the average residence time of grains, it is often\nsubstantially younger than the maximum age, which better reflects the overall duration over\nwhich the soil column has undergone space weathering and gardening.\nFor the Chang’e-5 and Chang’e-6 soils, the exposure ages we originally cited corresponded to\naverage values, which was indeed not the most appropriate metric. Following the reviewer’s\nrecommendation, we have now replaced all CE5, CE6, and Apollo 17 (73002) exposure ages\nwith their maximum exposure ages:\n CE5: ~2.2 Ma\n CE6: ~3.2 Ma\n Apollo 17 (73002): ~7.0 Ma\nFor CE5 and CE6, these maximum ages are derived from the impact gardening model\nconstrained by local topography and regolith overturn rates (Zhang et al., 2025b), whereas the\nApollo 17 (73002) maximum exposure age is determined directly from SEP track\nmeasurements (McFadden et al., 2024).\nWe greatly appreciate the reviewer’s guidance, which has strengthened both the clarity and the\nphysical relevance of our interpretation.\ngardening and the surface soil exposure time. The in-situ gardening timescale increases in the\npanels to the right with grains with a higher degree of exposure being introduced into the system\nwhile previously space-weathered grains migrate downward. The distributions on the right\ncorrespond to their two respective depths (McFadden et al., 2024).\nRevision: We have revised Lines 325–328 to clarify how regolith gardening manifests itself\nin terms of exposure age:\n“Another critical component of space weathering is regolith “gardening” driven by meteorite\nimpacts, which is commonly expressed through the surface soil maximum exposure age, where\nsoils with longer exposure durations experience more extensive overturn and reworking75.”\nWe have also carefully updated all exposure age values throughout the manuscript to reflect\nthe maximum exposure ages for CE5, CE6, and Apollo 17 (73002).\n17. Paragraph 18: Please define “syn-formation”\nResponse: Very good catch of loose language. After careful reassessment, we agree that the\nterm “syn-formation gas” is not sufficiently precise and may cause unnecessary ambiguity. The\nintended meaning is volatile gas escaping during agglutinate formation. We have therefore\nrevised the wording accordingly to improve clarity and ensure more accurate scientific\nexpression.\nRevision: We have revised Lines 404–406 as follows:\n“The comparatively lower internal porosity observed in larger CE6 agglutinates is consistent\nwith more efficient volatile gas escape under open-system conditions.”\n18. Elaborate on transient temperatures. Do the authors mean diurnal heating? Would the\nauthors suggested transient temperature realistically influence the temperatures which very\nquickly melt and quench regolith?\nResponse: Thanks for asking for further clarification. As the reviewer correctly notes, diurnal\nheating on the lunar surface is insufficient to melt regolith, where the maximum daytime\ntemperature (~389 K) and minimum nighttime temperature (~86 K) reported by Cremers et al.\n(1972) are far below the melting point of silicates.\nIn our manuscript, however, “transient temperatures” refer not to diurnal thermal cycling but\nto the extreme, short-lived temperature spikes generated during hypervelocity micrometeorite\nimpacts. Laboratory impact experiments show that micrometeorite impacts can generate flash\ntemperatures of several thousand Kelvin (Eichhorn, 1978). Furthermore, recent silicon isotope\nmodeling provides quantitative estimates of these transient melt temperatures for lunar regolith:\nCE5 soils likely experienced impact-induced temperatures of ~1,200–2,000 K, whereas CE6\nand Apollo soils reached even higher temperatures (~2,500–5,000 K) (Zhang et al., 2025a).\nThese temperatures are fully sufficient to melt and partially vaporize regolith, thereby\ngenerating agglutinates.\nWe have now clarified this distinction in the revised text and explicitly stated that the transient\ntemperatures discussed in our study refer to those of hypervelocity impacts, not from diurnal\nheating.\nRevision: Revised Lines 406-409:\n“Micrometeorites with higher impact velocities and larger masses generate higher transient\ntemperatures32, which lower melt viscosity and promote rapid bubble ascent and vesicle\ncoalescence, thereby facilitating volatile gas venting before cooling85-87 (Fig. 5).”\nWe have also revised lines 412–417 to explicitly clarify the estimated transient temperatures\nfor CE5 and CE6, and to note that these temperatures are sufficient to melt regolith and generate\nagglutinates.\n“Previous research has shown that CE5 samples experienced relatively lower temperatures\n(1,200–2,000K, ref. 32) induced by micrometeorite impacts compared to Apollo soil samples88,\nalthough such temperatures are already sufficient to melt regolith and generate agglutinates.\nFor CE6 samples, the discovery of oldhamite89 and silicon isotope modeling suggests that the\nglass formation temperatures are likely comparable to those documented in Apollo samples,\nincluding Apollo 17 (2,500–5,000K, ref. 32,88).”\n19. I am confused as to why the authors compare their results to 73002. Why would we expect\nthis Apollo sample to be similar/different? The Apollo missions were largely equatorial and\nthe Chang’e missions landed at much higher latitudes. If the transient temperature is\nmicrometeorite flux or diurnal heating, which are latitudinally dependent, why is 73002\nsamples similar to the Chang’e 6 samples?\nResponse: Fair question that other readers may also wonder about, so thanks for asking. Indeed,\nthe Apollo sites are largely equatorial, whereas the Chang’e 5 and Chang’e 6 landing sites lie\nat higher latitudes, and we agree that micrometeorite flux itself exhibits latitudinal dependence.\nHowever, in our interpretation, the factor most relevant to the internal structure of agglutinates\nis not the micrometeorite flux, but rather the transient melt temperature generated during\nindividual hypervelocity impacts. These temperatures are governed predominantly by impact\nvelocity and mass, rather than by latitude-dependent flux.\nAs noted in previous studies, CE5 soils experienced comparatively lower impact-induced\ntemperatures relative to Apollo soils (Nie et al., 2024). In contrast, silicon isotope modeling\nindicates that the glass-formation temperatures for CE6 regolith are comparable to those\ndocumented for Apollo soils, rather than to CE5 (Zhang et al., 2025a). Based on these findings,\nwe hypothesized that the higher formation temperatures associated with more energetic\nmicrometeorite impacts at the CE6 site could have facilitated more efficient volatile escape,\nyielding the lower agglutinate void ratios observed for CE6 relative to CE5.\nBecause our interpretation already relies on Apollo regolith as a benchmark for impact-induced\nmelt temperatures, Apollo soils provide an appropriate reference dataset for evaluating\nagglutinate porosity. Moreover, in the previous review round, Reviewer #1 specifically\nencouraged us to incorporate Apollo agglutinate data to further substantiate our interpretation.\nFollowing this suggestion, we selected a Apollo 17 sample specifically because its thermal\nhistory is explicitly incorporated into the modeling framework we have just mentioned in Nie\net al. (2024). Using the same methodology applied to CE5 and CE6, we extracted the 3D\ngeometries of 42 agglutinates from 73002 and derived their void ratios. The resulting mean\nvoid ratio (e = 0.10) is nearly identical to that of CE6 and substantially lower than that of\nA17\nCE5. Coupled with the evidence for similar melt-formation temperatures between CE6 and\nApollo soils, this comparison provides additional support for our interpretation.\nRevision: We have clarified this logic in Paragraph 18 by strengthening the connection\nbetween impact-induced melt temperatures of Apollo soils and our rationale for selecting\nApollo 17 (73002) as a reference sample. Corresponding text in lines 412–417 has been revised\nto explicitly state that Apollo 17 is included in the glass-formation temperature assessments\ncited from previous studies, thereby making the comparison with CE6 clearer and more\ncoherent.\n“Previous research has shown that CE5 samples experienced relatively lower temperatures\n(1,200–2,000K, ref. 32) induced by micrometeorite impacts compared to Apollo soil samples88,\nalthough such temperatures are already sufficient to melt regolith and generate agglutinates.\nFor CE6 samples, the discovery of oldhamite89 and silicon isotope modeling suggests that the\nglass formation temperatures are likely comparable to those documented in Apollo samples,\nincluding Apollo 17 (2,500–5,000K, ref. 32,88).”\n20. At the end of this paragraph the authors mention potential contributions to vesicle\nformation in agglutinates. The authors mention melt viscosity and cooling/welding history.\nThese can be further elaborated on considering that melt compositions tend towards that of\nmafic compositions due to low melting temperatures of basaltic components and that the\ncooling history of agglutinitic melt is extremely fast (quenched).\nResponse: Thanks for this very constructive suggestion. We fully agree that vesicle formation\nin agglutinates is closely linked to both melt composition and cooling history, and that these\naspects deserve clearer elaboration.\nPrevious geochemical studies show that CE5 regolith contains substantially higher abundances\nof mafic compositions (42.0 wt% pyroxene, 5.7 wt% olivine, and 4.5 wt% ilmenite) compared\nwith CE6, which contains only 33.3 wt% pyroxene, 0.5 wt% olivine, and 1.6 wt% ilmenite (Li\net al., 2022; Li et al., 2024). Because these phases have relatively low melting temperatures,\nagglutinitic melts derived from CE5 soils are expected to be compositionally more mafic. As a\nconsequence, CE5 agglutinitic melts likely cooled extremely rapidly, which would have limited\nthe time window for volatile escape. Under such rapid cooling, vesicles that would otherwise\nmigrate, coalesce, or collapse may instead become trapped within the melt interior. The\ncombined effects of mafic-rich melt compositions and ultra-fast quenching therefore provide a\nplausible explanation for why CE5 agglutinates may have retained a higher internal porosity.\nRevision: We appreciate the reviewer highlighting this mechanism, and we have incorporated\nthis clarification into the revised manuscript in Lines 420-423:\n“An alternative explanation for the higher porosity observed in CE5 agglutinates is that CE5\nregolith contain a higher proportion of mafic components24,25, leading to lower melting\ntemperatures and extremely rapid cooling of agglutinitic melts, which may trap more volatile\ngas within the agglutinate interiors.”\n21. Paragraph 19: Line 414-415: The authors mention that abrasion preferentially removes\nthe fragments within agglutinates. How would this affect the Ir value if the vesicles are inherent\nto melt, not the fragments.\nResponse: Thanks for the insightful comment. We apologize that the phrasing in the previous\nmanuscript may have caused confusion regarding how abrasion and fragmentation affects Ir\nvalues.\nTo clarify, vesicles within agglutinates can be broadly categorized into three groups:\n(i) vesicles formed within the glassy matrix\n(ii) vesicles located at interfaces between the glassy matrix and incorporated fragments\n(iii) vesicles contained within fragments themselves\nThe third category is volumetrically minor—our Supplementary Figure 6 shows that most\nbasalt fragments in CE5 and CE6 have void ratios <2%, and previous studies (Goguen et al.,\n2024) similarly report very low particle porosity (<0.3%) in Apollo 11 and Apollo 14 soils.\nThus, the vast majority of vesicles in agglutinates belong to categories (i) and (ii).\nAbrasion during gardening preferentially removes the solid fragments embedded within\nagglutinates. This process exposes or truncates vesicles located at fragment-glass interfaces,\neffectively converting many of these irregular vesicles into open cavities that are no longer\ncounted toward the internal vesicle statistics used to calculate Ir. As a result, the surviving\nvesicles in bulk soil agglutinates are dominated by those formed within the glassy matrix,\nwhich tend to be more regular in morphology (as detailed in our response to Comment #9).\nThis mechanism likely explains why bulk soil agglutinates exhibit higher Ir values than their\nlarger counterparts.\nRevision: Revised Lines 440-443:\n“At the bulk soil scale, this process reduces the abundance of internal fragments, exposes or\nremoves irregular vesicles, and leaves behind agglutinates dominated by regular vesicles\nformed within the glassy matrix, thereby producing higher Ir values (Fig. 3a).”\n22. Do CE6 agglutinates generally contain more fragments? Why might this be the case?\nResponse: Both are good questions. We apologize that our previous phrasing may have\ninadvertently created the impression that CE6 agglutinates generally contain more fragments.\nThis is not the case.\nTo clarify, we distinguish two categories of agglutinates in our study:\n Larger (selected) agglutinates (>200 μm)\n Bulk soil agglutinates (<200 μm)\nFor larger agglutinates, the distributions of Ir values for CE5 and CE6 (Response Fig. 4a,\nupper panel shown below) show nearly identical means, indicating no systematic difference in\nfragment volumetric abundance between CE5 and CE6 for this size class.\nFor the bulk soil agglutinates, however, Response Figure 4b shows that CE6 exhibits a\ndistribution shifted toward smaller Ir. Because Ir inversely correlates with fragment abundance,\nthis pattern suggests that CE6 bulk soil agglutinates may contain more fragments on average.\nWe recognize that the original legend in Supplementary Fig. 7 (“CE5 agglutinates” vs. “CE6\nagglutinates”) was overly general and may have contributed to this misunderstanding. We have\ncorrected this to specify “bulk soil agglutinates.”\nRegarding the possible cause of this difference, we propose the following mechanism: for CE5,\nlarger agglutinates contain higher internal porosity and larger vesicles (as detailed in our\nresponse to Comment #20). Their fragments are connected by relatively limited amounts of\nglassy matrix. Under long-term gardening, shear stresses can induce breakage and preferential\nremoval of fragment-rich regions, leaving behind more glass-dominated structures with higher\nIr values. For CE6, larger agglutinates contain more densely packed fragments embedded in a\ncontinuous glass matrix. Abrasion dominates their modification, allowing internal fragment\nstructures to be better preserved. As a result, CE6 bulk soil agglutinates retain slightly more\nfragments, consistent with their lower Ir values.\nImportantly, we emphasize that this interpretation is inferred from Ir-based observations. While\nthis explanation is plausible, it is not the only possible scenario. Future work with larger\ndatasets is required to better quantify the relationship between Ir and Fr, as well as to test this\nproposed mechanism more rigorously.\nResponse Figure 4. a Void ratios and internal regular vesicles fraction (Ir) of selected larger\nagglutinates from CE5 (red) and CE6 (blue) lunar samples. The average Ir values of the two\ngroups are nearly identical, as indicated by vertical solid lines in the upper kernel density plot.\nPentagons indicate average Ir and void ratios of numerous bulk soil agglutinates for CE5 (red;\n235 particles) and CE6 (blue; 555 particles), where the higher Ir values observed in CE5\nagglutinates may be attributed to differences in internal structure. b Ir distribution of CE5 and\nCE6 bulk soil agglutinates.\nRevision: Revised Lines 443-447:\n“In contrast, CE6 larger agglutinates contain more densely packed fragments embedded in a\ncontinuous glass matrix, and their evolution is governed primarily by abrasion (Supplementary\nFig. 8e, f). These more strongly welded structures are more resistant to fragment loss, which\nmay help preserve internal fragments in bulk soil agglutinates and result in slightly lower Ir\nvalues compared with CE5 bulk soil agglutinates.”\nWe have also updated Supplementary Figure 7 to explicitly indicate that the statistics\nrepresent bulk soil agglutinates.\n23. Paragraph 20: Line 437-438: Generally, when correlating surface exposure proxies, be\nspecific as to which ones. In this context, what exactly was grain size fraction compared to.\nResponse: Good point and thanks for requesting this. After revisiting the relevant literature\n(McKay et al., 1974), we confirm that the correlation we referred to concerns the relationship\nbetween mean grain size (<1 cm) and the Is/FeO maturity index, which serves as the specific\nsurface exposure proxy in this context. We appreciate the reviewer’s suggestion, as our original\nphrasing did not make this sufficiently explicit.\nRevision: We have revised Lines 465-467 to clearly specify the surface exposure proxy used\nin the comparison:\n“For impact fragmentation, previous studies have shown a correlation between the Is/FeO value,\na surface exposure proxy, and mean particle size in regolith samples with particle sizes <1 cm\n(ref. 72).”\n24. Paragraph 21: Line 452-454: I would specify that this saturation affects the bulk size\nfraction, as opposed to the entirety of the regolith (i.e. all size fractions).\nResponse: Thanks for requesting this helpful clarification. You are absolutely right that our\ncurrent dataset does not allow us to demonstrate that morphological saturation applies to all\nsize fractions of the regolith. Our conclusions are constrained to the bulk soil fraction (20–200\nµm), which is the size range directly analyzed in this study. We appreciate the reviewer’s\nsuggestion and have revised the text accordingly to avoid overgeneralization.\nRevision: We have updated Lines 480-482 to explicitly limit the inference to the bulk soil\nfraction:\n“Moreover, the data imply that space weathering-induced morphological modification may\nhave reached a saturation state within the bulk soil fraction of the youngest and least weathered\nCE5 regolith46, with the onset of saturation thus occurring before ~2.2 Ma.”\n25. 73002 was found to be largely composed of highlands material with small amounts of\nbasaltic material found throughout, mostly concentrated near the top 3 cm (Neuman et al., 2025:\nRevealing the Moon's Taurus‐Littrow Landslide via Integrated Analysis of Pristine Apollo 17\nSoil Core 73001/2). I would clarify that 73002 is compositionally distinct from pure mare\nregolith and elaborate on this when making comparisons to Chang'e samples.\nResponse: Important and constructive comment. Indeed, Apollo 73002 is compositionally\ndistinct from pure mare regolith, as it contains a substantial proportion of highlands-derived\nmaterial with basaltic components concentrated mainly in the uppermost few centimeters, as\npointed out by the reviewer with the timely reference (Neuman et al., 2025). This distinction\nis essential when comparing 73002 with the Chang’e-5 and Chang’e-6 mare soils.\nWe have now explicitly incorporated this clarification into the revised manuscript. At the same\ntime, we emphasize that despite these compositional differences, our morphometric results\nshow that large basalt particles (500–700 µm) from 73002 exhibit strong morphological\nsimilarity to those from CE5 and CE6. This agreement provides further support for our\ninference that the saturation state may persist even for larger particles in more strongly space-\nweathered soils, and even in regolith such as 73002 that includes more complex lithologic\nmixtures.\nRevision: As suggested, we have elaborated on these points (Lines 482–486):\n“To further test this potentially compelling hypothesis, we examined Apollo 17 sample 73002,\ncollected from the upper ~20 cm of the Light Mantle landslide deposit at the base of the South\nMassif in the Taurus–Littrow Valley, a site at the mare-highland boundary characterized by a\nsubstantial proportion of highlands material and basalts of multiple ages90,100.”\nAs well as in revised Lines 492–495:\n“Morphometric results show that the saturation state may persist even for larger particles in\nmore strongly space-weathered soils, and even in 73002 regolith that is not pure mare material\nbut instead contains more complex lithologic mixtures (Supplementary Fig. 9).”\n26. In Morris et al., 2022, they found that the in situ reworking zone of 73002 had an upper\ndevelopment age of ~17 Ma. The 3.0 Ma age found in SEP track densities was the average of\nexposure ages from individual particles in the core, and not reflective of the overall time\nregolith had been subject to regolith gardening. Therefore, it would be more appropriate to\ncompare the maximum exposure ages of a soil to evaluate the saturation timescales of bulk soil\nmorphology.\nResponse: We sincerely thank the reviewer for this valuable clarification. We fully agree that\nmaximum exposure age, rather than average exposure age, provides a more appropriate basis\nfor evaluating the saturation timescales of bulk soil morphology.\nRegarding the ~17 Ma upper development age reported in McFadden et al. (2024), we carefully\ntraced its derivation. Morris (1978) established a relationship between the depth of the in-situ\nreworking zone and its development timescale using combined Is/FeO and 26Al profiles from\nApollo cores:\nDr 2.2\n\nt\n0.45\nwhere Dr represents the depth of the in-situ reworking zone (in cm), and t denotes the timescale\nfor its development. Based on an estimated reworking depth of ~8 cm reported by McFadden\net al. (2024), the corresponding in-situ reworking zone development timescale is calculated to\nbe ~17 Ma.\nHowever, this value is not equivalent to the maximum exposure age. As we discussed in our\nresponse to Comment #16, the maximum exposure age of ~7.0 Ma (McFadden et al., 2024) is\nmore directly tied to the 73002 uppermost regolith that experiences the particle reshaping\nprocesses relevant to morphological saturation.\nHowever, the depth of the in-situ reworking zone for 73002 remains somewhat\nuncertain, e.g., ~8 cm in McFadden et al. (2024) vs. ~9.5 cm in Morris et al. (2022). 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Decision letter