Widespread ilmenite contributions to the surface water cycle in lunar Procellarum KREEP Terrane

Domain: earth_science
Published: 2025-08-15
Editorial outcome: minor_revision

Abstract
The role of widespread ilmenite in lunar mare regions in the abundance and diurnal variations of surficial OH/H2O remains controversial. Here, we report the water content and hydrogen isotopes in the rims of lunar ilmenites from Chang’e-5 soil samples using an ion microprobe. Ilmenite rims exhibit higher water contents (~730−3,700 ppm) and lower δD values (−884 to −482‰) than that of the lunar mantle, indicating a predominantly endogenic origin from solar-wind (SW) implantation. Our data further reveal that although ilmenite and silicate minerals overlap in the δD vs. H2O diagram, almost all ilmenites fall above those of silicates with SW-like δD values. This signature is consistent with the drastic difference in vesicle abundance between ilmenite and silicate minerals. Thus, the lower water content in ilmenite rims most likely reflects a faster dynamic equilibrium between SW-hydrogen implantation and outgassing than in other silicates. These findings suggest that ilmenite may play a critical role in the surface water cycle of lunar maria within the Procellarum KREEP Terrane. This is also crucial for assessing the in-situ resource utilization potential of the region, highlighting the need to reevaluate ilmenite as a viable resource for future lunar exploration.

Introduction
Water on the Moon is of paramount importance for understanding the formation and evolution of the lunar interior and for future in-situ resource utilization (ISRU)1,2. The abundance of water in the lunar mantle provides constraints on the Moon-forming giant impact3, the subsequent crystallization of the lunar magma ocean4, and the longevity of volcanism on the Moon5. By contrast, the lunar surface is covered by a layer of fine-grained regolith with a thickness up to ~12-15 meters6. This regolith layer can contain abundant water7–9, and the lunar regolith could contain water ice, which are essential resources for building a lunar base and long-term life support10. The ISRU will drastically reduce the reliance on supplies from Earth. The water resources can be further used to produce oxygen/hydrogen for fuel11. Infrared spectrometry data provide direct evidence for the presence of OH/H2O on the lunar surface12–14, with both species (OH and H2O) commonly referred to as water. This finding was further confirmed by the Stratospheric Observatory for Infrared Astronomy (SOFIA), which detected molecular water at high latitudes on the Moon15. The laboratory analyses of Apollo and Chang’e-5 (CE5) lunar samples suggested that both indigenous source16–19 and exogenous sources such as solar wind (SW) implantation7–9,20, carbonaceous chondrites21,22, and comets23 have contributed to the distribution of OH/H2O on the lunar surface. On a global scale, the distribution of OH/H2O on the lunar surface, determined from the Moon Mineralogy Mapper (M3) reflectance spectra, is observed to correlate with latitude or temperature, with higher concentrations at higher latitudes24. This phenomenon was interpreted to be mainly caused by SW implantation, which is later verified by laboratory analyses of returned lunar samples. The data show that the OH/H2O concentrations in the space-weathered rims of CE5 silicate grains from the middle latitude are indeed higher than the values measured in Apollo soil samples from low latitudes8,9,25, supporting the latitude-dependent conclusion drawn from remote sensing data. On the other hand, the time-of-day-dependent variations in OH/H2O were also observed by spectral observations, which are also attributed to the SW implantation24,26,27. These implanted SW-H is considered to combine with O to form OH or H2O, which is subsequently lost from soil grains through temperature-dependent diffusion and photolysis26,28.

However, it remains controversial whether the variations of OH/H2O content on the lunar surface depend on the chemical compositions. Some studies show that the low-Ti regions commonly have higher water contents than high-Ti mare basalts29,30. A relatively larger time-of-day-dependent variation of surficial OH/H2O in Ti-rich areas has been displayed by comparing with other low-Ti mare regions29,30. Other studies suggest that there are some areas with water anomalies in the mare regions of the Moon, but their TiO2 content does not differ significantly from the surrounding areas31. The lunar maria are mainly composed of plagioclase, pyroxene, olivine, and ilmenite, and the Ti-rich regions usually represent a high abundance of ilmenite32. However, previous studies on H isotopes in Apollo and CE5 lunar soil grains have focused only on silicate minerals, agglutinates, and glasses, but these different silicate phases do not show significant differences in the abundance of H (refs. 7–9,20,23,25,33,34). By contrast, only a few studies were taken to investigate the water contents and H isotopes in the returned lunar ilmenite grains’ rims34–36. One ilmenite from sample 71501, measured for H isotopes and H-depth profiles with SIMS, exhibited slight enrichment in deuterium (D), indicating a non-solar H component36. The atom probe tomography measurements on an Apollo 17 ilmenite revealed H-bearing molecular ions, such as OH+ and H2O+, in the space-weathered rims, which are likely sourced from the SW35. The latest nanoscale secondary ion mass spectrometry (NanoSIMS) analyses on two ilmenite grains from the CE5 drill core samples show one ilmenite has a δD value of 110 ± 1172‰ (2σ), and the other displays a SW-like H isotopic ratio (δD = −884 ± 140‰; 2σ)34. To our knowledge, this is the only reported ilmenite grain that has a SW-like H isotopic composition. However, they have not compared the oxides and silicate phases because the data on ilmenite are limited. In addition, the microstructures on these ilmenites have not been analyzed. Further systematic analysis of ilmenite is therefore required to understand the interaction between SW implantation and ilmenite and the fate of the hydration components.

In this study, we conduct a comprehensive analysis to investigate the water contents, hydrogen isotopes, and microstructures in the space-weathered lunar ilmenite grains returned by the CE5 mission from northern Oceanus Procellarum37. The CE5 lunar soil is submature-mature based on the characteristics of the in-situ spectrum and grain size distribution38,39 and consists mainly (>90%) of local mare basalt40,41. The CE5 lunar soils consist of ~4–5% ilmenite by volume, indicative of a low-Ti basalt32. We aim to (i) measure and characterize the water distribution in the rims of lunar ilmenite, (ii) compare the differences between ilmenite and silicate minerals, and (iii) discuss the effects of ilmenite on the water cycle in the Procellarum KREEP Terrane (PKT).

Results
The measured H and D concentrations were used to estimate the water contents and isotopic compositions of ilmenite rims. Moreover, the H isotopes can be used to trace the origins of lunar surface water. After NanoSIMS analysis, the microstructures were studied to understand the effects of space weathering on oxides under different conditions and to compare with the silicate minerals.

Water content and hydrogen isotopic composition of ilmenite rims
The hydrogen concentrations with depth and isotopes in CE5 ilmenite grains were measured using NanoSIMS (Supplementary Fig. 1). It should be noted that the NanoSIMS used here cannot differentiate between H species such as H2, OH, H2O, or other compounds. Conventionally, the H concentrations measured with NanoSIMS are converted to H2O equivalents by weight for discussion. To characterize the depth-profiling data, we determined the depths of NanoSIMS-generated craters in ilmenite grains from two polished basalts (Supplementary Fig. 2) and calculated the sputtering rate to be 0.84 ± 0.08 nm/cycle (1SD), which is slightly lower than the sputtering rate in silicate minerals (~1 nm/cycle)8.

The results show that, similar to previously reported CE5 silicate minerals, the water content in ilmenite grains decreases from the surface towards inside, but the average water content of which is about 2–4 times lower than that in silicate phases (Fig. 1). The maximum water contents within the 100-nm surface layer vary mainly from approximate 1500–5000 ppm, with only two ilmenite grains show slightly higher water contents of 5600 and 6400 ppm, respectively (Fig. 1a).

Based on the simultaneously measured D and H abundances, we calculated the δD (defined as [(D/H)sample/(D/H)SMOW − 1] × 1000, see “Methods” section) in surface layers of ilmenite (~100 nm depth). In addition, we also corrected the water contents and δD values using a cosmic-ray exposure (CRE) age of 50 Myr previously used by Hu et al.5 and He et al.7 and the reported D and H production rate by cosmic rays42. This correction has a negligible effect on the water content (less than 1 ppm; Supplementary Table 1). In addition, since a ~10-nm-thick vapor-deposited layer is sometimes observed on ilmenite surfaces (Supplementary Fig. 3), we also subtracted the water content associated with this layer. The results show that the average water content at 10–100 nm depth decreased by ~10% compared to the unsubtracted data, while the H isotopic composition remained consistent within the uncertainty (Supplementary Table 1). Therefore, the reported water contents in the 0–100 nm layer likely represent the upper limit of the ilmenite rims. These values, along with their corresponding H isotopes, were still used in the subsequent discussions. With the exception of one ilmenite from CE5 core soils with mantle-like H isotopes34, ilmenite grains have corrected δD values that range from −884 ± 70‰ to −482 ± 38‰ (1σ) (Supplementary Table 1). Interestingly, almost all δD values from the thirteen ilmenite grains are consistently lower than that of the lunar mantle, which represents the endmember purely from endogenic hydrogen. In contrast, comparative analysis of SW-derived rims (δD < −400‰) reveals that while the δD values for ilmenites fall within the range of silicate minerals (olivine, pyroxene, and plagioclase), they systematically exceed those of ~60% of silicate minerals, which display much lower δD signatures (Fig. 2). This difference becomes more evident when combined with their water contents (as shown in Fig. 2), resulting in two distinct clusters in the δD vs. H2O content diagram.

Abundant npFe0 particles and vesicles in the rims of ilmenite
Space-weathered rims on four representative ilmenites (S404-09, S404-14, S404-21, and S503-04) are enriched in nanophase metallic iron (npFe0) particles and vesicles (Fig. 3 and Supplementary Fig. 4), consistent with the characteristics of previously studied space-weathered lunar ilmenites43–46. In sample S404-09, the SW-damaged rim, about 150−165 nm thick, is abundant in elongate npFe0 particles averaging ~13.01 ± 6.01 nm in length and ~5.94 ± 2.23 nm in width (N=108, number of npFe0 particles), which are randomly distributed within the top ~70 nm surface layer (Fig. 3a–c). Some large vesicles (~10 nm in diameter) are also present in the rim of S404-09 (Fig. 3b, c).

In sample S404-21, the space-weathered rim is about 85–100 nm thick, and the npFe0 particles in this grain are relatively small and mainly distributed in the uppermost ~40 nm layer. These npFe0 particles have an average length of ~9.79 ± 5.61 nm and an average width of ~4.92 ± 1.86 nm (N=45) (Fig. 3f, g). Large spherical or subspherical vesicles, with an average diameter of 9.14 ± 5.25 nm (N=71), are observed on the uppermost layer (within the top 40 nm) of this grain. Similarly, the other two ilmenites (S404-14 and S503-04) also have a >100 nm-thick SW-damaged zone and contain a ~50-nm-thick upper layer enriched in npFe0 and planar defects (Supplementary Fig. 4). These characteristics indicate a degree of deeply damaged rim based on the classification by Guo et al.46. In addition, we observed a Si-rich vapor-deposited layer (~10–15 nm thick) on two ilmenite grains (Supplementary Fig. 3), consistent with previous results from Apollo and Chang’e-5 lunar soils43,44,46.

Numerous slender and nano-scaled vesicles are also present in the subsurface layer (~40–100 nm) (Fig. 3g). EELS spectra acquired from the substrate ilmenite, npFe0, and the large vesicle in sample S404-21 did not show a diagnostic peak of the H K-edge at the low energy-loss of about 13 eV, while a helium signal (~22 eV) was detected in both vesicles and npFe0 particles within the space-weathered rim (Fig. 3i). By contrast, no helium was detected in the vesicles of sample S404-09, probably due to the perforation of the vesicles by ion beam cutting during sample preparation or electron beam irradiation (Fig. 3e). It should be noted that the space-weathered rims of silicate minerals (olivine and pyroxene) contain npFe0 particles but no vesicles (Supplementary Figs. 5 and 6). Neither hydrogen nor helium signals were detected in the weathered rims of pyroxene and olivine (Supplementary Figs. 5d and 6d). When integrated with previous results from Gu et al.44 and Guo et al.46, the data reveal clear microstructural distinctions between ilmenite and silicate minerals, with ilmenite exhibiting significantly more vesicles and npFe0 particles.

Solar flare tracks are observed in all analyzed minerals, including ilmenite, olivine and pyroxene (Supplementary Fig. 7). The track densities, estimated by counting the tracks within the uppermost zone of the four ilmenite grains, were 6.93 × 10^10 (S404-09), 5.02 × 10^9 (S404-14), 1.32 × 10^9 (S404-21), and 2.19 × 10^10 (S503-04) tracks/cm2, respectively, which are comparable to those of silicate minerals (5.3 × 10^9–8.5 × 10^10; Supplementary Table 2). Since there is no experimentally-determined track production rate for ilmenite and pyroxene, we assumed that both have the same rate as olivine and anorthite reported by Keller et al.47. The estimated ages are provided in Supplementary Table 2. It should be noted that as ilmenite is more resistant to amorphization by ion irradiation than silicates due to its close-packed crystal structure48, the actual solar-flare-track exposure time for ilmenite may be similar to or even longer than our estimated values.

Origin of water and H isotopes in ilmenite rims
The corrected δD values of the uppermost H-enriched zone (~100 nm) in our ilmenites are significantly lower than those of the lunar mantle (Fig. 2). To produce such a signature, a low-δD source is required. The reservoirs in the Solar System such as water-rich carbonaceous chondrites (−200 to +800‰)21,49,50 and comets (0 to +2000‰)51,52 generally have positive δD values10, while only the SW is measured to have extremely low δD value (~−997‰; ref. 53). From this standpoint, it suggests that SW is probably the main cause for the H isotopic ratios in ilmenites. This interpretation is also supported by the presence of abundant npFe0 particles and vesicles in the weathered rims of ilmenites, which were thought to be the diagnostic characteristics of SW irradiation46. In fact, the low-δD values (<−400‰) found in CE5 and Apollo silicate minerals and glasses have previously been explained by the contribution from SW implantation8,9,33,34. The ilmenite, whose H isotopes are similar to those of the lunar mantle, was probably only exposed to the SW for a very short time or not at all34. Therefore, we conclude that SW irradiation is a critical source for lunar surface water.

More importantly, we note that although ilmenite and silicate minerals overlap in the δD vs. H2O plot, almost all ilmenites fall above those of silicates with SW-like δD values (Fig. 2). The observed difference is further evidenced by their distinct microstructural characteristics (as detailed in the next section). These distinct H isotopic signatures in ilmenite grains likely stem from varying exposure durations to SW irradiation and/or differential outgassing of H-bearing species after implantation34. However, the exposure time estimated based on solar flare tracks for ilmenites (~0.03–1.57 Myr; Supplementary Table 2) is comparable to those of the silicate minerals (~0.12–1.95 Myr; ref. 8 and this study). Furthermore, we find that both the water content and δD values of the analyzed ilmenite rims remain almost unchanged with increasing exposure time (Supplementary Fig. 8). Specifically, the water contents in ilmenite rims are consistently lower and the δD values are consistently higher, approximately paralleling those of the silicate minerals (Supplementary Fig. 8). These observations therefore rule out the exposure time as the main cause. In contrast, an early study performed ion irradiation experiments on ilmenites and silicate materials (plagioclase and simulated glass) and found that ilmenite stored about twice as much deuterium as other target materials under the same condition and exhibited a higher loss of implanted ions than other target materials after heating54. These results suggest ilmenite grains can contain comparable concentrations of SW-implanted hydrogen to silicates, leading us to infer that lunar ilmenite rims could form similar water contents to those in silicate materials.

Our further modeling shows that the degassing of H-bearing phases from a silicate-like endmember with low δD and high water content could significantly decrease the water content but increase the H isotopes (Fig. 2). Most of the ilmenite data fall within the range defined by the degassing in the form H2, H2O and pure H (Fig. 2). Given these observations, why do ilmenite soil grains have systematically lower water content together with heavier H isotopes in their topmost ~100-nm-depth layer compared to most of the silicate minerals? We speculate that this difference is most likely related to the nature of the minerals, meaning that ilmenite water may not be retained as readily in the weathered rims as in silicate minerals.

Microstructural differences between ilmenite and silicate modulate the formation and retention of SW-derived water
The observed differences between ilmenite and silicate minerals in terms of H isotopes and water contents closely correlate with their crystal structures. Ilmenite, with lower binding energies for Fe-O (407) and Ti-O (667) than Si-O (800) in silicates (numbers in brackets are the bond dissociation energies in kJ/mol)55, can be more easily reduced with H in experimental settings to produce water more efficiently and yield a metal product of iron [FeTiO3(s) + H2(g) ⇌ Fe(s) + TiO2(s) + H2O(g)]56–58. This mineral is also a potential material for the effective extraction of oxygen and has been extensively studied for ISRU purposes59. TEM measurements reveal abundant npFe0 particles in the space-weathered rims of ilmenites (Fig. 3), indicating that a large amount of SW-implanted H should be present to involve the formation of npFe0 particles. In addition, vesicles are one of the features that can form when a mineral grain is exposed to the space weathering environment, and it has been suggested that the SW is particularly important in the development of these features, implanting H and He ions that concentrate in these void spaces43,60. The trapped H may react with O to form OH and H2O. Recently, Kling et al.61,62 detected H, OH, and H2O in the weathered rims of Apollo olivine and clinopyroxene grains based on the results of EELS and APT analyses, and they suggested that vesicles and other complex microstructural features like defects may be sites where water can be trapped and stored in higher concentrations. Similarly, Bradley et al.60 detected molecular H2O in the vesicles of an interplanetary dust particle (IDP) and in experimentally irradiated samples using the EELS detector. From this standpoint, we infer that the abundant and large vesicles in the weathered rims of ilmenites should have generated more water compared to the silicate minerals. A recent study also proposed that ilmenite is a highly efficient material for producing water63. However, the solar component in the space-weathered rims of ilmenite detected by EELS and NanoSIMS is predominantly helium, with rare H species (Fig. 3), consistent with the results from other previously reported lunar ilmenites35,43,45. Combined with the H isotopic results, we suggest that the SW-induced OH/H2O in the space-weathered rim of ilmenite is more readily lost. A schematic model is then proposed to illustrate the behaviors of SW-implanted H in ilmenite rims, in comparison with silicate minerals (Fig. 4).

Multiple factors may trigger the loss of OH/H2O in weathered rims of ilmenites. The water formed in the weathered rims could be released as the temperature rises26,64–66, with the lunar surface temperature of up to ~380 K (ref. 67). When the gas pressure (e.g., noble gases, H2, H2O) within vesicles exceeds their percolation threshold, as shown in the experimental study on H-irradiation of alumina68, this feature could accelerate the outgassing of H-containing species into space. Losses of volatile gases can also occur through micrometeorite impacts and SW sputtering. Thus, although ilmenite grains could form more water, they struggle to efficiently retain it. Future high-energy ion implantation experiments on oxide and silicate minerals can test our hypothesis and may help interpret the remote sensing observations.

Implications for the diurnal variation of water in the lunar mare regions
The new data of H isotopes and water contents measured in the rims of lunar ilmenites have important implications for understanding the lunar surface water cycle. Previous studies using remote sensing data showed diurnal OH/H2O variations across most of the lunar surface, particularly for the lunar mare regions, such as in Mare Crisium, Mare Serenitatis, Mare Nectaris, Mare Tranquillitatis, and western Oceanus Procellarum29,30. They further found that the high-Ti regions generally have a weaker OH/H2O absorption band than the low-Ti basalts, especially at lunar midday, and that high-Ti mare areas show much stronger variations in 3-µm band depth throughout the lunar day compared to low-Ti maria29,30. The amplitude of OH/H2O adsorption bands in the lunar highlands, which have even lower TiO2 content, is generally lower. Though the mechanism for these variations remains uncertain, it has been proposed that TiO2-rich regions tend to adsorb SW-implanted H primarily into low-energy binding states, facilitating a periodically occurring time-of-day-dependent desorption and reabsorption of OH/H2O (ref. 29).

As shown in Fig. 5, we compared the maps of mineral abundances (olivine, clinopyroxene, plagioclase, and TiO2 content) with the IBD 3µm maps over a lunar day in the PKT. The IBD 3µm index quantifies the amount of OH/H2O, derived from the M3 spectral data calibrated in a refined way by a newly developed physical thermal model69. Our extended comparison of ten regions in the PKT reveal that the diurnal variations in IBD 3µm (morning-noon and evening-noon) positively correlate with both TiO2 and clinopyroxene, but negatively with plagioclase (Fig. 5g). The inverse relationship between plagioclase and water diurnal variation indicates that the presence of plagioclase likely inhibits water diurnal variation, which aligns with experimental results: experiments on SW implantation suggested that plagioclase can capture more H+ than olivine or pyroxene to form OH/H2O65,70. By contrast, water diurnal variations show positive correlations with both TiO2 (predominantly from ilmenites as shown in Supplementary Fig. 9) and clinopyroxene, indicating that both minerals probably influence water variations. Since clinopyroxene generally has comparable water concentrations to olivine and plagioclase in lunar soils71, we infer that ilmenite—the dominant Ti-rich oxide, constituting up to 20% of lunar maria32—may significantly contribute to the observed diurnal water variations in the PKT. However, factors such as latitude and regolith maturity have also been shown to influence lunar surface water cycling24, which must be considered when interpreting localized diurnal water variations.

Our laboratory analyses of CE5 ilmenite demonstrate that these minerals can form additional OH/H2O through SW implantation. However, most of this hydrogen is subsequently removed via diffusion/thermal evaporation and photolysis. This observation likely supports the view that SW-induced hydrogen is adsorbed into binding states of low energy29, though future ilmenite-specific experiments are required to investigate OH/H2O formation and release mechanisms. In other words, compared to other candidate water reservoirs such as lunar glasses and plagioclases7,65, our findings suggest that the widespread ilmenites, particularly in lunar maria, do not serve as potential water reservoirs, despite their well-documented capacity to retain He and noble gases72,73. Therefore, our results shed light on the dynamic interactions between SW and oxide minerals and underscore the imperative to reassess ilmenite’s viability as a strategic resource for future lunar exploration.

Methods
Samples
Two CE5 lunar soil fractions (CE5C1000YJFM00404 and CE5C0600YJFM00503; hereafter abbreviated as S404 and S503, respectively) from the China National Space Administration were used in this study. The preservation and preparation of samples were carried out in a Class 1000 clean laboratory under a bench at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Seven ilmenite grains from sample S503 and four ilmenite grains from sample S404 were manually selected under a binocular microscope. These ilmenite grains have the size ranging from 112 × 114 µm to 368 × 370 µm (Supplementary Fig. 1). For comparison, two silicate grains were also collected from sample S404 and used for microstructural analysis: one pyroxene grain with a size of 282 µm × 326 µm and one olivine grain with a size of 274 µm × 442 µm (Supplementary Fig. 1). The grains from different soil fractions were pressed into two indium mounts, with their relatively flat sides facing upwards. Furthermore, we selected two basalt clasts from sample S404, and then embedded them in epoxy resin and polished them. The ilmenites from these two basalts were used to determine the sputtering rate of NanoSIMS analysis.

Scanning electron microscopy
The surface tomography of the ilmenite grains from sample S503 was performed with a Nova NanoSEM 450, and the grains from sample S404 were observed using a Thermo Scientific Apreo S scanning electron microscope. Both instruments are located at IGGCAS. There are no differences between these two devices when it comes to observing sample surface tomography. Sample mounts were coated with a ca. 10-nm-thick layer of gold before being placed in the instrumental chamber. We used a CBS detector for both instruments to capture the surface topography. The working parameters were an accelerating voltage of 10–15 kV and an electron beam current of 1.6 nA, as shown at the bottom of the BSE images (Supplementary Fig. 1). The optimal working distance was 9–10 mm.

NanoSIMS analysis
D/H ratios and H concentrations of the standards, including San Carlos olivine, SWIFT MORB glass, Durango apatite, Kovdor apatite, and CE5 lunar soil grains, were measured by a CAMECA NanoSIMS 50L installed at IGGCAS. The vacuum in the analysis chamber and the conditions for instrumental analysis are largely similar to our previous studies8,34. During the sample measurements, the vacuum in the analysis chamber was better than 3 × 10−10 torr. We first found a flat area using a low Cs+ ion beam. Then, each 7 µm × 7 µm area was rastered by a Cs+ primary beam of ~500 pA with a beam size of 0.8–1 µm. The secondary anions 1H−, 2H−, and 18O− were simultaneously collected by electron multipliers. An electron gun was applied to compensate for the sample surface charge. The counting time was set to 0.541 s/cycle, and the measurements on the standards and samples consisted of 500–1000 cycles.

Two ilmenites from CE5 basalt clasts were used to determine the sputtering rate. Using an atomic force microscope, we determined the depth of the craters (Supplementary Fig. 2) and estimated the average sputtering rate for ilmenite (Supplementary Table 3).

Terrestrial San Carlos olivine grains were used to monitor the background and the potential surface contamination (Supplementary Fig. 10). The 90% of oxygen intensity, when stable, was used as the starting position for the depth-profiling surface (Supplementary Fig. 10). Then, the H counts of samples were calculated by subtracting the H counts of San Carlos olivine. The H counts on the surface of San Carlos olivine is less than 6% of that of the ilmenite soil grains (Supplementary Fig. 11). In addition, the H counts were converted to H2O content using the corrected-H/O ratios multiplied by the slope of the calibration line (Supplementary Fig. 12a). The calibration line was determined from measuring the standards San Carlos olivine (H2O = 1.4 ppm)74, SWIFT MORB glass (H2O = 0.258 wt.%, δD = −73 ± 2‰)75, Durango apatite (H2O = 0.0478 wt.%, δD = −120 ± 5‰)23, and Kovdor apatite (H2O = 0.98 ± 0.07 wt.%, δD = −66 ± 21‰)76. Corrections for instrumental mass fractionation (IMF) on D/H ratios were performed on the Kovdor apatite standard using αIMF = (D/H)measured / (D/H)recommended (Supplementary Fig. 12b). We calculated the D/H ratios in the uppermost ~100 nm (~120 cycles), since D counts of the ilmenite rims are too low to determine. The H isotopes are expressed in delta notation: δD (‰) = [(D/H)sample / (D/H)SMOW − 1] × 1000, where SMOW refers to the Vienna Standard Mean Ocean Water, which has a D/H ratio of 1.5576 ± 0.0001 × 10−4 (ref. 77). The δD values are reported with 1σ, which is derived from counting statistics. More technical details can be traced in Xu et al.8. The results are given in Supplementary Table 1.

Raman spectroscopic analysis
Mineral phases were identified using a LabRam HR Raman microscope at IGGCAS. A solid-state continuous-wave laser emitting at 532 nm was connected to the instrument via an optical fiber. A laser power of ~1 mW was focused on the bottom of the craters produced by NanoSIMS, and we observed the craters using an optical microscope with a ×50 magnification lens. We used a piece of single-crystal silicon as the reference to calibrate the Raman shifts. The Raman spectra from 200 to 1200 cm−1 were shown, and the monospectral collection time was ~10 s and the total accumulation time was 30 s. The diagnostic spectra made it possible to identify different phases (Supplementary Fig. 13).

Transmission electron microscopy (TEM)
The microstructures of space-weathered rims of one olivine, one pyroxene, and four ilmenite grains were examined by TEM. The ultrathin foils were cut and thinned using a Zeiss Auriga Compact focused ion beam (FIB) microscopy with a high voltage range of 5–30 kV and a beam current of 100 pA–2 nA. The slices are ~100 nm in thickness, ~5 µm in width, and ~10 µm in length. These ultrathin foils were observed using a JEOL JEM-2100 TEM. High-resolution TEM imaging (HRTEM) and selected area electron diffraction (SAED) were used to investigate crystallographic structures.

The foils were cleaned with Ar-plasma for 5 min and then analyzed using a scanning transmission electron microscope (STEM)78. High-angle annular dark-field (HAADF) imaging, electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS) analyses were performed using a FEI Titan Cubed Themis G2 300 spherical aberration-corrected STEM at Tianjin University of Technology. This TEM is equipped with a field emission gun (FEG) and integrated four Super-EDS detectors, which operated at 300 kV and ~90 pA with a convergent semiangle of 21.4 mrad. To detect the helium and H species, EELS acquisitions were performed with a collection semiangle of 56.5 mrad. The energy resolution for EELS is ~0.4 eV, as determined by the full-width at half maximum (FWHM) of the zero-loss peak (ZLP). Gatan Digital Micrograph software was used to perform peak alignment based on shifts in the ZLP, which was used to compensate for systematic energy drift during the test.

Calibration of spallation-produced D and H
The NanoSIMS-measured D/H ratios were corrected by the effects of cosmic-ray-induced spallation, with a D production rate of 2.17 × 10−12 mol/g/Myr (ref. 42) and an H production rate of 4 × 10−10 mol/g/Myr (ref. 79). In this work, we reported the corrected water contents and δD values of CE5 soil grains assuming a cosmic-ray exposure age of 50 Myr, which was adopted from a previous work on water content of CE5 basalts5,7. According to our estimates, the influence of spallation on water content is negligible when compared with the high water content on the surfaces of the soil grains.

Outgassing model
The SW-implanted H in the mineral rims can be lost by diffusion and/or thermal evaporation, which can be modeled by using the Rayleigh fractionation equation, expressed as R = R0 × f^((M1/M2)−1). Here, R and R0 represent the final and initial D/H ratio, respectively; f denotes the fraction of H in the residual phase; M1 and M2 are the masses of the degassed phase. Two scenarios proposed by Sharp et al.80 were considered here to model the changes in H concentrations and isotopes. It is assumed that the degassing of H-containing phases occurs in the form of H2 (M1 = 2.016 for H2 and M2 = 3.022 for HD), and H2O (M1 = 18.015 for H2O and M2 = 19.021 for HDO)80. In addition, a special case was also modelled in which we only consider the loss of H-bearing phase in the form of pure H, but do not include D. Based on the reported values for silicate minerals, the initial endmember is set to be 12,000 ppm for H2O and −950‰ for δD.

Data availability
All data generated or analyzed during this study are included in this published article and supplementary information files. The data generated in this study have been deposited in the Figshare database under accession code (https://doi.org/10.6084/m9.figshare.27094405). Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Widespread ilmenite contributions to the surface water cycle in lunar Procellarum KREEP Terrane” submitted to Nature Communications investigates two ilmenite grains from returned lunar samples with respect to water content, hydrogen isotope abundances, sputtering rate, npFe abundance and distribution, and helium in vesicles. Several laboratory methods are used to investigate the samples, including NanoSIMS, SEM, Raman spectroscopy, and TEM. This paper offers valuable insights into lunar resource utilization and the dynamics of the diurnal volatile cycle, making it highly relevant for future lunar missions. The methodology is appropriate and clearly presented, and the language and writing are consistently adequate. While the conclusions are supported by the data, the authors could strengthen the paper by more effectively connecting the key ideas and themes to create a more cohesive narrative. Specific suggestions for improvement are detailed below.\nAbstract:\nThe abstract requires revision to provide a more comprehensive and engaging summary of the study.\nMotivation: Explicitly state why ilmenite grains were chosen for this investigation (e.g., their abundance, potential role in water retention, relevance to ISRU).\nClarity: Define δD for a broader audience, explaining that it represents the ratio of deuterium to hydrogen in a sample compared to a standard.\nConsistency: Use either δD or D/H consistently throughout the abstract for clarity. δD is generally preferred as it provides a more readily interpretable value.\nISRU Context: Instead of abruptly introducing in-situ resource utilization, rephrase to emphasize the connection between the findings and the resource potential of lunar ilmenite. For example: \"...of lunar maria, which is crucial for assessing the in-situ resource utilization (ISRU) potential of the region. These results necessitate a reevaluation of ilmenite as a potential resource for future lunar missions.\"\nIntroduction:\nLine 36-37: I agree, but these two themes (lunar interior, ISRU) are never discussed in the Introduction. I suggest adding a short paragraph for each in the introduction.\nLine 48-50: Citation needed.\nLine 55-57: The loss processes are usually assumed to be diffusive loss and photolysis (see e.g., Farell et al., 2015; Grumpe et al., 2019). Importantly, micrometeoroid impacts are therein also considered to be a source process for the production/delivery of hydroxyl. Furthermore, “geological processes”, at least in my mind, also implies geological time scales, while these might be processes that happen over a lunar day. I would recommend reworking this sentence.\nLine 66ff: However, previous studies … focused only on …\nLine 69-70: Please cite the few studies directly here as well.\nLine 78: Highlight the improvements of this study compared to Tian et al., 2024. In which way are the samples analyzed therein different?\nResults and Discussion:\nThis section would greatly benefit from a brief introduction to the subsection listing for each why these properties were measured, what are the implications.\nLine 100: New paragraph “The results show that, ….”\nLine 113-115: This is indeed interesting. But this sentence and the following one seem like they are dropped in results without context. I would split this sentence up to give more weight to each part of the sentence. “Interestingly, almost all deltaD values are lower than that of the lunar mantle, which represents the endmember purely from endogenic hydrogen. In contrast, the values for ilmenite are almost always higher compared to those of silicate minerals (…) that are representative of SW-derived reservoirs (Fig. 2).” And then a short sentence about the interpretation of the results should follow here as well to give context. Furthermore, the discussion in the \"Origin of water H isotopic variations in ilmenite\" section feels disconnected from these results. Ensure a smoother transition and a clearer link between the data and its interpretation.\nLine 116-117: Furthermore, …. (Fig. 2), which indicates …?\nLine 133: Line break after “grain” to separate the npFe and vesicle results more clearly.\nLine 140: In addition, the -> It should be noted that the\nLine 140: rims of silicate minerals\nLine 143: delete “of silicates”\nLine 155: To enhance clarity, it would be helpful to elaborate on the causal connection between low values in chang’e 5 samples and chondrites and comets. Please rephrase to make the implications for these samples clearer.\nLine 159-161: This conclusion should be supported by clearer interpretation of the results. Why do these results lead you to the conclusion? An additional sentence here to explain the interpretation is appropriate.\nLine 171: Olivine and pyroxenes are also silica.\nLine 187: Similarly, ilmenite can be used more effectively for the oxygen production (e.g., Schlueter et al., 2020). In this context it should briefly be mentioned why this is relevant and pick up the theme of in-situ resource utilization opened in the introduction.\nLine 210: And Grumpe et al. 2019\nMethods:\nLine 246: What are the grain sizes of the two grains selected?\nLine 249: Replace “On the other hand” with “furthermore” or something similar, because this is not really a contrast to the previous sentence.\nLine 254-255: What are the differences between the two instruments? Why were two different instruments used?\nLine 292: with a x50 magnification lens.\nLine 329-331: Citation needed.\nFigure 2: Clearly state which degassing model was used for which dashed line.\nFarrell, W. M., Hurley, D. M., & Zimmerman, M. I. (2015). Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects. Icarus, 255, 116-126.\nGrumpe, A., Wöhler, C., Berezhnoy, A. A., & Shevchenko, V. V. (2019). Time-of-day-dependent behavior of surficial lunar hydroxyl/water: Observations and modeling. Icarus, 321, 486-507.\nSchlüter, L., & Cowley, A. (2020). Review of techniques for In-Situ oxygen extraction on the moon. Planetary and Space Science, 181, 104753.", "reviewer_2": "Thank you for the opportunity to review the manuscript by Xu et al., entitled ‘Widespread ilmenite contributions to the surface water cycle in lunar Procellarum KREEP terrane’. The paper reports on the correlated study of ilmenite grains returned by the Chang’e 5 mission using TEM, EELS and NanoSIMS to suggest that ilmenite should contain more water (OH / H20) than measured and has thus produced and lost H in a manner that would support the water cycle on the Moon. While this is an interesting idea, currently the data do not appear to support this conclusion. We detail our main concerns with the paper, along with a number of line-by-line comments, below. We feel that the manuscript requires a significant rewrite and reinterpretation of the data and are not convinced that the main premise of the paper would be supported following this revision. We would, however, be happy to review a revised and reinterpreted manuscript, though ultimately the findings may not be of interest to the broad readership of Nature Communications.\nMajor Comments\nInterpretation of NanoSIMS data: The major premise of the paper revolves around the statement on lines 162 and 163: “More importantly, we note that ilmenite and silicate minerals form two distinct clusters in the δD vs. H2O content plot, with minimal overlap”. Unfortunately, we do not agree with this comment. Figure 2 clearly shows overlap between the new ilmenite data points and existing silicate phases and does not appear distinct from the range of values previously published from lunar regolith. As such, we do not believe from the presented data that the ilmenite grains in the Chang’e-5 returned sample represent a unique reservoir compared to other silicate phases within the sample. In addition, we would argue that the analysed grains do contain elevated water abundances (and lower D/H ratios) when compared to lunar mantle grains, suggesting enrichment through solar wind H delivery and implantation, which contradicts the authors key statements.\nPresentation of NanoSIMS data: It is challenging to ascertain the quality of the NanoSIMS data presented in supplementary table S1. The authors do not show any measurements on either the San Carlos olivine standard used for background correction, not any additional reference materials used to ascertain the quality of measured D/H. The authors should really report additional raw details, including 1H and 2H (counts and rate), 13C, and 16O (to understand corrections applied), along with the 2H/1H ratio and associated error. This is especially true for the San Carlos standard, as currently it is impossible to compare the authors data with literature values, further undermining the key take aways of the manuscript.\nInterpretation of solar flare tracks: The authors state on line 166 to line 168 that “solar flare tracks (0.02-0.18 Myr) indicate significant solar wind irradiation on ilmenite surfaces”. However, comparison of ilmenite data with that reported from pyroxene and olivine (Supplementary Table S2) suggest comparable, or significantly less, solar track formation than within the silicate minerals. This relationship is also clearly visualised in Figure S4, where ilmenite appears to contain fewer identified tracks. Assuming this evidence demonstrates that ilmenite has undergone less significant space weathering than silicate minerals, you would not expect significantly elevated production of OH within the altered rims. If the authors analysed 5 TEM images for each particle, the rest of this data should be shown, especially if it supports the authors arguments. Otherwise, it is challenging to accept that the grain population presented in the manuscript have undergone comparable degrees of space weathering, placing further uncertainty on the interpretation of the anticipated H producing in ilmenite.\nLine by line comments\nTitle: The authors highlight KREEP terrane in the title, but the idea is not really revisited in the paper?\nL41-45: long unbroken sentence with inadeuate referencing.\nL48: Studies on apatite in Change 5 basaltic clasts?\nL61-62: Include measurement of H and D/H from samples (e.g. low and high Ti bsaalts) not just spectral data.\nL67: The authors never specify regolith only for this section. As such, it is missing lots of data on hard rock samples from Apollo, etc.\nL84: in-situ should be italicised.\nL110-111: Correction of 50 ma age?!? Based on Hu et al? If so, cite. Otherwise this seems random.\nL148-149: Not comparable? Olivine appears much more densely packed based on the reported statistics and figures. And show all TEM images of the grains that the authors have collected, which may support the original interpretation.\nL162-163: Minimal overlap? Figure 2 shows more than 50% of ilmenite data overlaps with the silicate mineral bubble. Additionally, some literature olivine and plagioclase plot above the ilmenite field. I think this undermines the point of the paper (see major points above).\nL172: Not comparable exposure times. Olivine = 0.7 Ma, ilmenite c. 0 Ma, based on the authors own figures. This makes it challenging to compare the reported track density and ages.\nL177-178: All ilmenite do not have heaver isotopes or less water? The nuance of the data, beyond issues raised, should be more carefully discussed within the manuscript.\nL189-190: large amounts of water should be present. IT IS! Lunar mantle ilmenite have basically none\nL199: IDP?\nFig S4: Should show the other images. Otherwise, does not seem comparable.\nMany thanks,\nLee White & Moni Boruah", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "A point-by-point reply to the comments and questions of the reviewers is given below.\nReply to Reviewers\nReviewer #1 (Remarks to the Author):\nThe manuscript entitled “Widespread ilmenite contributions to the surface water cycle in lunar Procellarum KREEP Terrane” submitted to Nature Communications investigates two ilmenite grains from returned lunar samples with respect to water content, hydrogen isotope abundances, sputtering rate, npFe abundance and distribution, and helium in vesicles. Several laboratory methods are used to investigate the samples, including NanoSIMS, SEM, Raman spectroscopy, and TEM. This paper offers valuable insights into lunar resource utilization and the dynamics of the diurnal volatile cycle, making it highly relevant for future lunar missions. The methodology is appropriate and clearly presented, and the language and writing are consistently adequate. While the conclusions are supported by the data, the authors could strengthen the paper by more effectively connecting the key ideas and themes to create a more cohesive narrative. Specific suggestions for improvement are detailed below.\nAbstract:\nThe abstract requires revision to provide a more comprehensive and engaging summary of the study.\nMotivation: Explicitly state why ilmenite grains were chosen for this investigation (e.g., their abundance, potential role in water retention, relevance to ISRU).\nClarity: Define δD for a broader audience, explaining that it represents the ratio of deuterium to hydrogen in a sample compared to a standard.\nConsistency: Use either δD or D/H consistently throughout the abstract for clarity. δD is generally preferred as it provides a more readily interpretable value.\nISRU Context: Instead of abruptly introducing in-situ resource utilization, rephrase to emphasize the connection between the findings and the resource potential of lunar ilmenite. For example: \"...of lunar maria, which is crucial for assessing the in-situ resource utilization (ISRU) potential of the region. These results necessitate a reevaluation of ilmenite as a potential resource for future lunar missions.\"\nResponse: Thanks for your suggestion. In the revised version, we have rewritten the abstract. In addition, due to the word limit in the abstract, we did not define the δD in this section, but to present the definition in the Results and Methods section.\nIntroduction:\nLine 36-37: I agree, but these two themes (lunar interior, ISRU) are never discussed in the Introduction. I suggest adding a short paragraph for each in the introduction.\nResponse: As suggested, a short paragraph for lunar interior and ISRU has been added in the introduction, that is “The abundance of water in the lunar mantle provides constraints on the Moon-forming giant impact3, the subsequent crystallization of the lunar magma ocean4 and the longevity of volcanism on the Moon5. By contrast, the lunar surface is covered by a layer of fine-grained regolith with a thickness up to ~12-15 meters6. This regolith layer can contain abundant water7-9 and water ice, which are the essential resources for building a lunar base and for long-term life support10. The ISRU will drastically reduce the reliance of supplies from Earth. The water resources can be further used to produce oxygen/hydrogen for fuel11”.\nLine 48-50: Citation needed.\nResponse: The references of Xu et al., PNAS, 2022 and Zhou et al., NC, 2022 have been cited in the manuscript.\nLine 55-57: The loss processes are usually assumed to be diffusive loss and photolysis (see e.g., Farrell et al., 2015; Grumpe et al., 2019). Importantly, micrometeoroid impacts are therein also considered to be a source process for the production/delivery of hydroxyl. Furthermore, “geological processes”, at least in my mind, also implies geological time scales, while these might be processes that happen over a lunar day. I would recommend reworking this sentence.\nResponse: We agree with this comment. On the basis of your suggestion, the sentence here has been modified as follows: “These implanted SW-H is considered to combine with O to form OH or H O, which is subsequently lost from soil grains 2 through temperature-dependent diffusion and photolysis26,28”.\nLine 66ff: However, previous studies … focused only on …\nResponse: Modified as suggested.\nLine 69-70: Please cite the few studies directly here as well.\nResponse: As suggested, three studies (Hashizume et al., 2000 Science; Greer et al., 2020 MAPS; Tian et al., 2024 GRL) were added in the revised version. Following this sentence, the details of these three papers are present.\nLine 78: Highlight the improvements of this study compared to Tian et al., 2024. In which way are the samples analyzed therein different?\nResponse: Thanks for pointing this out. Based on your suggestion, we have rewritten these sentences to make this point clearer in the revised version: “To our knowledge, this is the only reported ilmenite grain that has a SW-like H isotopic composition. However, they have not compared the oxides and silicate phases because the data on ilmenite are limited. In addition, the microstructures on these ilmenites have not been analyzed. Further systematic analysis of ilmenite is therefore required to understand the interaction between SW implantation and ilmenite and the fate of the hydration components.\nIn this study, we conduct a comprehensive analysis to investigate the water contents, hydrogen isotopes, and microstructures in the space-weathered lunar ilmenite grains returned by the CE5 mission from northern Oceanus Procellarum”\nResults and Discussion:\nThis section would greatly benefit from a brief introduction to the subsection listing for each why these properties were measured, what are the implications.\nResponse: As suggested, a brief introduction of “The measured H and D concentrations were used to estimate the water contents and isotopic compositions of ilmenite rims. Moreover, the H isotopes can be used to trace the origins of lunar surface water. After NanoSIMS analysis, the microstructures were studied to understand the effects of space weathering on oxides under different conditions and to be compared with the silicate minerals.” were added in the revised version.\nLine 100: New paragraph “The results show that, ….”\nResponse: Revised.\nLine 113-115: This is indeed interesting. But this sentence and the following one seem like they are dropped in results without context. I would split this sentence up to give more weight to each part of the sentence. “Interestingly, almost all deltaD values are lower than that of the lunar mantle, which represents the endmember purely from endogenic hydrogen. In contrast, the values for ilmenite are almost always higher compared to those of silicate minerals (…) that are representative of SW-derived reservoirs (Fig. 2).” And then a short sentence about the interpretation of the results should follow here as well to give context.\nResponse: Based on your suggestion, we have split this sentence into two parts, i.e. “Interestingly, almost all δD values from the thirteen ilmenite grains are consistently lower than that of the lunar mantle, which represents the endmember purely from endogenic hydrogen. In contrast, although the δD values for ilmenites overlap with the silicate minerals (olivine, pyroxene and plagioclase), they are almost always higher compared to the low-δD (< -800‰) endmember of silicate minerals (Fig. 2). This difference becomes more evident when combined with their water contents (as shown in Fig. 2), resulting in two distinct clusters in the δD vs. H O content diagram. ” in the revised version. The interpretation of the results is 2 not added here, as we have split the “Results and Discussion” section into two parts in the revised version, namely “Results” and “Discussion” in the revised version. The interpretation of the results is added in the first subsection (“Origin of water and H isotopes in ilmenite rims”) of the “Discussion” part.\nFurthermore, the discussion in the \"Origin of water H isotopic variations in ilmenite\" section feels disconnected from these results. Ensure a smoother transition and a clearer link between the data and its interpretation.\nResponse: I apologize for the unclear title in the original manuscript. In fact, the first two subsections “Water content and hydrogen isotopic composition of ilmenite rims” and “Identification of helium in npFe0 and vesicles in the rims of ilmenite” are the only results, while the last three subsections are the discussion parts. In the revised version, we have split the “Results and Discussion” into two parts, i.e. “Results” and “Discussion”, to make it clearer.\nLine 116-117: Furthermore, …. (Fig. 2), which indicates …?\nResponse: Thanks for pointing this out. We have carefully considered this content and found that the expression may not be reasonable. Thus, we decided to remove this sentence in the revised version.\nLine 133: Line break after “grain” to separate the npFe and vesicle results more clearly.\nResponse: Modified as suggested.\nLine 140: In addition, the -> It should be noted that the\nResponse: Modified as suggested.\nLine 140: rims of silicate minerals\nResponse: Modified as suggested.\nLine 143: delete “of silicates”\nResponse: We removed this in the revised version.\nLine 155: To enhance clarity, it would be helpful to elaborate on the causal connection between low values in Chang’e 5 samples and chondrites and comets. Please rephrase to make the implications for these samples clearer.\nLine 159-161: This conclusion should be supported by clearer interpretation of the results. Why do these results lead you to the conclusion? An additional sentence here to explain the interpretation is appropriate.\nResponse: Based on your two suggestions, we have rewritten this paragraph to make the discussion more rigorous and clear. The new paragraph is “The corrected δD values of the uppermost H-enriched zone (~100 nm) in our ilmenites are significantly lower than that of the lunar mantle (Fig. 2). To produce such a signature, a low-δD source is required. The reservoirs in the Solar System such as water-rich carbonaceous chondrites (-200 to +800‰)21,50,51 and comets (0 to +2,000‰)52,53 generally have positive δD values10, while only the SW is measured to have extremely low δD value (~ -997‰; ref. 44). From this standpoint, it suggests that SW is probably the main cause for the H isotopic values in ilmenites. This interpretation is also supported by the presence of abundant npFe0 particles and vesicles in the weathered rims of ilmenites, which was thought to be the diagnostic characteristics of SW irradiation48. In fact, the low-δD values (< -400‰) found in CE5 and Apollo silicate minerals and glasses have previously been explained by the contribution from SW implantation8,9,33,34. The ilmenite, whose H isotopes are similar to those of the lunar mantle, was probably only exposed to the SW for a very short time or not at all34. Therefore, we conclude that SW irradiation is a critical source for lunar surface water.”.\nLine 171: Olivine and pyroxenes are also silica.\nResponse: Thanks for this minder. The “silica” here refers to “glass phase”. We have removed this in the revised version.\nLine 187: Similarly, ilmenite can be used more effectively for the oxygen production (e.g., Schlueter et al., 2020). In this context it should briefly be mentioned why this is relevant and pick up the theme of in-situ resource utilization opened in the introduction.\nResponse: As replied above, several sentences about ISRU have been added in the introduction. In this part, we have added one transition sentence, i.e., “This mineral is also a potential material for the effective extraction of oxygen and has been extensively studied for ISRU purposes”. (Line X-X)\nLine 210: And Grumpe et al. 2019\nResponse: As suggested, this reference has been added in the revised version.\nMethods:\nLine 246: What are the grain sizes of the two grains selected?\nResponse: The pyroxene grains has a size of 282 × 326 μm and the size for olivine is 274 × 442 μm (see figure below).\n(These two BSE images are also provided in Supplementary Fig. 1)\nLine 249: Replace “On the other hand” with “furthermore” or something similar, because this is not really a contrast to the previous sentence.\nResponse: We replaced “On the other hand” with “Furthermore” as suggested.\nLine 254-255: What are the differences between the two instruments? Why were two different instruments used?\nResponse: When examining the sample surface tomography, there were no differences between the Nova NanoSEM 450 and Thermo Scientific Apreo S. This is simply because the samples were observed by two different individuals and they happened to use these two devices.\nLine 292: with a x50 magnification lens.\nResponse: Modified as suggested.\nLine 329-331: Citation needed.\nResponse: We have added the reference of “Sharp et al., 2013 EPSL A hydrogen-based oxidation mechanism relevant to planetary formation” in the revised version.\nFigure 2: Clearly state which degassing model was used for which dashed line.\nResponse: Thanks for pointing this out. In the revised version, we have added the following sentence to clarify: “Outgassing of H-bearing phases from the minerals into space is also modelled and shown in dashed lines with different colours, i.e., pink for H, green for H and blue for H O.” The dashed lines are clearly marked 2 2 with different colors.\nReviewer #2 (Remarks to the Author):\nThank you for the opportunity to review the manuscript by Xu et al., entitled ‘Widespread ilmenite contributions to the surface water cycle in lunar Procellarum KREEP terrane’. The paper reports on the correlated study of ilmenite grains returned by the Chang’e 5 mission using TEM, EELS and NanoSIMS to suggest that ilmenite should contain more water (OH/H O) than measured and has thus produced and lost H 2 in a manner that would support the water cycle on the Moon. While this is an interesting idea, currently the data do not appear to support this conclusion. We detail our main concerns with the paper, along with a number of line-by-line comments, below. We feel that the manuscript requires a significant rewrite and reinterpretation of the data and are not convinced that the main premise of the paper would be supported following this revision. We would, however, be happy to review a revised and reinterpreted manuscript, though ultimately the findings may not be of interest to the broad readership of Nature Communications.\nMajor Comments\nInterpretation of NanoSIMS data: The major premise of the paper revolves around the statement on lines 162 and 163: “More importantly, we note that ilmenite and silicate minerals form two distinct clusters in the δD vs. H2O content plot, with minimal overlap”. Unfortunately, we do not agree with this comment. Figure 2 clearly shows overlap between the new ilmenite data points and existing silicate phases and does not appear distinct from the range of values previously published from lunar regolith. As such, we do not believe from the presented data that the ilmenite grains in the Chang’e-5 returned sample represent a unique reservoir compared to other silicate phases within the sample. In addition, we would argue that the analysed grains do contain elevated water abundances (and lower D/H ratios) when compared to lunar mantle grains, suggesting enrichment through solar wind H delivery and implantation, which contradicts the authors key statements.\nResponse: (1) We do not emphasize that the ilmenite of Chang'e-5 is a special reservoir, but that the ilmenite from the Moon is special. Since only a few ilmenite particles were analyzed in the Apollo mission, we cannot use them for comparative analysis. This is also our original intention in measuring the H contents and isotopic ratios in this study. (2) We regret for the misunderstanding in the original manuscript. We admit that the high water content and low D/H in ilmenites compared to the lunar mantle are caused by the solar wind, which we stated in the first subsection of the Discussion section. In the revised version, in order to make this viewpoint more clear, we have carefully reviewed and revised the logic and content of the paper. (3) The new finding of our paper is that although the water in the weathered rims of ilmenite originates from solar wind, their H isotopes and water contents are clearly different from those of silicate minerals, which can have even higher water concentrations with δD as low as -990‰ (figure below). This difference is most likely related to the intrinsic properties of ilmenite and silicate minerals. We have reorganized and rewritten the corresponding content.\n(This figure is also provided in the revised manuscript as Figure 2)\nPresentation of NanoSIMS data: It is challenging to ascertain the quality of the NanoSIMS data presented in supplementary table S1. The authors do not show any measurements on either the San Carlos olivine standard used for background correction, not any additional reference materials used to ascertain the quality of measured D/H. The authors should really report additional raw details, including 1H and 2H (counts and rate), 13C, and 16O (to understand corrections applied), along with the 2H/1H ratio and associated error. This is especially true for the San Carlos standard, as currently it is impossible to compare the authors data with literature values, further undermining the key take aways of the manuscript.\nResponse: Thanks for this suggestion. In the revised version, we have provided the measurement results of the San Carlos olivine, which were used for the background correction (figure below). For the correction, the measured H-depth profiles of the samples were subtracted from the corresponding H-depth profiles of San Carlos.\n(The raw data of 1H, D, 18O and H/O are plotted, which is provided inn the Supplementary material)\nWe also provide a figure to compare the measured H concentrations of three representative samples with those of San Carlos olivine (figure below). This figure shows that the samples have much higher water contents than that in San Carlos olivine.\n(This figure is also provided in the Supplementary material)\nWe measured three standards for D/H correction, namely SWIFT MORB glass, Durango apatite and Kovdor apatite. Based on the measured D/H ratios of these three standards, we can determine the IMF used for D/H correction (figure below). In this work, we used the IMF value of KOV apatite for the correction. This information can be found in the Methods section.\n(The IMF values of the three standards, which is also provided in the Supplementary material)\nInterpretation of solar flare tracks: The authors state on line 166 to line 168 that “solar flare tracks (0.02-0.18 Myr) indicate significant solar wind irradiation on ilmenite surfaces”. However, comparison of ilmenite data with that reported from pyroxene and olivine (Supplementary Table S2) suggest comparable, or significantly less, solar track formation than within the silicate minerals. This relationship is also clearly visualised in Figure S4, where ilmenite appears to contain fewer identified tracks. Assuming this evidence demonstrates that ilmenite has undergone less significant space weathering than silicate minerals, you would not expect significantly elevated production of OH within the altered rims. If the authors analysed 5 TEM images for each particle, the rest of this data should be shown, especially if it supports the authors arguments. Otherwise, it is challenging to accept that the grain population presented in the manuscript have undergone comparable degrees of space weathering, placing further uncertainty on the interpretation of the anticipated H producing in ilmenite.\nResponse: During the review, we carefully checked the TEM results and found that we calculated the solar flare tracks from 0 nm to a depth of 2µm randomly. Since the distribution of solar flare tracks from the surface to the depth is not homogeneous, we modified and recalculated the solar flare tracks at the depth of 0-500 nm according to the method described in Keller et al. (2021 MAPS). We also added two more ilmenite grains to obtain a more convincing results (figure below). Our new results show that the four ilmenites we analyzed have exposure times of 0.03 Myr, 0.11 Myr, 0.50 Myr and 1.57 Myr, respectively, which are indeed comparable to those of silicate minerals (see table below).\n(The six TEM images show the solar flare tracks of four ilmenites, one olivine and one pyroxene, which can also be found in the supplementary material)\nSample Numbers Measured Track δ Solar flare track\nType Source\nname of tracks area (nm2) (track cm-2) age (Myr)\nS404-09 Ilmenite 207 298586 6.93 × 1010 1.57 ± 0.13 This study\nS404-14 Ilmenite 24 478400 5.02 × 109 0.11 ± 0.01 This study\nS404-21 Ilmenite 6 454405 1.32 × 109 0.03 ± 0.003 This study\nS503-04 Ilmenite 58 264560 2.19 × 1010 0.50 ± 0.04 This study\nS404-02 Pyroxene 21 363296 5.78 × 109 0.13 ± 0.01 This study\nS404-16 Olivine 82 250048 3.28 × 1010 0.75 ± 0.06 This study\nS07-08 Olivine 58 1,099,927 5.3 × 109 0.12 ± 0.01 Xu et al. (2022)\nS07-11 Olivine 113 470,655 2.4 × 1010 0.55 ± 0.05 Xu et al. (2022)\nS08-10 Pyroxene 186 519,674 3.6 × 1010 0.82 ± 0.07 Xu et al. (2022)\nS07-16 Plagioclase 230 269,781 8.5 × 1010 1.95 ± 0.17 Xu et al. (2022)\nTo further understand the influence of exposure time on water content and H isotopes, we also have created a new graph. The results indicate that both water content and H isotopes for ilmenites are consistent with increasing exposure time, even when the exposure time reaches > 1 Myr. One of the possibilities is that the water in the space-weathered rims of studied ilmenites may reach a steady state. The ilmenites appear to parallel the silicate minerals. This observation further emphasizes the difference between silicate minerals and ilmenites.\n(This figure is also provided in the Supplementary material)\nLine by line comments\nTitle: The authors highlight KREEP terrane in the title, but the idea is not really revisited in the paper?\nResponse: Thanks for this comment! In the revised version, we have extended the content of the implications of our findings in the last section. We have added a figure to compare the distribution of minerals and water in the Procellarum KREEP Terrane area. By comparison, we find that the diurnal variations of water are closely related to the distribution of TiO (ilmenite). Thus, our study provides 2 robust evidence for the water cycle in the PKT.\n(This new figure is also provided in the main text)\nL41-45: long unbroken sentence with inadequate referencing.\nResponse: In the revised version, we have rewritten this sentence as “The laboratory analyses of Apollo and Chang’e-5 (CE5) lunar samples suggested that both indigenous source (Boyce et al., 2010; Hauri et al.,2011; Hui et al., 2013; Saal et al., 2008) and exogenous sources such as solar wind (He et al., 2023; Liu et al., 2012; Xu et al., 2022; Zhou et al., 2022), carbonaceous chondrites (Barnes et al., 2016), and comet (Greenwood et al., 2011; Zhou et al., 2024) have contributed to the distribution of OH/H O on the lunar surface” to make the meaning clearer.\nResponse: These studies refer to the weathered rims of silicate minerals but not to apatite. In the revised version, we have rewritten this paragraph and the original sentence has been removed. We have added the prefix “space weathered rims” before the “silicate phases” to make it clearer.\nL61-62: Include measurement of H and D/H from samples (e.g. low and high Ti basalts) not just spectral data. (reference?)\nResponse: In this sentence, we focus on talking about the relationship between the distribution of water on the lunar surface and its composition, while the laboratory analysis of the lunar basalts with low and high Ti content most likely reflects the water inside the Moon. In addition, the contribution of indigenous water on the surface is unclear. In this case, we decided not to add the literature analyses of the lunar low-Ti and high-Ti samples.\nL67: The authors never specify regolith only for this section. As such, it is missing lots of data on hard rock samples from Apollo, etc.\nResponse: As suggested by the reviewer, we have added a short sentence to introduce the term “regolith”, namely “By contrast, the lunar surface is covered by a layer of fine-grained regolith with a thickness up to ~12-15 meters6.”. Also, to clarify, we tried to use “soil grains” when we mentioned the H isotopic studies of Apollo samples and also checked this expression throughout the manuscript.\nL84: in-situ should be italicised.\nResponse: Revised as suggested.\nL110-111: Correction of 50 ma age?!? Based on Hu et al? If so, cite. Otherwise this seems random.\nResponse: Yes, the correction is 50 Myr, which is based on the study by Hu et al. (2021 Nature) and He et al. (2023 NG). In the revised version, we have added these two references.\nL148-149: Not comparable? Olivine appears much more densely packed based on the reported statistics and figures. And show all TEM images of the grains that the authors have collected, which may support the original interpretation.\nL172: Not comparable exposure times. Olivine = 0.7 Ma, ilmenite c. 0 Ma, based on the authors own figures. This makes it challenging to compare the reported track density and ages.\nResponse: As replied above, our new results show that the four ilmenites have an exposure time of 1.57 ± 0.13 Myr, 0.11 ± 0.01 Myr, 0.03 ± 0.003 Myr and 0.50 ± 0.04 Myr, where the three olivines have an exposure time of 0.75 ± 0.06 Myr, 0.12 ± 0.01 Myr and 0.55 ± 0.05 Myr, the two pyroxenes have an exposure time of 0.13 ± 0.01 Myr and 0.82 ± 0.07 Myr and the plagioclase has an exposure time of 1.95 ± 0.17 Myr. The data are provided in Supplementary Table 2.\nL162-163: Minimal overlap? Figure 2 shows more than 50% of ilmenite data overlaps with the silicate mineral bubble. Additionally, some literature olivine and plagioclase plot above the ilmenite field. I think this undermines the point of the paper (see major points above).\nResponse: We agree with this comment! We reconsidered the δD values in the rims of soil grains and grouped them into two categories, δD < -400‰ and > -400‰, as classified in Tian et al. (2024 GRL). Among them, the samples with δD < -400‰ were considered to contain major contribution from SW implantation, which is the only reservoir to have the extremely low δD value. For those grains with δD < -400‰ (Fig. 2), we can find out that the ilmenite and the silicates only partly overlap, while the majority of silicate phases exhibit much higher water content and lower δD values. In the revised version, we carefully considered this issue and revised it to a more reasonable expression, namely “In contrast, although the δD values for ilmenites overlap with the silicate minerals (olivine, pyroxene and plagioclase), they are almost always higher compared to the low-δD (< -800‰) endmember of silicate minerals (Fig. 2). This difference becomes more evident when combined with their water contents (as shown in Fig. 2), resulting in two distinct clusters in the δD vs. H O content diagram.” “More importantly, we note 2 that although ilmenite and silicate minerals are overlapped in the δD vs. H O plot, 2 almost all analyzed ilmenites consistently form a distinct cluster as mentioned above (Fig. 2).”. In the revised version, we have modified this point but it did not change our main conclusion.\nL177-178: All ilmenite do not have heaver isotopes or less water? The nuance of the data, beyond issues raised, should be more carefully discussed within the manuscript.\nResponse: We agree with this comment! Only one data point from Tian et al. (2024) is found to deviate from the cluster, but the difference between ilmenite and silicate is still clearly recognizable. This deviating data point showed a low water content and mantle-like H isotopic composition, which represents a fresh grain or experiencing very short-scale SW irradiation. In addition, the degassing model in Figure 2 has also been modified, which can interpret most of the data. Adjusting the H isotopes and water contents of the initial endmember can cover all other data.\nL189-190: large amounts of water should be present. IT IS! Lunar mantle ilmenite have basically none\nResponse: Here the phrase “there should be a large amount of H present” is used to interpret the observation of abundant nanoscale iron particles. This sentence is logical and a smooth transition to the introduction of our conclusion.\nL199: IDP?\nResponse: The term “IDP” is the abbreviation for “interplanetary dust particle”. We have added it in the revised version.\nFig S4: Should show the other images. Otherwise, does not seem comparable.\nResponse: As replied above, we have removed the original images and put the new TEM images.\nWe would like to thank the referee again for taking the time to review our manuscript.\nA point-by-point reply to the comments and questions of the reviewers is given below.\nReply to Reviewers\nReviewer #4 (Remarks to the Author):\nThe manuscript presented a study that aimed to address controversial observations regarding the water content and its variation in high-Ti mare regions. The work particularly examined the water content and hydrogen isotopes in ilmenite grains. A hypothesis was proposed based on the analysis of Chang’E 5 samples to explain orbital observations of water content and variations in these high-Ti mare regions at PKT. Water absorptions at different local times, as seen in the M3 data, were also examined. The stronger temporal variation of water content in high-Ti regions was attributed to the greater abundance of vesicles observed in ilmenite compared to silicates. Overall, the work is very interesting and novel, and the manuscript is well written.\nI have two concerns. First, I agree with Reviewer 2 that, even from the new Figure 2, it remains unclear that “their H isotopes and water contents are clearly different from those of silicate minerals.” It is recommended to highlight the drastic difference in the number of vesicles observed in ilmenite versus silicate minerals. I understand that the hydrogen isotope information is critical for arguing the faster diffusion of hydrogen in ilmenite compared to silicates, but it appears that the entire hypothesis is based on the difference in vesicle abundance.\nReply: We thank the reviewer for the suggestion. The differences in microstructural characteristics (particularly vesicle abundance) between ilmenite and silicate minerals have been highlighted in relevant sections of the Abstract, Results, and Discussion. This highlight enhances the coherence and clarity of the manuscript's logic and content.\nSecond, in Figure 5, the temporal variation of water content in the mare region seems to correlate even better with clinopyroxene and olivine than with TiO2. It may be helpful to use scatter plots to quantify how well each of these minerals and elements correlates with the water variation. While it is highly possible that space-weathered ilmenite on the lunar surface may exhibit faster diffusion of solar wind hydrogen than other silicate minerals, it remains unclear whether the difference in diffusion rates is detectable in the M3 data or other orbital datasets. This uncertainty is particularly notable given that: 1) regions rich in ilmenite may also be rich in pyroxene (both CPX and OPX), and 2) the lunar surface water content can also be affected by latitude and regolith maturity, while these factors are sometimes hard to separate.\nReply: We sincerely appreciate the reviewers' suggestions, which have prompted deeper reflection on this issue. Based on the suggestions, we have made the following modifications: Firstly, we have quantified the abundances of olivine, clinopyroxene, plagioclase and the content of TiO for the 10 points marked by 2 black-rimmed ellipses to characterize their relationships with temporal variations in water content (see figure below). These results demonstrate that the temporal variation of water content in the mare region exhibits a good positive correlation with both TiO and clinopyroxene but a negative correlation with plagioclase. 2 There is no correlation between water content and olivine. The positive correlation between water variation and TiO , and the negative correlation between water 2 variation and plagioclase are consistent with our expected results. For plagioclase, experimental studies have shown that the OH/H O formed by solar-wind 2 implantation in the lunar soil is likely to be mainly preserved in plagioclase (Tang et al., 2021 Icarus 359, 114322) because the structural OH/H O in plagioclase 2 formed by H⁺ irradiation is more stable than in other silicates (Zeng et al., 2021, 560, 116806).\n(As suggested, this figure has been included as panel (g) of Figure 5 in the revised manuscript)\nWe have first examined whether the TiO content is controlled by clinopyroxene 2 abundance. We compiled Apollo basalt data and found that TiO content in lunar 2 basalts (i.e., mare regions) is predominantly controlled by ilmenite rather than clinopyroxene (see figure below), consistent with our prediction. The TiO -rich 2 regions associated with abundant Cpx/Opx (as noted by the reviewer) may derive from impact ejecta or be related to their origins. Current solar-wind irradiation experiments on pyroxene and analyses of lunar pyroxene grains show no significant difference in OH/H O formation/release between pyroxene and other 2 silicates. Thus, although clinopyroxene shows a positive correlation with diurnal water variation, it likely has minor contributions to the lunar surface water cycle.\n(The data of Apollo basalts are from Hallis et al., 2014, Geochimica et Cosmochimica Acta, 134, 289–316. This figure is provide in the revised version as a supplementary figure)\nSecondly, we agree that surface hydration is also influenced by latitude and regolith maturity. For different localized regions, varying regolith maturities have the potential to affect the degree of diurnal water variation (morning-noon- evening). Based on these suggestions, we have toned down ilmenite's singular role in governing mare water variations and highlighted the effects of latitude and regolith maturity. Related discussion has been added to the Implications section and other appropriate sections.\nReviewer #5 (Remarks to the Author):\nThe authors have well addressed the comments provided by the referees, and presented a clear, logical manuscript. I suggest it be published after some minor revisions.\nIn Figure 2, the δD vs. H2O plot of ilmenites exhibits a distinct cluster and shows a slightly positive correlation between H2O content and δD, which is different from silicate minerals. This phenomenon is very interesting. I think the result not only is related to the escape of water in ilmenite, but also to the reducing effect of solar wind protons on ilmenite. Because the escape of water will cause a negative correlation between H2O content and δD, like the silicate minerals. This may require another process to explain. The protons (or H2) react with ilmenite to form npFe0 and produce H2O. The hydrogen isotopes undergo fractionation during the reaction process, allowing the heavier D to remain in the ilmenite. Of course, this is a hypothesis for the authors’ reference.\nReply: After careful examination of Figure 2, we find that, apart from two data points slightly deviating from the main cluster, the remaining ilmenite data do not exhibit a clear positive correlation trend. Additional future data would be optimal to test this potential correlation.\nRegarding the reviewer's hypothesis that “protons (or H ) react with ilmenite 2 to form npFe0 and produce H O, with hydrogen isotope fractionation during the 2 reaction allowing the heavier D to remain in the ilmenite”: given the extremely low deuterium (D) content in the solar wind and the low D abundance in the lunar soil samples themselves, we think that hydrogen isotope fractionation caused by the proposed reduction process to form a positive trend is unlikely. Continuous implantation would induce progressively lighter hydrogen isotopic composition, most likely forming a negative trend. In the revised manuscript, we clarify this by adding to Figure 2’s caption that the slight positive correlation is likely controlled by two data points.\nLine 165—Line 169 and corresponding supplementary information\nThe author uses the track production rate (4.4 ± 0.4 × 10^4 tracks cm⁻² yr⁻¹ at 1 AU) provided by Keller to calculate the exposure age of samples and discuss their relationship to water content. However, Keller said this track production rate is only valid for olivine and anorthite at depths < 0.5 μm. Therefore, directly using this rate to calculate the exposure age of ilmenite and pyroxene directly is incorrect. If the author wants to establish an accurate relationship between exposure ages and water contents, they should choose a suitable track production rate for ilmenite and pyroxene; at least provide some explanations. In addition, all the calculations should be kept at depths < 0.5 μm. Otherwise, this rate should be amended according to different depths. (Please refer to the detailed description of Keller's paper on page 10).\nReply: We thank the reviewer for raising this point. To date, track production rates have been determined only for olivine and anorthite. In this work, to enable comparison of cosmic-ray exposure time across different minerals, we applied these rates to both ilmenite and pyroxene. However, given that ilmenite possesses a close-packed crystal structure as also mentioned by the reviewer, it is likely more resistant to solar wind irradiation than silicate minerals. Therefore, the actual track production rate for ilmenite may be lower than that of silicates, and thus its true exposure time is likely longer than our calculated value. In turn, this further supports our conclusion that the low water content observed on the surface of Chang'e-5 ilmenite grains cannot be attributed to a short exposure time.\nOlivine is a nesosilicate and pyroxene is an inosilicate; although there may be potential variations in track production rates, both have silicate structures. We therefore assumed an identical production rate for pyroxene to that of olivine. We have incorporated this discussion into the revised manuscript.\nI notice the author shows some microstructure evidence to illustrate the space weathering rim, all caused by solar wind irradiation. However, the surface of the lunar grains is also easily containing vapor deposit layer (see Gu et al., 2022), which can contain solar wind water. It is better to add chemical composition by EDS for all samples to make sure if there are splash melts or vapor deposits layers on the ilmenite surface and exclude their effects on the water contents.\nReply: Thanks for the suggestion. During the revision, we analyzed three FIB ultra-thin foils by EDS; the results are presented below. These elemental mappings reveal a Si-rich vapor-deposited layer with a thickness of 10-15 nm. The presence of this vapor-deposited layer is consistent with previous observations in Apollo (Burgess and Stroud, 2018 GCA) and Chang’e-5 lunar soils (Guo et al., 2024 CEE).\n(TEM images of three ilmenite grains and the selected regions for EDS mapping)\nAlthough not all ilmenite grains were analyzed, we uniformly corrected for the vapor-deposited layer on all grain surfaces, considering the ubiquitous presence of this vapor deposit. Therefore, water contents and hydrogen isotopic compositions within the 10–100 nm depth range were recalculated by subtracting the ~10 nm thickness (see Table below). As a consequence, compared to the unsubtracted data, the average water content at 10–100 nm decreased by approximately 10%, while hydrogen isotopic compositions remained similar within analytical uncertainty. Based on these results, and considering that the vapor-deposited layer is also subject to solar wind irradiation, the unsubtracted data (i.e., CRE age-corrected water content and hydrogen isotope data) were still used for subsequent discussion in the revised version.\nSample Phase Ave. δD 1σ Ave. H2O δD in 0- 1σ Ave. H2O δD in 10- 1σ\nname H2O in 0- (‰) content in 100 nm (‰) content in 100 nm (‰)\ncontent 100 100 nm (ppm) (corrected 100 nm (ppm) (corrected\nin 0- nm (ppm) (corrected after 50 (corrected after 50\n100 nm (‰) (corrected after 50 Myr) after 50 Myr)\n(ppm) after 50 Myr) Myr)\nS503-01_2 Ilmenite 1268 -613 51 1268 -618 58 1163 -610 56\nS503-02_1 Ilmenite 2000 -678 43 2000 -681 49 1879 -659 50\nS503-03_2 Ilmenite 3714 -480 34 3713 -482 38 3491 -468 37\nS503-04_2 Ilmenite 1336 -538 62 1336 -543 66 1229 -580 64\nS503-05_1 Ilmenite 1793 -761 33 1792 -764 36 1660 -779 35\nS503-06_1 Ilmenite 1639 -546 49 1639 -550 53 1560 -558 52\nS503-07_2 Ilmenite 976 -810 40 976 -816 44 875 -816 44\nS404-09_1 Ilmenite 1732 -726 32 1732 -730 34 1628 -737 34\nS404-09_2 Ilmenite 1575 -773 38 1575 -777 42 1486 -759 42\nS404-09_3 Ilmenite 2808 -608 36 2808 -611 39 2694 -613 38\nS404-10_1 Ilmenite 2550 -624 29 2550 -627 30 2384 -644 30\nS404-10_2 Ilmenite 3377 -669 24 3377 -671 26 3230 -677 26\nS404-14_1 Ilmenite 2054 -718 33 2054 -721 35 1816 -749 35\nS404-14_2 Ilmenite 1910 -745 32 1910 -749 35 1780 -745 35\nS404-14_3 Ilmenite 1815 -764 30 1814 -768 33 1627 -789 32\nS404-21_1 Ilmenite 833 -789 40 833 -797 43 771 -806 42\nS404-21_2 Ilmenite 734 -723 50 734 -732 52 674 -768 50\nS404-21_3 Ilmenite 729 -764 42 729 -773 46 673 -777 45\nS404-21_4 Ilmenite 767 -722 44 767 -730 46 702 -755 46\n(The three columns highlighted in yellow represent newly calculated values that exclude the surface contribution from vapor deposits)\nLine 197—Line 200: The authors declare that similar implantation depths between ilmenite, pyroxene, and olivine should have similar solar wind water contents. However, TRIM only considers the random collision of ions with atoms in target material, ignoring effects like crystalline structure. Previous research shows that ilmenite is more resistant to solar wind irradiation than silicate minerals (see Li et al., 2013). The authors infer ilmenite grains should contain a similar abundance of SW-implanted hydrogen as other silicate minerals, but I think this conclusion is questionable. Under the same condition of H+/D+ irradiation, the water content of different silicate minerals and glass also varies (Tang, et al., 2021; Zeng et al., 2024), and the ability of H preservation in ilmenite is likely to be different, resulting in different water contents of diverse minerals.\nReply: The question of whether ilmenite can incorporate hydrogen from solar wind implantation at levels comparable to silicate minerals is indeed significant. Currently, both simulation experiments on hydrogen implantation in ilmenite and analyses of lunar ilmenites are relatively limited. During the review process, we conducted a thorough literature search and identified a study published in the year of 1986 that performed hydrogen ion implantation experiments on ilmenite and silicates (plagioclase and glass). The results (as shown in the figure below, left panel) demonstrate that under the same irradiation conditions, the implantation depths of hydrogen in ilmenite are similar to those in plagioclase. More importantly, ilmenite retained approximately twice as much deuterium as the other target materials. This indicates that, although ilmenite may be more resistant to ion irradiation-induced amorphization than silicates, it is still capable of storing significant amounts of hydrogen.\nOn the other hand, heating experiments on the irradiated minerals revealed that hydrogen loss occurred more rapidly from ilmenite compared to silicates (as shown in the figure below, right panel). Collectively, these experimental results support the results and conclusions presented in our study. We have incorporated the relevant discussion of this prior work into the revised manuscript.\n(These two images are from Blanford et al., 1986 JGR “On-line” analyses of simulated solar wind implantation of terrestrial analogs of lunar materials)\nThe authors suggest that the ilmenite may contribute significantly to the observed diurnal variations in water content in the high-Ti mare regions. However, the 3-µm band of ilmenite seem not obvious, which makes it difficult to explain the spectral variations in 3-µm band depth. Please clarify this.\nReply: As suggested by Reviewer #4, we have plotted a new figure of “mineral abundances versus diurnal water content” to characterize the correlation. The new figure shows that both the TiO content (primarily hosted in ilmenite) and 2 clinopyroxene are positively correlated with water variations (see the detailed replies above). The main text has been modified accordingly to clarify this.\nL55: ref. 23 supports carbonaceous chondrites rather than comets.\nReply: As suggested, we have moved this reference after “carbonaceous chondrites”.\nL143, L147, L149: the “n” needs to explain its meaning.\nReply: It refers to “number of nanophase iron particles”, which has been clarified in the revised text."}, {"round": 2, "reviewer_comments": {"reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The manuscript presented a study that aimed to address controversial observations regarding the water content and its variation in high-Ti mare regions. The work particularly examined the water content and hydrogen isotopes in ilmenite grains. A hypothesis was proposed based on the analysis of Chang’E 5 samples to explain orbital observations of water content and variations in these high-Ti mare regions at PKT. Water absorptions at different local times, as seen in the M3 data, were also examined. The stronger temporal variation of water content in high-Ti regions was attributed to the greater abundance of vesicles observed in ilmenite compared to silicates. Overall, the work is very interesting and novel, and the manuscript is well written.\nI have two concerns. First, I agree with Reviewer 2 that, even from the new Figure 2, it remains unclear that “their H isotopes and water contents are clearly different from those of silicate minerals.” It is recommended to highlight the drastic difference in the number of vesicles observed in ilmenite versus silicate minerals. I understand that the hydrogen isotope information is critical for arguing the faster diffusion of hydrogen in ilmenite compared to silicates, but it appears that the entire hypothesis is based on the difference in vesicle abundance.\nSecond, in Figure 5, the temporal variation of water content in the mare region seems to correlate even better with clinopyroxene and olivine than with TiO₂. It may be helpful to use scatter plots to quantify how well each of these minerals and elements correlates with the water variation. While it is highly possible that space-weathered ilmenite on the lunar surface may exhibit faster diffusion of solar wind hydrogen than other silicate minerals, it remains unclear whether the difference in diffusion rates is detectable in the M3 data or other orbital datasets. This uncertainty is particularly notable given that: 1) regions rich in ilmenite may also be rich in pyroxene (both CPX and OPX), and 2) the lunar surface water content can also be affected by latitude and regolith maturity, while these factors are sometimes hard to separate.", "reviewer_5": "The authors have well addressed the comments provided by the referees, and presented a clear, logical manuscript. I suggest it be published after some minor revisions.\nIn Figure 2, the δD vs. H2O plot of ilmenites exhibits a distinct cluster and shows a slightly positive correlation between H2O content and δD, which is different from silicate minerals. This phenomenon is very interesting. I think the result not only is related to the escape of water in ilmenite, but also to the reducing effect of solar wind protons on ilmenite. Because the escape of water will cause a negative correlation between H2O content and δD, like the silicate minerals. This may require another process to explain. The protons (or H2) react with ilmenite to form npFe0 and produce H2O. The hydrogen isotopes undergo fractionation during the reaction process, allowing the heavier D to remain in the ilmenite. Of course, this is a hypothesis for the authors’ reference.\nLine 165—Line 169 and corresponding supplementary information\nThe author uses the track production rate (4.4 ± 0.4 × 10^4 tracks cm⁻² yr⁻¹ at 1 AU) provided by Keller to calculate the exposure age of samples and discuss their relationship to water content. However, Keller said this track production rate is only valid for olivine and anorthite at depths < 0.5 μm. Therefore, directly using this rate to calculate the exposure age of ilmenite and pyroxene directly is incorrect. If the author wants to establish an accurate relationship between exposure ages and water contents, they should choose a suitable track production rate for ilmenite and pyroxene; at least provide some explanations. In addition, all the calculations should be kept at depths < 0.5 μm. Otherwise, this rate should be amended according to different depths. (Please refer to the detailed description of Keller's paper on page 10）\nI notice the author shows some microstructure evidence to illustrate the space weathering rim, all caused by solar wind irradiation. However, the surface of the lunar grains is also easily containing vapor deposit layer (see Gu et al.,2022), which can contain solar wind water. It is better to add chemical composition by EDS for all samples to make sure if there are splash melts or vapor deposits layers on the ilmenite surface and exclude their effects on the water contents.\nLine 197—Line 200: The authors declare that similar implantation depths between ilmenite, pyroxene, and olivine should have similar solar wind water contents. However, TRIM only considers the random collision of ions with atoms in target material, ignoring effects like crystalline structure. Previous research shows that ilmenite is more resistant to solar wind irradiation than silicate minerals (see Li et al., 2013). The authors infer ilmenite grains should contain a similar abundance of SW-implanted hydrogen as other silicate minerals, but I think this conclusion is questionable. Under the same condition of H+/D+ irradiation, the water content of different silicate minerals and glass also varies (Tang, et al., 2021; Zeng et al., 2024), and the ability of H preservation in ilmenite is likely to be different, resulting in different water contents of diverse minerals.\nThe authors suggest that the ilmenite may contribute significantly to the observed diurnal variations in water content in the high-Ti mare regions. However, the 3-µm band of ilmenite seem not obvious, which makes it difficult to explain the spectral variations in 3-µm band depth. Please clarify this.\nL55: ref. 23 supports carbonaceous chondrites rather than comets.\nL143, L147, L149: the “n” needs to explain its meaning."}, "author_response": "A point-by-point reply to the comments and questions of the reviewers is given below.\nReply to Reviewers\nReviewer #5 (Remarks to the Author):\nThe revised manuscript is significantly improved. I would suggest additional minor revision.\nL47 “This regolith layer can contain abundant water and water ice”. Water ice can only exist in polar lunar soil. This sentence is prone to causing ambiguity and needs to be revised to be clearer.\nReply: Thanks for the suggestion. We have modified this sentence to “This regolith layer can contain abundant water7-9, and the polar lunar regolith could contain water ice, which are essential resources for building a lunar base and long-term life support”.\nFig. 3 The layout of Fig. 3 should be optimized, as the spacing between the panels appears inconsistent.\nReply: In accordance with your suggestion, we have optimized the image spacing to enhance the overall layout."}]

Decision letter