Geological evidence for extensive basin ejecta as plains terrains in the Moon’s South Polar Region

Domain: earth_science
Published: 2024-07-10
Editorial outcome: minor_revision

Abstract
Water ice and other volatiles that accumulated in the Moon’s polar regions are among the top priority targets for lunar exploration, due to their significances in both lunar geology and extraterrestrial resource utilization. Locating suitable landing sites and determining the provenance of sampled/measured surface materials are critical for future landed missions. Here, we map over 800 sites of plains terrains in the Moon’s south polar region, with a total surface area of ~46,000 km2. Orbital measurements and analog studies show that most of these plains have apparently higher albedo and lower iron content than volcanic mare plains, suggesting an origin of ejecta-induced debris flows from distant impact craters, especially from the Schrödinger basin. Our findings suggest that the entire lunar south polar region probably have experienced contributions from distant basin materials. We recommend these plains as priority landing sites for future exploration of lunar polar volatiles and early bombardment history.

Introduction
Water ice and other frozen volatiles are thought to have accumulated in the permanently shadowed regions in polar regions of the Moon due to the extremely cold environments over geologically long periods of time1–4. Some of these volatiles may have been delivered to the lunar surface since the very early stages of solar system history, and they are therefore of scientific significances for solar system studies, e.g., the source and migration of solar system volatiles, and formation and evolution of planets5,6. Moreover, these ice deposits can be potentially harvested as a resource to enable sustained human exploration to the Moon and beyond in space7. In addition, the lunar samples collected in this context provide fundamental information about the history of the Moon and the provenance (origin) of both the samples8 and the associated volatiles9. As a consequence, these polar volatiles have emerged as high priority targets for lunar science and exploration5,10.

In the coming decade, dozens of orbital and landed missions are aiming to explore and assess lunar polar volatiles, especially in the southern polar region. For instance, NASA’s Artemis III mission (currently scheduled for launch in the mid-2020’s) is planned to send astronauts to explore the southern polar region of the Moon11,12. Moreover, China and Russia are jointly proposing the International Lunar Research Station, a long-term research station near the lunar south pole13. In addition to frozen volatiles, the south polar region of the Moon is also adjacent to the South Pole-Aitken (SPA) basin, the largest and oldest preserved lunar impact basin14,15, providing an unparalleled opportunity to study deep-seated lunar materials16 and the early bombardment history of the Moon.

The design and implementation of these lunar south polar missions all require precursor scientific understanding of the lunar southern polar region, in particular the geological characteristics and evolution of the candidate landing sites. These geological investigations will provide critical mission support, including landing and roving safety, provenance of the in-situ measured and returned samples, and modification processes experienced by the polar volatile deposits, support well demonstrated by the very successful Apollo missions17.

As shown in the imaging and topographical maps (Fig. 1), also in recent8 and earlier17,18 geological mapping, the southern polar region of the Moon lacks the extensive mare plains like those in the landing areas of many previous missions that provide flat terrains to ensure safe landing. Instead, the southern polar region surface is dominated by feldspathic highland terrains overprinted by abundant impact craters of a wider range of dimensions (up to ~300 km in diameter) and ages (from pre-Nectarian to Copernican8,18), and ejecta from these craters and basins19, a terrain resembling the Apollo 16 site and other heavily cratered terrain on the Moon (e.g., the Feldspathic Highlands Terrane20). This results in the very rugged long-wavelength topography (Fig. 1; median roughness (root mean square height) ~66 m at 1.82 km baseline) and a complex geological history. Locating suitable surface landing sites is critical for mission design and planning, as well as scientific understanding of the mission results.

Preliminary morphological examination, and previous geological mapping efforts8,21–23 identify many small (relative to typical lunar mare plains) plains terrains in the southern polar region of the Moon (Fig. 1), especially on the floors of many craters, for instance, the Amundsen crater (Fig. 2a, b). This configuration is morphologically similar to many plains-filled craters in the equatorial regions of the Moon23,24, such as the mare-filled Colombo crater (~79 km diameter, centered at 15°S, 46°E; Fig. 2c) and the light plain-filled Plummer crater (~73 km diameter, 25°S, 155°W; Fig. 2d).

Compared with the rugged topography in most areas of the lunar south polar region (median slope ~8° at 60 m baseline and median roughness ~25 m at 660 m baseline; Fig. 1 and Supplementary Fig. 1b), these polar plains are topographically very flat (median slope ~3° at 60 m baseline and median roughness ~7 m at 660 m baseline; Fig. 2b and Supplementary Fig. 1b), serving as attractive exploration sites for surface landing and roving lunar polar missions. However, a set of fundamental questions remain to be further investigated about these polar plains terrains: (1) What is their spatial distribution across the southern polar region of the Moon? (2) What are their detailed geological characteristics, including topography, morphology, composition, and chronology? Are they similar to or different from the well-known mare basaltic plains? (3) What is their origin and emplacement mechanism? Are they also volcanic in origin as are many shallowly buried cryptomaria, or basaltic mare plains on the Moon with different mineralogy and higher albedo? (4) Are these polar plains promising sites for lunar polar volatile exploration? What can we learn from these polar plains about the stratigraphy, emplacement, and age of volatile deposits at lunar southern polar region? These questions are critical for the design and planning of future lunar polar exploration efforts.

In this contribution, we focus on the plains terrains in the lunar polar region southward of 80°S latitude. We employ the latest high-resolution and high-precision topographic and other data sets to access the spatial distribution and detailed characteristics of these polar plains. These analyses are then combined with analog studies to investigate their formation mechanism and potential source regions. Finally, we discuss the science and exploration value of these polar plains, including both volatile prospecting and other scientific opportunities.

Results
Distribution of plains terrains in the lunar south polar region
Using high-resolution LOLA topography and derived maps, we undertook a thorough search for, and mapping of, plains terrains in the southern polar region of the Moon. We identified 873 sites of plains terrains with surface areas ≥1 km2 (Fig. 3; limited by the 20–1000 m pixel sizes of the various datasets, see Methods) in the region shown in Fig. 1. Smaller plains units were also found but are not cataloged and characterized in this work. The surface area of these mapped plains ranges over nearly four orders of magnitude, up to over 8000 km2, with a median value of 9.3 km2 (Supplementary Fig. 2). Areally extensive plains occurrences include those emplaced on the floor of large craters, e.g., Schrödinger (8138 km2), Drygalski (2584 km2), and Amundsen (2322 km2). The histogram of surface areas is characterized by a leptokurtic distribution, with a positive skewness toward larger areas (Supplementary Fig. 2). Smaller plains are much more common than larger ones: ~53% of the plains are smaller than 10 km2 and ~86% are smaller than 50 km2. All the mapped plains have a total surface area of about 46,700 km2, constituting ~12.8% of the entire study region (Fig. 1).

A significant part (~57%) of these plains are located on the floors within craters, especially for many relatively larger-area plains occurrences (Fig. 3). These crater-interior plains are generally characterized by very low slopes (typically <2°), smooth surface textures (topographical roughness typically <10 m at 660 m baseline), and distinct contacts with the more rugged and steep crater walls (Fig. 4a). They usually have an approximately circular outline defined by the sloped wall terrains. In addition, we also observed many plains terrains (~43%) at locations outside distinct impact craters (Fig. 3). These inter-crater plains have slightly elevated topographical slopes (though mostly <5°), while they still retain smooth surface textures (Fig. 4b), similar to those of crater-interior plains. Many of these inter-crater plains appear to be superposed on the underlying sloped terrains, often with elongated outlines. In these cases, their spatial boundaries from adjacent terrains are often more gradational.

We note that a portion of the polar plains that we mapped, were also mapped as light plains by ref. 23 (Fig. 3). Whereas our mapping effort has identified significantly more plains than previous work: only 95 sites of plains ≥1 km2 were mapped by ref. 23 in the lunar south polar region, with a total surface area of ~29,700 km2. This discrepancy is mainly attributed to the new LOLA datasets with much higher resolution (20 m/pixel) and better spatial coverage employed in this work, compared with the 100 m/pixel LROC WAC data used by ref. 23, with considerable shadowed areas, especially for regions south of 85°S latitude (Fig. 1a). In addition, we mapped these plains at various scales from 1:200,000 to 1:400,000, compared with the 1:300,000 scale by ref. 23.

Geological characteristics
We first referred to the gridded LOLA data and derived maps (Figs. 1b and 2b and Supplementary Fig. 3) to examine the topographic characteristics of the plains terrains in the lunar south polar region. We plotted the histograms of the elevation, slope, and roughness values for the polar plains and the entire south polar region (Supplementary Fig. 1). The plains terrains are characterized by a wide range of topographic elevations from about −7 km to about 6.7 km relative to the mean lunar radius (1737.4 km), while the elevation of the majority (~75%) of these plains range from −5 km to −2 km (Supplementary Fig. 1a), with peak values around −4.7 km. Compared to the entire southern polar region (peak values around −2.3 km), the plains generally lie at lower elevations, as in the cases of those on the impact crater floors. The histogram plot shows an approximately unimodal distribution pattern for the entire southern polar region, while a number of peaks are observed for the mapped plains terrains. This comparison suggests that these plains are likely to have been emplaced after the formation of the primordial highland terrain in the southern lunar polar region and that their topography is superimposed on that of the underlying terrain. The slope-frequency distribution of both the plains terrain and the entire southern polar region exhibits a distinctive unimodal structure, with a downward roll-over at slopes near zero, a common slope signature for natural surfaces. Expectedly, the polar plains are characterized by very flat slopes, with ~74% of the surface flatter than ~5° and most-frequent slopes around 2°, compared with ~4° for the entire southern polar region (Supplementary Fig. 1b). In addition, these plains terrains have lower roughness values at all three baselines (Supplementary Figs. 1c, d, and 3).

We then investigated the surface optical albedo and iron contents25–27 of the mapped polar plains, and their comparison with those of other typical lunar terrains. In the histogram plots, we observed that the polar plains have nearly identical 1064 nm albedo and iron contents as those of the entire southern polar region (excluding the mapped plains), with median albedo around 0.32 and FeO abundance around 7.2 wt% (Fig. 5). The global light plains mapped by ref. 23 and the entire Moon also have similar most-frequent values of LOLA albedo and FeO content, while they also contain a considerable portion with lower albedo and higher iron content. (Note that the small discrepancy in the most-frequent FeO abundance values between the south polar region and the global Moon (median value: 7.2% vs. 6.5%; Fig. 5b) are likely attributed to the usage of different datasets (Lunar Prospector vs. Kaguya + LRO), as no such discrepancy is observed if the same Lunar Prospector data alone is analyzed (Supplementary Fig. 4).) The maria regions, however, display lower albedo (median value ~0.18) and elevated FeO abundance (median value ~16 wt%), distinctly different from the polar plains terrains (Fig. 5).

Finally, we conducted a systematic survey of the surface geomorphology and textures of the polar plains mapped in this work. We identified two major types of morphologies for these plains terrains. The first type is represented by those emplaced on the floor of the Amundsen crater (Fig. 4a). These Amundsen-type plains terrains are characterized by very low slopes and show flat, smooth surface textures, with little topographic relief. The major features superposed on these plains are many small craters, including both circular primary craters and elongated or irregular secondary craters and clusters. Many of these plains occur on the floors of larger craters, while some are emplaced on terrains outside craters, and usually have slightly steeper slopes (e.g., Fig. 4b).

Another type of plains is represented by those occurring on the floor of Schrödinger basin (Fig. 4c). These Schrödinger-type plains are also generally flat, but their surface is populated by abundant small mounds and hummocky features. A network of fractures and graben has been identified by many previous studies on the Schrödinger basin floor28. We find that such fractures are uniquely observed in the plains within Schrödinger basin, and not in plains elsewhere in the southern polar region. The Schrödinger fractures are likely to have formed by dike propagation-induced shallow magmatic intrusion29, and are not genetically related to the plains terrains, except due to the emplacement of some dark-halo pyroclastic deposits and small mare patches. As a result, these fractures/grabens are not designated among the shared properties of Schrödinger-type plains. On the basis of this classification scheme, we assessed the morphological type of each plain terrain and mapped its distribution (Supplementary Fig. 5). We found that these polar plains are dominantly Amundsen-type and only 9 occurrences of Schrödinger-type plains are identified (Supplementary Fig. 6a–i). All these Schrödinger-type plains are emplaced within impact craters, and none are observed outside craters. Except for the Schrödinger basin (312 km diameter), all the other host craters are much smaller, including Shackleton crater (21 km diameter) at the lunar south pole (Supplementary Fig. 6g).

Origins and source regions
On the surface of terrestrial planetary bodies, the presence of plains terrains generally indicates the formation of a gravitational equipotential surface in the geological past, which typically involves the emplacement of fluidized materials, for instance, fluvial sediments on Earth and Mars. Considering the unique geological and environmental conditions of the lunar surface, especially the nearly anhydrous and vacuum conditions, many plain-formation geological process involving water and volatiles can be excluded and the possible candidates mainly include volcanic and impact-related processes. Our optical albedo and iron abundance measurements show clearly that these polar plains are apparently different from volcanic mare deposits (Fig. 5). This comparison indicates that these polar plains are unlikely to be volcanic basaltic mare plains. We also examine the surface albedo, morphology, and composition of the mapped plains in detail (see Methods) and do not observe any dark-halo impact craters (consistent with ref. 30) or mafic-rich regolith signature (Supplementary Fig. 7), showing that these polar plains are not underlain by basaltic materials (cryptomaria). (It should be noted that it is challenging to identify surface albedo variations in the polar regions using typical conventional optical images due to considerable shadows and typically very low Sun illumination.) This observation is also consistent with the deficiency of volcanic deposits detected in the polar regions of the Moon8,31,32. Impact cratering is known to produce various kind of materials and deposits, and some are able to flow during their emplacement. The most common impact-derived fluidized materials include impact melt flows33,34 and impact debris flows consisting of pre-impact substrate and ejecta materials35–37. How to distinguish between the two kind of impact materials for the nature and origin of our mapped polar plains terrains? We address this issue through geomorphology and analog studies of impact-derived plains terrains elsewhere on the Moon.

We first focus on the more prevalent Amundsen-type plains terrains with very smooth surface textures. Amundsen is a typical lunar complex crater with a diameter of 103 km and lies ~170 km from the south pole. We undertook a survey of lunar impact craters with sizes (90–110 km diameter) similar to Amundsen crater (n = 187 according to the catalog of ref. 38) and containing interior plains terrains. We found similar smooth plains on the floors of a number of impact craters, for instance, Parenago (26°N, 109°W, 95 km diameter) and Kibal’chich (3°N, 147°W, 92 km; Fig. 4d). The plains terrain on the floor of Kibal’chich have been mapped previously by ref. 23 as light plains (Supplementary Fig. 8). The spatial distribution of the Kibal’chich plains and other adjacent plains demonstrate that they are radial to the ~930 km-diameter Orientale basin (Supplementary Fig. 8), supporting the interpretation of ref. 23 that they may originate from fluidized basin ejecta materials from the Orientale basin-formation impact. On the basis of these analog and process studies, we suggest that Amundsen-type polar plains are likely to be ejecta deposits from distant impact craters or basins. It should be acknowledged that impact melt materials could have been produced during the impact cratering process and emplaced on the primitive crater floor, which would have been buried by and mixed with subsequent ejecta materials. These melt materials can be potentially sampled/measured by future landed missions to date the host crater and been compared with previous crater population dating results8. We will investigate the potential source craters of these plains terrains in the latter part of this section.

We then turn to the Schrödinger-type plains terrains with characteristic mound features. As the second youngest (after Orientale15) impact basin on the Moon, the 315 km-diameter Schrödinger basin is regarded as one of the prototypes for studying large impact basins. We found that the light plains radial to the Orientale basin generally lack such mound features, for example, those emplaced within the 350-km-diameter Schiller-Zucchius (unofficial name) impact basin in the northeast of Zucchius crater (Supplementary Fig. 9), suggesting that the Schrödinger-type polar plains are probably not emplaced basin ejecta materials. In addition, we did not find craters or basins with a size comparable to Schrödinger that host similar mound-bearing plains terrains. Alternatively, these Schrödinger-type plains resembles many units of the Maunder (inside the Inner Rook Ring) and Montes Rook (between the Cordillera and Outer Rook Rings) Formation of the Orientale basin, which predominately occur in topographically low regions, and contain mainly of flat-lying materials and km-scale blocks (Fig. 4e). The Maunder and Montes Rook Formation units have been generally interpreted as clast-rich melt materials produced during the basin-formation impact39,40. We also found that these Schrödinger-type plains are morphologically very similar to the floor terrains of some very young (Copernican) craters, for instance, Copernicus (10°N, 20°W, 96 km in diameter; Fig. 4f), Tycho (43°S, 11°W, 85 km), and Ohm (18°N, 114°W, 62 km). At Copernicus crater, abundant small mound features (up to ~1–2 km in size) are observed along the marginal portions of the crater floor, which were interpreted as broken, un-melted rock fragments embedded in the impact melt deposits34, a common occurrence in young, unmodified lunar craters33,41. From these observations, we suggest that Schrödinger-type polar plains terrains are likely to represent local impact-related blocks and melt materials. Similar mound-bearing plains were not observed within impact craters of comparable size of Schrödinger because Schrödinger is among the youngest basins on the Moon and the primordial surface features of the impact melts within other older basins have been topographically degraded.

In summary, the two morphological types of polar plains terrains are interpreted to have different formation mechanisms and source regions: the smooth Amundsen-type plains are composed of mixtures of local substrate and ejecta materials from distant impact craters/basins and the mound-bearing Schrödinger-type plains are local impact melt-related materials associated with the host craters. This interpretation is consistent with the stratigraphy of the host craters of these plains (Supplementary Fig. 5). Most of the Schrödinger-type plains terrains (n = 9; Supplementary Fig. 6a–i) occurs within relatively younger craters (compared to the host craters of Amundsen-type plains): two Eratosthenian-aged craters, five Imbrian-aged craters, and two Nectarian-aged craters (from the geological maps8,18). Their relatively younger ages make it possible to preserve the primary impact melt-related morphology. Most of the Amundsen-type plains occur in ancient (Nectarian and Pre-Nectarian) terrains, either crater floors or inter-crater terrains, as only huge impact basins (which are all very ancient: Imbrian or older) are able to produce sufficient ejecta materials to form the extensive plains terrains.

The geological provenance of the Schrödinger-type plains is local and related to the host impact crater, while the Amundsen-type plains consist of mixtures of both local substrate and ejecta materials delivered from distant impact craters/basins35,36. What are the source regions of the distal materials? Examination of the surface image and geological map8,18 shows that no crater or basin is both larger and younger than the 315 km Imbrian-aged Schrödinger basin in the vicinity of the south polar region of the Moon. On the basis of this observation, we suspect that the Schrödinger basin may have been among the major sources of these fluidized ejecta materials.

To test this hypothesis, we conducted a case study of the Nectarian-aged Amundsen crater and calculated the theoretical ejecta thickness (Supplementary Table 1) at the center of the Amundsen floor (85°S, 88°E) from all lunar impact craters/basins ≥10 km in diameter (from the Robbins database42) using an empirical ejecta thickness model of Pike43 with distance and deposition corrections (see methods in ref. 44 and references therein). We found that among the craters with a comparable or younger age of Amundsen (from the geological map of ref. 18), ejecta from Schrödinger basin is the thickest (~61 m), significantly thicker than the second largest ejecta contribution from Orientale basin (~13 m). The delivered ejecta materials from Schrödinger basin would have excavated and mixed with local materials with a depth up to ~183 m (Supplementary Table 1), which together formed the ejecta deposit flows. No impact craters (up to ~2.9 km in diameter) superposed on the Amundsen floor plains are observed to excavate the underlying primitive crater floor substrates which are probably more cohesive than the surface fine-grained debris flow materials, suggesting the emplaced ejecta material are at least ~250 m thick (the maximum excavation depth of the superposed craters: 0.084 * diameter45), close to the total thickness of local and foreign materials (Supplementary Table 1). We then calculated the areal abundance of plains terrains with distance from Schrödinger basin (Supplementary Fig. 10) as previous investigations of the Orientale basin found that the areal abundance of ejecta flows as light plains from the host basin reached ~20% as distance from the crater center increased to ~4 radii, and the areal abundance dropped below 10% beyond 4 radii46. We found that within one radius from the Schrödinger basin center, the areal abundance of plains terrains is very low (~5%; Supplementary Fig. 11), as the ejecta terrain is dominated by a hummocky, continuous ejecta deposit. The areal abundance increases to ~15% from one to two radii and remains at this high level out to a distance of four radii (Supplementary Fig. 11). We also investigated the areal abundance of light plains22 moving away from Schrödinger in the northern direction (opposite to the southern polar region), and observed similar patterns, thought the areal abundance of light plains stars to drop at locations relatively closer to the basin rim (~3 radii), likely due to the coverage of some plains by mare deposits (Supplementary Figs. 12 and 13). This observation is generally consistent with the interpretation of Orientale ejecta light plains46, supporting the hypothesis that Schrödinger basin is the major source crater for many smoother plains terrains in the lunar southern polar region. In addition, the maximum areal abundance of ejecta plains from Schrödinger (~15%) is slightly smaller than that of Orientale basin (~20%), indicating that other craters/basins might have also contributed considerable amounts of ejecta in the south polar region19, especially the Orientale basin, as it is stratigraphically younger than Schrödinger basin and its ejecta should have been superposed on and mixed with the Schrödinger ejecta.

Finally, we investigated the surface model ages of the Amundsen floor plains and Schrödinger basin using superposed crater population analyses. Are their ages similar or different? We utilized the catalog of kilometer-scale impact craters from ref. 42 to analyze the crater size-frequency distribution (CSFD) characteristics of the Amundsen floor plains and the Schrödinger impact melt deposits. Possible secondary craters were excluded according to their spatial distribution and morphological patterns (Supplementary Fig. 14). The resultant cumulative CSFD plot shows that the crater density of the two areas is comparable in the 1–2 km diameter range. For craters >2 km in diameter, the Schrödinger floor has a relatively higher cumulative crater density than the Amundsen floor, but the difference is generally within statistical errors (gray bars). Chronological fitting of crater populations yields similar model ages (~3.8–3.9 Ga, with difference well within the uncertainties of the crater population dating technique47) for floor terrains of Amundsen and Schrödinger basins (Supplementary Fig. 14).

In conclusion, the analysis results from ejecta thickness, areal abundance distribution, and crater population chronology all support Schrödinger basin as a major source region for many smooth plains terrains at southern polar region of the Moon.

Methods
Mapping of plains terrains
We identified and mapped plains terrains in the region within 10° latitude of the lunar south pole (i.e., the extent shown in Fig. 1) using maps of shaded relief, topographical slopes, and roughness calculated from high-resolution (20 m/pixel) altimetric gridded data from the Lunar Orbiter Laser Altimeter (LOLA)56. A shaded relief model of the lunar surface was produced from the LOLA topography data via mimicking the solar illumination effects (315° azimuth and 10° altitude angles). Such simulated shaded relief maps are commonly used for geomorphological analysis22,57, especially for lunar polar regions where conventional optical images often contain a large percentage of shaded areas. A topographic slope map was calculated using a moving 3 × 3 pixel window (60 m baseline) to identify topographically flat terrains. In addition, topographic roughness maps were independently employed to map flat and smooth areas. In this work, the root mean square (RMS) of height/elevation over the analysis baseline was calculated as the topographical roughness. We derived roughness maps over a wide range of baselines and found that the combination of roughness at baselines of 660 m, 1020 m, and 1820 m is particularly useful for identifying plains terrains. Roughness maps at these three baselines were used to generate a pseudo-color image with 1820 m-baseline in the red channel, 1020 m-baseline in the green, and 660 m-baseline in the blue (Supplementary Fig. 3). As plains terrains should be smooth at all the three baselines, they, therefore, appear dark (i.e., low roughness values at each baseline) in the color composite roughness map23. We manually delineated the boundary of each plains terrain at various scales (from 1:200,000 to 1:400,000) from LOLA topography-derived maps.

Geological characterization of plains terrains
We then analyzed the detailed geological characteristics of the mapped plains terrains using various types of orbital remote sensing data sets. LOLA gridded topography, and the derived slope and roughness maps are employed again to examine the topography of lunar polar plains occurrences and their comparison with the entire southern polar region. The LOLA shaded relief map, with assistance of the optical image mosaic with minimum shadows acquired by the Lunar Reconnaissance Orbiter (LRO) Wide-Angle Camera (WAC)58, were used to analyze the detailed morphology of the mapped polar plains terrains. We compared their morphology with that of analog sites elsewhere on the Moon to explore their potential substrate properties and emplacement mechanisms. In addition, the catalog of kilometer-scale impact craters from ref. 42 was used to investigate the crater populations superposed on representative polar plains to infer their chronology and potential source regions. During the crater population analysis, possible secondary impact craters or non-impact pits were eliminated on the basis of their known characteristics and morphologies59,60. Empirical radial thickness models of impact ejecta materials (ref. 44 and references therein) were utilized to access the potential material provenance of representative polar plains.

We also characterized the surface optical albedo of the mapped polar plains terrains and compared them with those of typical landforms of the Moon (southern polar region, mare, light plains, and the global Moon) using the LOLA active laser measurements25,61. This unique albedo data was acquired by the LOLA laser instrument through measuring the laser energy at the 1064 nm wavelength reflected from lunar surface. As LOLA carries its own light source, it has the unique ability of mapping the surface reflectivity of lunar surface that are obscured from solar illumination. This is especially useful for the lunar polar regions where the solar altitude is always very low and large areas are within permanent shadows. The gridded LOLA albedo data used in this work was sampled from original point measurements at a resolution of 1 km/pixel for polar regions and 10 pixels/degree (about 3 km/pixel at the equator) for the global Moon. In addition, we investigated and compared the surface iron elemental abundance of various lunar terranes using the gridded FeO map (pixel size of 0.5 degree, or ~15 km at the equator) acquired by the Lunar Prospector gamma ray spectrometer (GRS)27. In particular, the FeO abundance map of lunar polar regions (1 km/pixel) derived from the Kaguya Spectral Profiler (SP) and LOLA data and calibrated to the Lunar Prospector GRS FeO map26 was employed to characterize the surface chemical composition of the mapped polar plains, due to its significantly higher spatial resolution than Lunar Prospector GRS data. Optical albedo and iron content are key measurements for distinguishing typical lunar surface material types, as they are characterized by distinctly different optical and compositional signatures. Specifically, the LOLA albedo and WAC mosaic images are employed to search for impact craters with dark ejecta (dark-halo craters) on the mapped plains. We also use the Kaguya-SP and LRO-LOLA blended mineral abundance maps (1 km/pixel)26 to characterize potential mafic-rich regolith in the polar plains. The presence of dark-halo craters and mafic-rich regolith are important criteria to distinguish whether these plains are underlain by mare volcanic deposits (cryptomaria)62.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript \"Geological Evidence for Extensive Basin Ejecta as Plains Terrains in the Moon’s\nSouth Polar Region\" presents and analysis of distinct smooth plains units across the South Pole\nregion of the Moon, presenting clear argument for potential modes of origin and sources of these\nfeatures. The research presented in the manuscript is original, integrating across multiple datasets\nand using various models to develop a possible origin for the plains units found in the circumpolar\nregion. In general the data presented support the authors hypotheses, and the authors do a good\njob of explaining their interpretations. The figures presented in the manuscript are adequate, but\nthe authors may consider some revisions based on the comments below.\nLine 68: The use of Figure 1 to make the point that the South Pole is distinct from the other areas of\nexploration, is not quite clear from this figure especially since the data aren't geologic maps, so it\nmay be worth just saying that the area lacks the inferred volumes of volcanic material that have\nbeen sampled elsewhere?\nLines 68-77: It may be worth pointing out here that while the terrain is \"very rugged\" its no different\nthan other heavily cratered terrain on the Moon, like the FHT.\nLines 86 and 87: Here you compare the areas of high topography with the flat regions, it may be\nworth beginning to quantify what rugged and \"very flat\" are. The reason for this is to highlight that\nwile the \"rugged\" areas are indeed higher topographically, they are no different than any other\nportion of the ancient crust. There's a mis-conception in the community that the south pole is\nwildly different than the rest of the Moon (largely because of images like Figure 1a, which do a great\njob of showing the morphology, but also seem to suggest that the terrain is markedly different.\nLIne 82-85, 117: Figure 2: This figure is somewhat confusing, mixing shaded relief (hillside), slope,\nand albedo image data. I might suggest trying to directly compare Amundsen and either Aitken or\nColombo but not both.\nLine 127: I don't think you define it, but you should set what you mean by \"South Polar region\" as\nbeing areas poleward of 80 degrees south, just to be explicit.\nLine 130 - You should explain why you don't catalogue plains unites smaller than 1 km ^2.\nLine 141: Is there a way to define \"Smooth\" or is that subjective?\nLine 155-158: Are there any other reasons for the differences in the datasets used in the Meyer\nstudy vs. what is presented here? Are there differences in how units were defined?\nLine 190-200: Given that the various datasets used for this comparison (LP derived FeO, LOLA\nalbedo) have different spatial resolutions, are there any considerations for the cross-comparison of\nthe values we should be aware of?\nLine 244: Could you expand a little on why the Amundsen plains are not likely cyroptomare? Are\nCryptomare units included in Figure 5?\nLine 256: How many other craters did you examine for this comparison?\nLine 272: Did you examine any units within Orientale basin, or just those outside of the basin? How\nsimilar/dissimilar is the Montes Rook Formation of Orientale?\nLines 315-339: Here you calculate possible ejecta thicknesses to fill Amundsen, are there any\nmechanisms for calculating the actual thickness of the deposits in Amundsen? It does not appear\nthat there are any craters that excavate through the fill, but can you place a limit based on those\nthat are inside the crater?\nLine 350: It may be worth including a reference to figure S9 here.\nLine 356: This may not be a necessary figure for the main text.\nLine 348-387: It should be noted that the landing ellipse for Artemis may be much less than that of\nthe Luna 25 mission.\nOther notes: Are there any morphologies within the plains units that should be discussed? For\nexample, in the plains units between de Gerlache and Ibn Bajja are appear to resemble wrinkle\nridges?\nAre there any plains units within the 13 Artemis III Candidate regions, this might be worth\nmentioning in your Discussion section.", "reviewer_2": "This manuscript presents new mapping and analysis of light plains in the south polar region of the\nMoon. The authors interpret the origin of these plains as distal impact ejecta, especially from the\nSchrodinger basin, and discuss the potential of these plains to provide safe landing sites for future\nlanded exploration and the potential science advances that would come from investigating the\nplains.\nOverall, the new mapping results provide a modest update to past work, and will be of interest to\nthose examining the geology of the Moon’s south pole. The interpretations regarding the origin of\nthe plains are rather superficial, and do not substantially advance our understanding of their origin.\nAdditional consideration should be given to the contributions of the many other basins that will\nhave contributed to the origin of plains deposits in this region. Detailed comments follow.\nLines 68–75: The Moon’s south pole is not more rugged than other highland regions, and the text\nhere could perpetuate this myth that stems from the large shadows. “Very rugged” should be\nquantified or placed in context with other highland regions. Further, extensive flat terrain is not\nneeded to ensure a safe landing for many upcoming missions. The last sentence here is redundant\nwith earlier text in this section.\nLine 82–85: The comparison with mare-filled equatorial craters is fine, but the light plains in\nAmundsen and other polar craters are also morphologically similar to many equatorial light plains\nthat have previously been mapped. It is unclear why the comparison shown here is only to\nequatorial mare-filled craters. The introduction does not provide adequate context for the globally\nmapped light plains of, e.g., the Meyer et al. work that is cited later. The contrast stretch in Fig. 2c is\nalso very low; a better stretch would highlight the clear albedo difference between the mare and\nsurroundings.\nLine 87: The manuscript makes liberal use of the word “very” – e.g., here, “very flat” plains, but this\nword is open to wide interpretation. How much flatter, and over what baselines?\nLines 89–98: I am not sure that all of these questions “remain to be investigated.” This work\nprovides some modest advances, but all of these questions have been addressed to some degree\nin past work. (For example, it is generally well accepted that none of these regions are basalt or\ncryptomare, but that they originate as ejecta similar to the Apollo 16 region.)\nLine 111: add “(b)” before LOLA DEM. Was DEM defined earlier in the text?\nLine 128: With plains ≥1 km2, I think some of the data sets would not have sufficient resolution to\nreally characterize the smallest areas?\nLine 155: Did the scale at which Meyer et al. mapped features (1:300K) also contribute to the\ndifference?\nFigure 4: Image source? LROC?\nLine 173–175: Is there a typo here? It seems that –7000 to –6700 m is not a wide range and should\nmaybe be +6700 m based on Fig. S3a?\nLines 214–222: If the Schrodinger plains include distinctive features not shared with the other\nplains they are being grouped with, perhaps a Schrodinger is not an appropriate type example, and\ninstead one of the others could be used. Also, there is no figure given that provides a good view of\nthe two types of plains and the morphologies that distinguish them, which is needed (Fig. 4a and 4c\ncould potentially serve this purpose with additional info in the caption or arrows added to highlight\nkey features).\nLine 228: I’m surprised to see Shackleton mentioned here. Its floor is hummocky and not plains\nlike. It also doesn’t seem to be mapped as plains in Fig. 3.\nLine 237: Is “flowable” a word? Fluid? Materials that can flow to reach an ~equipotential surface?\nLine 249: The ballistic sedimentation process seems to include low-angle ejecta sweeping and\nchurning up the surface, so describing this as “impact ejecta flows” is a bit of a simplification or\ncould give readers the wrong mental image of how the plains may have been emplaced.\nLine 265: “On the basis of these analog and process studies” – which studies? Alternatives – such\nas melt from the Amundsen impact event – are dismissed rather out-of-hand. This is a large impact\ncrater and certainly part of the flat floor is due to impact melt. Maybe later impacts mixed/covered\nit, but surely some of the material is Amundsen melt. (Cryptomelt? Just kidding! No need to invent a\nterm, but a concept similar to cryptomare.) This section is example of some of the over-\nsimplification in this manuscript.\nLine 286: Many references on impact melt that could be helpful to readers if included here.\nLine 299: Typo - extra s on younger\nLine 301: Craters included range from Eratoshenian to Nectarian – it does not seem appropriate to\ndescribe their ages as “young” and it is surprising that a Nectarian-age crater has primary melt-\nrelated morphology preserved. There are only nine plains of this type – a supplemental figure that\nshows the morphology of each would be helpful. (Also n=9, but 2 Eratosthenian + 5 Imbrian + 1\nNectarian = 8.)\nLines 307–309: The description here ignores mixing in of local materials when the distal ejecta is\nemplaced (see, e.g., Oberbeck work and application by Petro and Pieters). It is not simply a blanket\nof distal ejecta material, but thought to be a mix that includes varying portions of the local\nsubstrate. I think this is an important point to highlight for sample provenance.\nLine 334 and surroundings: Schrodinger is likely a major source of light plains and one of the most\nrecent significant contributors, but the way this is presented is very definitive and ignores other\nlikely contributors to the plains in the region. There appears to be light plains/ejecta covering some\nof the mare within Schrodinger so some large impact event affected the region post-Schrodinger\n(Orientale?), Orientale definitely affected the area to some extent, and the many basins around the\narea must have also contributed. This should be described to acknowledge the complexity.\nLines 350–351: The ages of Schrodinger and Amundsen plains are similar, but it seems a stretch to\ncall them nearly identical. I’m a skeptic of the small model age error values, but 3.8±0.02 Ga does\nnot overlap 3.9±0.01 Ga, and this issue is not discussed. If those values are to be believed,\nSchrodinger is older than Amundsen and could not have contributed at all to the plains in the\ninterior. Further, the Schrodinger count area is complex with many secondaries. There is a ghost\ncrater (filled in with light plains, see screen grab in pdf) that suggests the area has been resurfaced\nafter some amount of time during which that crater formed post-formation of Schrodinger. If this is\nthe case, then the Schrodinger impact event is even older than the surface age this count area\nwould indicate.\nGeneral comment on “Results” section: includes lots of interpretation that should really be in the\n“Discussion” section.\nLine 365: popular targets for future/planned exploration (none of these missions to the south polar\nregion have yet happened)\nLine 366: Large-scale rugged topography compared to what – the mare? ~Similar to highlands.\nLine 396: “areally extensive areas” – remove areally\nLine 412: The sentence starting with “In addition” is a non sequitur from what is discussed in the\nrest of the paragraph.\nLine 421: “Adequate solar energy” will be mission dependent.\nLine 428 (and abstract and line 431 and 438): I think describing the ejecta as “foreign” and having\n“contaminated” is the wrong framing. Ejecta materials from possibly distant impact craters/basins,\nwith similar highland composition. This is true basically anywhere in the lunar highlands. The final\nparagraph here again discusses local material being “buried” but local material is thought to be\nincorporated into the deposit via ballistic sedimentation.\nLines 467–471: I believe this red-green-blue method is similar to Meyer et al., if so should be\nreferenced.\nLine 505: Lunar Prospector data resolution is altitude dependent, 0.5 degrees must be equatorial\ndegrees because the resolution is more like 30 km not 2.6 km. Thus using LP FeO data is likely not\nreally appropriate for distinguishing among the compositions of small plains deposits."}, "author_response": "Response: We sincerely thank reviewer #1 for the very supportive and helpful comments. All\nthese comments are responded to below.\nLine 68: The use of Figure 1 to make the point that the South Pole is distinct from the other areas of\nexploration, is not quite clear from this figure especially since the data aren't geologic maps, so it may be worth just saying that the area lacks the inferred volumes of volcanic material that have\nbeen sampled elsewhere?\nResponse: Yes, the reviewer is correct, and we have clarified this in the revised manuscript.\nLines 68-77: It may be worth pointing out here that while the terrain is \"very rugged\" its no different\nthan other heavily cratered terrain on the Moon, like the FHT.\nResponse: Yes, we agree and have incorporated this point.\nLines 86 and 87: Here you compare the areas of high topography with the flat regions, it may be worth\nbeginning to quantify what rugged and \"very flat\" are. The reason for this is to highlight that wile the\n\"rugged\" areas are indeed higher topographically, they are no different than any other portion of the\nancient crust. There's a mis-conception in the community that the south pole is wildly different than the\nrest of the Moon (largely because of images like Figure 1a, which do a great job of showing the\nmorphology, but also seem to suggest that the terrain is markedly different.\nResponse: We have quantified the topographic slopes of the mapped plains and the entire\nsouthern polar region using both median values (slope: ~3° vs. ~8°; roughness at 660 m\nbaseline: ~7 m vs. 25 m) and histogram plots (Supplementary Fig. 1) in the revised manuscript.\nYes, we agree with reviewer that the southern polar region is topographically similar to “any\nother portion of the ancient crust”, as we have addressed in the response to the “Lines 68-77”\ncomments above.\nLine 82-85, 117: Figure 2: This figure is somewhat confusing, mixing shaded relief (hillside), slope, and\nalbedo image data. I might suggest trying to directly compare Amundsen and either Aitken or\nColombo but not both.\nResponse: Thanks for this very helpful suggestion.\nWe have labelled the location and source data for the Fig. 2 maps, and removed the Aitken map\nin the revised manuscript. We also added a map for the Plummer crater that contains light\nplains on the floor, according to one of the comments (Line 82–85) from Reviewer #2.\nLine 127: I don't think you define it, but you should set what you mean by \"South Polar region\" as\nbeing areas poleward of 80 degrees south, just to be explicit.\nResponse: We have defined the studied area (southward of 80° latitude) in the last paragraph of\nthe Introduction section of the revised manuscript.\nLine 130 - You should explain why you don't catalogue plains unites smaller than 1 km ^2.\nResponse: We have added the explanation that the minimal size of the mapped plains units is\nlimited by the 20–1000 m/pixel spatial resolution of the various data sets used in this work (see\nrevised Methods).\nLine 141: Is there a way to define \"Smooth\" or is that subjective?\nResponse: We have referred to the RMS height roughness calculated in this work to quantify\nthe smoothness (topographical roughness typically < 10 m at 660 m baseline) in the revised\nmanuscript.\nLine 155-158: Are there any other reasons for the differences in the datasets used in the Meyer study\nvs. what is presented here? Are there differences in how units were defined?\nResponse: We addressed the point that the difference in the mapping scale (1:300K vs. various\nscales from 1:200K to 400K) was additionally one of the reasons, but the major reason remains\nto the new LOLA data sets with much higher resolution (20 m/pixel) and better spatial coverage\nemployed in this work, compared with the 100 m/pixel LROC WAC data used by Meyer et al.,\nwith considerable shadowed areas.\nThe definition of plains units in this work is similar to that of the Meyer study, and we have\nspecifically added the citation to the Meyer study in the Methods section.\nLine 190-200: Given that the various datasets used for this comparison (LP derived FeO, LOLA\nalbedo) have different spatial resolutions, are there any considerations for the cross-comparison of the\nvalues we should be aware of?\nResponse: Thanks for this very constructive suggestion.\nWe have reanalyzed the FeO abundance of the polar region using the new FeO abundance map\nof lunar polar regions derived from the Kaguya Spectral Profiler (SP) and LOLA data and\ncalibrated to the Lunar Prospector GRS FeO map (Lemelin et al., 2022), which has the same 1\nkm/pixel spatial resolution of the LOLA albedo map.\nIn addition, we analyzed the FeO abundance of non-polar regions (the global Moon, maria, and\nlight plains) using the LP GRS data, which indeed has a much lower resolution. Cross-\ncomparison between the two FeO datasets (cf. Fig 5b and Supplementary Fig. 4) indeed reveals\na small discrepancy in the FeO abundance (e.g., 7.2% vs. 6.5% median value for the southern\npolar region). We have discussed this issue in the revised manuscript (Line 207-212).\nLine 244: Could you expand a little on why the Amundsen plains are not likely cyroptomare? Are\nCryptomare units included in Figure 5?\nResponse: Thanks for this very constructive comment concerning cryptomeria as a candidate\nfor the nature and origin of the Amundsen plains. We have addressed this concern through\nadditional data analyses and geological deductions in the revised manuscript:\n(1) We systematically search for impact craters with dark ejecta (dark-halo craters) that might\nindicate the presence of cryptomaria on the mapped polar plains using both LOLA albedo\nand WAC mosaic images;\n(2) We employ mineral abundance maps derived from the Kaguya Spectral Profiler (SP) and\nLOLA data to characterize potential mafic-rich regolith that might indicate the presence of\ncryptomaria in the polar plains;\n(3) On the basis of the presence/absence of dark-halo crater and mafic-rich regolith, we\ndetermine whether the evidence suggests that the mapped polar plains are underlain by\nmare volcanic deposits (cryptomaria);\nOur detailed investigations do not observe any dark-halo impact craters (consistent with\nIzquierdo et al., 2024, JGR-Planets, 129, e2023JE007867) or mafic-rich regolith signature in the\nplains terrains that we studied, providing no evidence of mare or cryptomaria for the origin of\nthese plains. This result is also consistent with the deficiency of volcanic deposits detected in\nthe polar regions of the Moon (e.g., Krasilnikov et al., 2023, Icarus, 394, 115422; Broquet &\nAndrews-Hanna, 2024, Icarus, 411, 115954; Head et al., 2023, Reviews in Mineralogy &\nGeochemistry, 89, 453-507).\nLine 256: How many other craters did you examine for this comparison?\nResponse: Thanks for this very helpful comment. We have examined all impact craters (n=187\naccording to the catalogue of Head et al. (2010, Science, 329, 1504-1507)) with sizes (90–110 km\ndiameter) similar to Amundsen crater (103 km diameter). We have added this description in the\nrevised manuscript.\nLine 272: Did you examine any units within Orientale basin, or just those outside of the basin? How\nsimilar/dissimilar is the Montes Rook Formation of Orientale?\nResponse: Thanks for this very insightful comment. We only investigated the units outside of\nthe Orientale basin in the initial manuscript.\nWe have examined the Maunder (inside the ~480 km diameter Inner Rook Ring) and Montes\nRook (between the ~930 km diameter Cordillera Ring and the ~620 km diameter Outer Rook\nRing) Formations of Orientale in the revision, and indeed found that many units are\nmorphologically similar with the Schrödinger-type plains terrains mapped in our study:\npredominately occurring in topographically low regions, and consisting mainly of flat-lying\nmaterials and km-scale blocks.\nThis new comparison provides additional important insights into the nature and emplacement\nmechanism of the Schrödinger-type polar plains. These units of the Maunder and Montes Rook\nFormations have been commonly interpreted as clast-rich melt materials deposited at or just\nbeyond the extent of the transient cavity for Orientale (Head, 1974; Scott et al. 1977), consistent\nwith our local impact melt interpretation for these Schrödinger-type plains. We have\nincorporated these new analyses and a new map of units in the Montes Rook Formation (Fig.\n4e) in the revised manuscript.\nLines 315-339: Here you calculate possible ejecta thicknesses to fill Amundsen, are there any\nmechanisms for calculating the actual thickness of the deposits in Amundsen? It does not appear that\nthere are any craters that excavate through the fill, but can you place a limit based on those that are\ninside the crater?\nResponse: Thanks for this very constructive comment. We have used the excavation depths\n(0.084 times crater rim-to-rim diameter) of the superposed impact craters (up to ~2.9 km in\ndiameter) to constrain the upper limit (~250 m) of the thickness of Amundsen floor plain\nmaterials.\nLine 350: It may be worth including a reference to figure S9 here.\nResponse: Yes, incorporated.\nLine 356: This may not be a necessary figure for the main text.\nResponse: We feel that these maps are essential for the analog studies to interpret the\nformation mechanism of the mapped plains terrains. To make such comparison more apparent\nand reduce the paper length, we have merged it into Fig. 4 in the revised manuscript.\nLine 348-387: It should be noted that the landing ellipse for Artemis may be much less than that of the\nLuna 25 mission.\nResponse: Thanks for this very useful suggestion. We have added a note that the landing\nellipse for NASA’s Artemis program is probably much smaller, at a scale of ~100 m (e.g.,\nChavers et al., 2020, 2020 IEEE Aerospace Conference, 1-9), in the revised manuscript.\nOther notes: Are there any morphologies within the plains units that should be discussed? For\nexample, in the plains units between de Gerlache and Ibn Bajja are appear to resemble wrinkle\nridges?\nResponse: Thanks for this very helpful comment. The wrinkle ridges in the lunar south polar\nregion definitely deserve dedicated studies, like this one between de Gerlache and Ibn Bajja, as\nmany lunar wrinkle ridges are usually interpreted to be related to underlying layered strata,\nespecially layered mare basalts. We have examined the distribution of wrinkle ridges using\nvarious data sets including images, topography, and spectroscopy, and found no evidence of\ncorrelation with buried mare basalts, consistent with the geological mapping of Krasilnikov et\nal. (2023, Icarus, 394, 115422).\nAre there any plains units within the 13 Artemis III Candidate regions, this might be worth mentioning in\nyour Discussion section.\nResponse: Thanks for this very constructive comment.\nWe examined the 13 Artemis III candidate regions (https://www.nasa.gov/news-release/nasa-\nidentifies-candidate-regions-for-landing-next-americans-on-moon/) and found that 8 of them\ncontained the plains terrains mapped in this work, though these plains are typically very small\nin size (several km) and occurs at the marginal low-lying portion of the candidate regions. We\nhave added these discussions and a new map (Supplementary Fig. 13) of the Peak Near\nShackleton region in the supplement to illustrate this observation.\nResponse: We sincerely acknowledge Reviewer #2 for the very supportive and constructive\ncomments. All these comments are responded to below.\nSpecifically, as we have responded to above, we have thoroughly addressed the concerns\nconcerning the interpretations regarding the origin of the plains, through additional data\nanalyses and geological deductions in the revised manuscript:\n(1) We systematically search for impact craters with dark ejecta (dark-halo craters) that might\nindicate the presence of cryptomaria on the mapped polar plains using both LOLA albedo\nand WAC mosaic images;\n(2) We employ mineral abundance maps derived from the Kaguya Spectral Profiler (SP) and\nLOLA data to characterize potential mafic-rich regolith that might indicate the presence of\ncryptomaria in the polar plains;\n(3) On the basis of the presence/absence of dark-halo crater and mafic-rich regolith, we\ndetermine whether the evidence suggests that the mapped polar plains are underlain by\nmare volcanic deposits (cryptomaria);\n(4) We explored alternative scenarios for the nature and origin of the studied plains terrains,\nincluding both exposed (maria) and obscured (cryptomaria) basaltic volcanism;\n(5) We also discussed the challenges and limitations for identifying surface albedo variations\nin the polar regions using typical conventional optical images due to considerable shadows\nand typically very low Sun illumination.\nOur detailed investigations do not observe any dark-halo impact craters (consistent with\nIzquierdo et al., 2024, JGR-Planets, 129, e2023JE007867) or mafic-rich regolith signature in the\nplains terrains that we studied, providing no evidence of mare or cryptomaria for the origin of\nthese plains. This result is also consistent with the deficiency of volcanic deposits detected in\nthe polar regions of the Moon (e.g., Krasilnikov et al., 2023, Icarus, 394, 115422; Broquet &\nAndrews-Hanna, 2024, Icarus, 411, 115954; Head et al., 2023, Reviews in Mineralogy &\nGeochemistry, 89, 453-507).\nLines 68–75: The Moon’s south pole is not more rugged than other highland regions, and the text here\ncould perpetuate this myth that stems from the large shadows. “Very rugged” should be quantified or\nplaced in context with other highland regions. Further, extensive flat terrain is not needed to ensure a\nsafe landing for many upcoming missions. The last sentence here is redundant with earlier text in this\nsection.\nResponse: Yes, we agree with the reviewer that “the Moon’s south pole is not more rugged\nthan other highland regions”, and we have incorporated this in the revised manuscript.\nWe have also quantified the “ruggedness” of the Moon’s south pole here and at multiple\nlocations in the revised manuscript, and reworded the last sentence.\nLine 82–85: The comparison with mare-filled equatorial craters is fine, but the light plains in Amundsen\nand other polar craters are also morphologically similar to many equatorial light plains that have\npreviously been mapped. It is unclear why the comparison shown here is only to equatorial mare-filled\ncraters. The introduction does not provide adequate context for the globally mapped light plains of,\ne.g., the Meyer et al. work that is cited later. The contrast stretch in Fig. 2c is also very low; a better\nstretch would highlight the clear albedo difference between the mare and surroundings.\nResponse: Thanks for these very constructive suggestions.\nWe have (1) added a map for an equatorial crater (Plummer, Fig. 2d) that was filled with light\nplains mapped previously (Meyer et al., 2020), (2) added adequate context for the globally\nmapped light plains (e.g., Wilhelms et al., 1979; Meyer et al., 2020) in the introduction section,\nand (3) remapped Fig. 2c using WAC morphological mosaic (other than the low-sun mosaic in\nthe initial manuscript) for better stretching the albedo difference.\nLine 87: The manuscript makes liberal use of the word “very” – e.g., here, “very flat” plains, but this\nword is open to wide interpretation. How much flatter, and over what baselines?\nResponse: We have quantified these descriptions here and in multiple locations in the revised\nmanuscript.\nLines 89–98: I am not sure that all of these questions “remain to be investigated.” This work provides\nsome modest advances, but all of these questions have been addressed to some degree in past work.\n(For example, it is generally well accepted that none of these regions are basalt or cryptomare, but that\nthey originate as ejecta similar to the Apollo 16 region.)\nResponse: Thanks for this very helpful suggestion. We have reworded it to “remain to be\nfurther investigated” to make it more accurate.\nAlso, as reflected by one of the Reviewer #1’s comments (Line 244), we are not sure “that they\nare not cryptomaria” is “generally accepted”. On the basis of discussions at the recent Lunar\nand Planetary Science Conference 55, we believe that some people in the community are still\nnot convinced, and thus we prefer to leave this as is.\nLine 111: add “(b)” before LOLA DEM. Was DEM defined earlier in the text?\nResponse: Thanks. As the term DEM was only used once in the manuscript, we have changed it\nto “gridded” in the revised manuscript.\nLine 128: With plains ≥1 km2, I think some of the data sets would not have sufficient resolution to really\ncharacterize the smallest areas?\nResponse: Thanks for this important comment.\nWe have reanalyzed the FeO abundance of the polar region using the new Kaguya-SP and LRO-\nLOLA blended FeO map (1 km/pixel, Lemelin et al., 2022), rather than the 0.5 degree/pixel LP\nGRS data used in the initial manuscript. Now, the various data sets used in this work have pixel\nsizes ranging from 20 m to 1 km, sufficient to resolve all plains ≥1 km2.\nLine 155: Did the scale at which Meyer et al. mapped features (1:300K) also contribute to the\ndifference?\nResponse: Yes, we agree. We have added this explanation in the revised manuscript.\nFigure 4: Image source? LROC?\nResponse: We have added descriptions of the image sources (LOLA shaded relief and LROC\nWAC mosaic) in each panel and the figure caption.\nLine 173–175: Is there a typo here? It seems that –7000 to –6700 m is not a wide range and should\nmaybe be +6700 m based on Fig. S3a?\nResponse: Thanks for the careful review. Yes, it should be +6700 m here. We have fixed this\nerror.\nLines 214–222: If the Schrodinger plains include distinctive features not shared with the other plains\nthey are being grouped with, perhaps a Schrodinger is not an appropriate type example, and instead\none of the others could be used. Also, there is no figure given that provides a good view of the two\ntypes of plains and the morphologies that distinguish them, which is needed (Fig. 4a and 4c could\npotentially serve this purpose with additional info in the caption or arrows added to highlight key\nfeatures).\nResponse: Yes, we agree with the reviewer on the distinctive features on the Schrödinger\nplains. However, the other mound-bearing plains (~1–102 km2) are over one order of magnitude\nsmaller than the Schrödinger plains (~8576 km2 mapped in a small part of the Schrödinger\nfloor). The other plains typically occupy only a very small portion of the floor of the host crater\n(Supplementary Figs. 5 and 6) and their morphological characteristics are not as apparent and\nrepresentative as the Schrodinger floor plains. Therefore, we prefer to retain Schrödinger as a\ntype example.\nWe have added descriptions and arrows in Figs. 4a and 4c to highlight key features of these\nplains.\nLine 228: I’m surprised to see Shackleton mentioned here. Its floor is hummocky and not plains\nlike. It also doesn’t seem to be mapped as plains in Fig. 3.\nResponse: Yes, the reviewer is correct that the Shackleton floor is mainly hummocky, but two\nvery small areas (both ~1.5 km2) are relatively flat and mapped as plains (Supplementary Fig.\n6g; also see figure below).\nThe Shackleton floor plains have actually been mapped (color patches) in Fig. 3, but it is too\nsmall to be seen in the initial manuscript. We have re-made the Fig. 3 map (add outlines) to\nmake it more visible.\nLine 237: Is “flowable” a word? Fluid? Materials that can flow to reach an ~equipotential surface?\nResponse: Thanks. We have replaced it with “fluidized”.\nLine 249: The ballistic sedimentation process seems to include low-angle ejecta sweeping and\nchurning up the surface, so describing this as “impact ejecta flows” is a bit of a simplification or could\ngive readers the wrong mental image of how the plains may have been emplaced.\nResponse: Thanks for this very insightful comment.\nWe agree that many ejecta materials would excavate and mix with preexisting terrain materials\nwhen they re-impacted the lunar surface. The mixtures of distant and local debris would move\nlaterally and form the plains terrains (e.g., Oberbeck, 1975, Reviews of Geophysics, 13, 337-\n362). We have re-worded it to “impact debris flows consisting of pre-impact substrate and\nejecta materials” to acknowledge the complexity.\nLine 265: “On the basis of these analog and process studies” – which studies? Alternatives – such\nas melt from the Amundsen impact event – are dismissed rather out-of-hand. This is a large impact\ncrater and certainly part of the flat floor is due to impact melt. Maybe later impacts mixed/covered\nit, but surely some of the material is Amundsen melt. (Cryptomelt? Just kidding! No need to invent a\nterm, but a concept similar to cryptomare.) This section is example of some of the over-\nsimplification in this manuscript.\nResponse: We meant the studies described earlier in this paragraph: comparison of the\nmorphologies of Amundsen-type plains terrains (Figs. 4a,b) and light plains radial to the\nOrientale basin (Fig. 4d), and the related geological deductions.\nYes, we agree with reviewer on the underlying melt materials, and we have added such\ndiscussions in the revised manuscript.\nLine 286: Many references on impact melt that could be helpful to readers if included here.\nResponse: Yes, we agree, and we have added the Osinski et al. (2011, EPSL, 310, 167-181)\nreference. Due to the reference number limit of the journal, we cannot add more references.\nLine 299: Typo - extra s on younger\nResponse: fixed.\nLine 301: Craters included range from Eratoshenian to Nectarian – it does not seem appropriate to\ndescribe their ages as “young” and it is surprising that a Nectarian-age crater has primary melt-\nrelated morphology preserved. There are only nine plains of this type – a supplemental figure that\nshows the morphology of each would be helpful. (Also n=9, but 2 Eratosthenian + 5 Imbrian + 1\nNectarian = 8.)\nResponse: Thanks for the very careful review.\nWe have (1) added a supplemental figure (Supplementary Fig. 6) that shows the morphology of\nall the 9 Schrödinger-type plains, and (2) re-checked our result and fixed the count error: 2\nEratosthenian + 5 Imbrian + 2 Nectarian = 9, in the revised manuscript.\nWe addressed the point that the host craters of Schrödinger-type plains are relatively younger\nthan those of Amundsen-type plains (Nectarian and Pre-Nectarian).\nThe two Nectarian-age craters are Idel'son (57 km diameter, centered at 81.2°S, 112.7°E;\nSupplementary Fig. 6f) and one unnamed crater between Casatus D and Newton E (19 km,\n78.6°S, 39.9°W; Supplementary Fig. 6a). We address the point that the stratigraphy of these\ncraters is from the geological maps of Fortezzo et al. (2020) and Krasilnikov et al. (2023a), and\nthese stratigraphical assignments are not of problem (e.g., McEwen et al., 1993, JGR, 98(E9),\n17207–17231; Neukum & König, 1976, LPSC; Xie et al., JGR-Planets, 125, e2019JE006112; Wang\net al., 2020, JGR-Planets, 125, e2019JE006091). For instance, the unnamed 19-km-diameter\ncrater has relatively pristine appearances on both LOLA shaded relief and WAC image maps,\nsuggesting that it is probably younger than the assigned Nectarian age.\nWe agree with the reviewers on the preservation of melt-related morphology in Nectarian-aged\ncraters, but impact melt morphologies have been previously mapped in Nectarian-aged craters\n(e.g., the Crisium basin; Spudis & Sliz, 2017, GRL, 44, 1260–1265; Runyon et al., 2020, JGR-\nPlanets, 125, e2019JE006024).\nWe have addressed these issues in the revised manuscript.\nLines 307–309: The description here ignores mixing in of local materials when the distal ejecta is\nemplaced (see, e.g., Oberbeck work and application by Petro and Pieters). It is not simply a blanket of\ndistal ejecta material, but thought to be a mix that includes varying portions of the local substrate.\nI think this is an important point to highlight for sample provenance.\nResponse: Yes, we totally agree with the reviewer on this very important point. We have\naddressed the mixing of local materials and implications for sample provenance throughout\nthe revised manuscript.\nLine 334 and surroundings: Schrodinger is likely a major source of light plains and one of the most\nrecent significant contributors, but the way this is presented is very definitive and ignores other\nlikely contributors to the plains in the region. There appears to be light plains/ejecta covering some of\nthe mare within Schrodinger so some large impact event affected the region post-Schrodinger\n(Orientale?), Orientale definitely affected the area to some extent, and the many basins around the\narea must have also contributed. This should be described to acknowledge the complexity.\nResponse: Thanks for this very constructive comment.\nYes, we agree with the reviewer that additional basins should have also contributed to the\nmapped plains materials, especially the Orientale basin, as it is younger and its ejecta should\nhave been superposed on and mixed with the Schrödinger ejecta.\nWe have acknowledged the complexity in the revised manuscript.\nLines 350–351: The ages of Schrodinger and Amundsen plains are similar, but it seems a stretch to\ncall them nearly identical. I’m a skeptic of the small model age error values, but 3.8±0.02 Ga does not\noverlap 3.9±0.01 Ga, and this issue is not discussed. If those values are to be believed, Schrodinger is\nolder than Amundsen and could not have contributed at all to the plains in the interior. Further, the\nSchrodinger count area is complex with many secondaries. There is a ghost crater (filled in with light\nplains, see screen grab in pdf) that suggests the area has been resurfaced after some amount of time\nduring which that crater formed post-formation of Schrodinger. If this is the case, then the Schrodinger\nimpact event is even older than the surface age this count area would indicate.\nResponse: Thanks for these very careful and constructive comments.\nWe agree with the reviewer on the issues concerning the small model age error values. We\nhave addressed that the difference between the model ages of Schrodinger and Amundsen\nplains are well within the uncertainties of the crater population dating technique (e.g., Fassett,\n2016 and many references therein) in the revised manuscript.\nWe have checked the detailed morphology of the potential “ghost” crater using high-resolution\nKaguya TC (10 m pixel size, upper figure) and LROC NAC (~1 m pixel size, middle figure)\nimages, and did not observe the embayment of the crater rim by the adjacent materials (an\nexample of a ghost crater at 40.39°N, 51.21°W is shown in the lower figure). We attribute its flat\nfloor to the highly degradation morphology of this crater.\nGeneral comment on “Results” section: includes lots of interpretation that should really be in the\n“Discussion” section.\nResponse: Thanks for this very helpful suggestion, and we have moved some interpretations to\nthe “Discussion” section.\nLine 365: popular targets for future/planned exploration (none of these missions to the south polar\nregion have yet happened)\nResponse: Yes, and we have reworded this.\nLine 366: Large-scale rugged topography compared to what – the mare? ~Similar to highlands.\nResponse: Yes, it is compared with mare plains that many previous missions landed on, and\nwe have reworded this.\nLine 396: “areally extensive areas” – remove areally\nResponse: Fixed.\nLine 412: The sentence starting with “In addition” is a non sequitur from what is discussed in the rest of\nthe paragraph.\nResponse: We have reworded it.\nLine 421: “Adequate solar energy” will be mission dependent.\nResponse: Yes, and we have reworded it.\nLine 428 (and abstract and line 431 and 438): I think describing the ejecta as “foreign” and having\n“contaminated” is the wrong framing. Ejecta materials from possibly distant impact craters/basins,\nwith similar highland composition. This is true basically anywhere in the lunar highlands. The final\nparagraph here again discusses local material being “buried” but local material is thought to be\nincorporated into the deposit via ballistic sedimentation.\nResponse: Thanks for this very helpful suggestion. We have reworded the manuscript using\n“distant” instead of “foreign” and “contributed” instead of “contaminated”.\nLines 467–471: I believe this red-green-blue method is similar to Meyer et al., if so should be\nreferenced.\nResponse: Yes, the reviewer is correct. We have added the citation to the Meyer study in the\nMethods section.\nLine 505: Lunar Prospector data resolution is altitude dependent, 0.5 degrees must be equatorial\ndegrees because the resolution is more like 30 km not 2.6 km. Thus using LP FeO data is likely not\nreally appropriate for distinguishing among the compositions of small plains deposits.\nResponse: Yes, we agree and have changed the description of the LP FeO data resolution to\n“pixel size of 0.5 degree, or ~15 km at the equator”.\nWe have reanalyzed the FeO abundance of the polar region using the new Kaguya-SP and LRO-\nLOLA blended FeO map (1 km/pixel, Lemelin et al., 2022), other than the 0.5 degree/pixel LP\nGRS data used in the initial manuscript. The new FeO data should have sufficient resolution to\nresolve all plains ≥1 km2 mapped in this work."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have made important updates that improved the manuscript.\nI think the authors may have missed updating the abstract - it still talks about \"ejecta flows\" and the\nsentence about \"the entire lunar south polar region should have been contributed by distant basin\nmaterials\" should be revised to better match the rest of the text.\nLine 249 - suggest changing to plural - Origins and Source Regions\nLine 342: update to \"What are the source regions of the distal materials?\" since the sentence before\nrefers to local and distal material.\nLine 358-360: How would you tell if a superposed crater excavated Amundsen crater floor vs.\nsuperposed plains? Compositions are likely very similar, I think it is not possible to make this claim.\nInstead you could note that if the light plains are as thick as predicted from ejecta models, none of\nthe superposed craters are large enough to have excavated through them (if that is the case).\nLine ~370: Another check of the Schrödinger idea would be to see if the abundance of light plains\nalso follows this pattern moving away from Schrödinger in another direction. I understand the\nauthors only mapped the polar area, but this should also be observed in the Meyer et al map.\nLine 396: I suggest changing from \"as the major source\" to \"as a major source\". Picky, but allows for\nother major sources like Orientale.\nLine 411: candidate landing regions, rather than sites."}, "author_response": "Response: Thanks for these very careful reviews.\nWe have changed \"ejecta flows\" to \" debris flows\", and “… should have been …” to “probably\nhave been” to better match the rest of the text (e.g., Lines 273 and 472 in the manuscript).\nLine 249 - suggest changing to plural - Origins and Source Regions\nLine 342: update to \"What are the source regions of the distal materials?\" since the sentence before\nrefers to local and distal material.\nResponse: Yes, we have incorporated these two revisions.\nLine 358-360: How would you tell if a superposed crater excavated Amundsen crater floor vs.\nsuperposed plains? Compositions are likely very similar, I think it is not possible to make this claim.\nInstead you could note that if the light plains are as thick as predicted from ejecta models, none of the\nsuperposed craters are large enough to have excavated through them (if that is the case).\nResponse: Yes, we agree that the compositions of the crater floor and superposed plains\nshould be very similar. While we think that the primitive crater floor substrates are probably\nmore cohesive than surface fine-grained debris flow materials. Impacts into these layered\ntargets could form craters with unique morphologies, e.g., crater floors of concentric features.\nWe have added this explanation in the revised manuscript.\nLine ~370: Another check of the Schrödinger idea would be to see if the abundance of light plains\nalso follows this pattern moving away from Schrödinger in another direction. I understand the\nauthors only mapped the polar area, but this should also be observed in the Meyer et al map.\nResponse: Thanks for this very constructive suggestion.\nWe have added the suggested analysis and indeed found similar patterns, thought the areal\nabundance of light plains stars to drop at locations relatively closer to the basin rim (~3 radii),\nlikely due to the coverage of some plains by mare deposits (Supplementary Figs. 13 and 14).\nWe have incorporated these analyses and interpretations in the revised manuscript.\nLine 396: I suggest changing from \"as the major source\" to \"as a major source\". Picky, but allows for\nother major sources like Orientale.\nLine 411: candidate landing regions, rather than sites.\nResponse: Yes, we have incorporated these two revisions."}]

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