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

The Moon's polar regions, containing water ice and other volatiles, are crucial for lunar geology and resource utilization. Future landed missions, such as NASA's Artemis III and the International Lunar Research Station, necessitate identifying suitable landing sites and understanding the provenance of surface materials. The lunar south pole, adjacent to the South Pole-Aitken (SPA) basin, offers unique opportunities to study deep lunar materials and early bombardment history. However, unlike the flat mare plains of previous landing sites, the south polar region is characterized by rugged, heavily cratered highland terrains. This study focuses on identifying and characterizing relatively flat plains terrains in this region to assess their suitability as landing sites for future missions. The research addresses key questions about the spatial distribution, geological characteristics (topography, morphology, composition, chronology), origin, and exploration value (volatile prospecting and other scientific opportunities) of these polar plains. The investigation uses high-resolution topographic and compositional data sets, combined with analog studies, to determine the formation mechanism and potential source regions of these plains. The successful Apollo missions serve as a model for the importance of geological understanding in supporting mission design and interpretation.

Previous geological mapping efforts have identified small plains terrains in the lunar south polar region, particularly within craters. These plains are morphologically similar to plains-filled craters in equatorial regions. While these polar plains appear topographically flat and attractive for landing, their origin and composition remain open questions. Are they volcanic, similar to cryptomaria, or are they impact-related? The existing literature provides some preliminary observations, but a comprehensive analysis of their spatial distribution, geological characteristics, and formation mechanism is lacking, making this study essential for future mission planning.

This study uses high-resolution data from the Lunar Orbiter Laser Altimeter (LOLA) to identify and map plains terrains south of 80°S latitude. The mapping process involved analyzing shaded relief, slope, and roughness maps derived from LOLA altimetry data at various scales (1:200,000 to 1:400,000). The researchers identified 873 plains terrains with surface areas ≥1 km², totaling ~46,700 km². The study categorized the plains into two major morphological types: Amundsen-type (smooth, flat surfaces) and Schrödinger-type (with mound features). Detailed geological characterization was conducted using LOLA topography, slope, and roughness data, as well as LRO Wide-Angle Camera (WAC) images. The surface optical albedo (1064 nm) was analyzed using LOLA active laser measurements, and iron content was determined using Kaguya-SP and LRO-LOLA blended data. The researchers searched for evidence of volcanic or impact origins by examining surface albedo variations, morphology, and composition (e.g., presence of dark-halo craters or mafic-rich regolith). Analog studies of impact-derived plains elsewhere on the Moon were employed to constrain the formation mechanism and source regions. Ejecta thickness models were used to estimate the contribution of ejecta from different impact craters/basins. Crater population analyses were performed to determine the surface model ages of the Amundsen floor plains and Schrödinger basin to compare their ages. The researchers used the catalog of kilometer-scale impact craters from Robbins (2019) and carefully excluded secondary craters and non-impact pits based on their characteristics. They used empirical radial thickness models of impact ejecta materials to assess the potential material provenance of representative polar plains. The data used includes LOLA topography and albedo, Lunar Prospector gamma-ray spectrometer data, LROC WAC image mosaic, and plains mapping data.

The study identified 873 plains terrains (≥1 km²) in the lunar south polar region, significantly more than previous studies. These plains cover ~46,700 km², or ~12.8% of the study region. ~57% are located within craters, particularly larger ones like Schrödinger, Drygalski, and Amundsen. Two main types of plains were identified: Amundsen-type (smooth, low-slope plains) and Schrödinger-type (plains with mound features). Amundsen-type plains are characterized by low slopes (<2°), smooth surface textures, and distinct boundaries with surrounding terrains. They are not volcanic in origin, as indicated by their albedo and iron content, which differ significantly from mare basalts. Analysis of similar smooth plains on the floors of other impact craters (e.g., Kibal'chich) suggests a distal origin from basin ejecta. The Schrödinger-type plains, characterized by mound features, were linked to local impact melt materials associated with their host craters. The smooth Amundsen-type plains are composed of a mixture of local substrate and ejecta from distant impact basins, primarily the Schrödinger basin. This is supported by ejecta thickness calculations, areal abundance distribution analysis, and crater population chronology, which shows similar ages for the Amundsen floor plains and Schrödinger basin (~3.8–3.9 Ga). The study found that the areal abundance of plains increases with distance from the Schrödinger basin, reaching ~15% at distances of one to four radii, consistent with the distribution of ejecta from the Orientale basin. The study also highlights that the Orientale basin likely contributed additional ejecta materials. 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 the southern polar region of the Moon.

The findings challenge the current focus on high-standing massifs as landing sites for lunar polar missions. While massifs offer favorable solar illumination, their steep slopes and limited flat areas pose challenges for landing and exploration. The identified plains terrains offer an alternative: extensive, flat areas suitable for landing, similar to mare plains. The Amundsen crater floor, for example, provides a large, relatively flat area with significant regions of permanent shadow (PSR) potentially containing water ice. Although solar illumination is less favorable than on massifs, substantial solar energy still exists in portions of these plains, and advanced nuclear batteries could mitigate this limitation. The study emphasizes the importance of considering the provenance of samples collected from these plains, as they represent a mixture of local and distant materials, particularly from the Schrödinger basin. Studying these materials offers opportunities to understand basin-formation impact processes and refine the lunar chronology.

This study provides crucial geological context for future lunar south polar missions. The mapped plains terrains, especially Amundsen-type plains, offer extensive, relatively flat areas suitable for landing and exploration. The findings support Schrödinger basin as a significant source of ejecta materials in the south polar region. Future research should focus on detailed in-situ measurements of these plains to characterize their composition, stratigraphy, and volatile content, as well as further investigation of the contribution of other impact basins. Careful consideration of the mixed provenance of materials in these plains is also necessary for accurate interpretation of scientific results.

The study relies on orbital remote sensing data, and in-situ measurements are necessary to confirm the findings. The interpretation of the plains' origin is based on indirect evidence, such as morphology and composition, and additional data could provide further constraints. The resolution of some datasets limits the detailed characterization of smaller plains terrains. The accuracy of the crater population-based age estimations is subject to the uncertainties inherent to this dating method.