Analyses of Lunar Orbiter Laser Altimeter 1,064‐nm Albedo in Permanently Shadowed Regions of Polar Crater Flat Floors: Implications for Surface Water Ice Occurrence and Future In Situ Exploration

Abstract

Potential water ice concentrated within the permanently shadowed regions (PSRs) near lunar poles is both scientifically significant and of value for future explorations. However, after decades of observations, the existence and characteristics of PSR water ice remain controversial. The 1,064‐nm laser reflectance measurements collected by the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO) provide a unique opportunity to detect and characterize PSR water ice. In this work, we focus on all major PSRs on the flat floors of lunar polar craters and analyze their detailed LOLA 1,064‐nm albedo and then compare this with the adjacent flat non‐PSRs. We find that the LOLA albedo of the majority of these PSRs is systematically higher than their adjacent non‐PSRs. Potential contributions of various factors to the observed LOLA albedo are individually quantitatively evaluated; we show that each of them is unable to account for the observed LOLA albedo anomalies and that the presence of surface water ice is the most likely explanation. Combined characterization of LOLA albedo and substrate impact cratering records (crater populations and depths) reveals that the inferred PSR water ices are in very small quantity (probably in the form of a surface frost layer or admixture with regolith) and are laterally heterogeneous in model ice concentration, ranging from negligible to ~6%. We recommend that these PSRs as priority targets for future surface in situ exploration endeavors, and a case assessment of Amundsen crater is presented. Plain Language Summary The possible presence of surface water ice within the permanently shadowed regions (PSRs) in lunar polar areas is important for both science and future lunar resource exploration. However, significant uncertainty still exists concerning both the presence and properties of water ice in lunar PSRs. A recent Moon‐orbiting laser instrument measures the laser brightness of the lunar surface, including areas that are permanently shadowed in visible light. These brightness data are very useful for studying the possible presence of water ice in PSRs, as water ice is much brighter than typical lunar surface soil. Our study finds that most of the PSRs on flat surface are generally brighter than the surrounding surface. We analyze many candidate reasons for this difference but find that the presence of water ice is the only explanation that can successfully explain the observations. We thus suggest that water ice very likely exists in many lunar PSRs. The surface water ice within PSRs is very thin and has different ice contents. These candidate ice‐containing PSRs on flat surfaces are optimal landing sites for future lunar exploration and are an important reference base for future lunar mission site selection.