Temporal Evolution of S-Band Circular Polarization Ratios of Kilometer-Scale Craters on the Lunar Maria

Circular polarization ratio (CPR) measurements from the Miniature Radio Frequency (Mini-RF) instrument provide information about how lunar craters evolve with time. In particular, S-band CPR data are sensitive to the rockiness and/or topographic roughness of the uppermost similar to 1m of the subsurface. We extracted CPR as a function of radial range for 6,206 unique craters superposed on the lunar maria with diameter D=0.8-2km and constructed median profiles aggregating craters into 13 different age classes. These aggregate profiles show systematic evolution of craters' CPR with time. The freshest craters (<200Ma) have ejecta exhibiting elevated CPR compared to the background maria out to distances of more than three crater diameters beyond the craters' rims. The extent and magnitude of this enhancement declines as craters age. Within crater interiors, the CPR signature initially increases for 0.4-0.6Ga and then declines. These observations provide constraints on rock breakdown and regolith development after crater formation. Additionally, our results demonstrate that the CPR evolution of crater interiors and ejecta are significantly decoupled. The CPR enhancement in crater ejecta fades faster than crater interiors, causing their overall CPR signature to look similar to anomalous craters whose interior CPR anomalies have been attributed in past work to the presence of water ice. Craters in this study became more anomalous-looking as they reach middle age (similar to 1.5-2.5Ga), as their interior and exterior regolith differ in rockiness as time passes. Our results support, but do not prove, that anomalous craters' CPR signatures can arise without requiring water ice. Plain Language Summary The Moon's surface is mottled with impact craters. When a crater roughly 1km in diameter forms, a bowl-shaped depression is excavated by the impact to similar to 200m deep, and the crater's surroundings are covered by rocky ejecta. Fragmented, rocky material also infills a new crater's interior, and bedrock is uplifted on the crater's walls. These rocks are good reflectors of radar, which causes the interior and ejecta deposits of craters to have a distinctive radar polarization signature, just as the polarization of visible light flips when hitting a mirror. Over billions of years, the rocks within a crater and surrounding it breakdown because of later smaller impacts and other processes. This causes a finer-grained regolith (a lunar soil) to develop. Rock-poor regolith-covered surfaces do not reflect radar as readily, so the distinctive polarization signature of craters fades with time. In this work, we use this behavior to study the evolution of crater characteristics. We see a very regular evolution in craters' radar signatures with time. The results help us understand the manner in which craters on the Moon change with time, and how rocks at the lunar surface and in the near subsurface breakdown.