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 ~1 m 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 = 0.8–2 km 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 (<200 Ma) 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 0.4–0.6 Ga 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 (~1.5–2.5 Ga), 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.

The surface of the Moon is covered by regolith, a layer of poorly sorted particles ranging in size from dust to boulders, the median particle size of which is very fine sand (Carrier et al., 1991). Because of its ubiquity, the regolith is primarily what we observe on the Moon with remote sensing instruments and is the material we interact with when exploring the lunar surface. Many of the important geological, geochemical, mineralogical, and geotechnical characteristics that define lunar regolith were established by the Apollo missions and its precursors (e.g., McKay et al., 1991). Modern remote sensing methods allow us to extrapolate regolith characteristics from landing sites to places that have not yet been explored in situ.The existing paradigm for regolith growth and evolution is that it is dominated by impact cratering and gardening (e.g., Costello et al., 2018;McKay et al., 1991;Shoemaker et al., 1967). Because of the similarity between impacts and explosions, this can be thought of as an explosive demolition process. Using the Neukum production function (NPF) for the Moon combined with scaling calculations to determine the size of impactors (Ivanov, 2001), the kinetic energy delivered by impacts over the last 3 Ga that formed 10 m ≤ D ≤ 500 m craters is ∼3-6 × 10 15 J/ km 2 , approximately equivalent to a megaton of TNT/km 2 , with ∼10 4 unique events/km 2 . This demolition process is why there is almost no bedrock exposed on the Moon's upper surface, the median grain size of the surface has been reduced to very fine sand, and regolith thickens with time.

Recent work using S-band (12.6 cm, 2,380 MHz) radar data from the LRO Miniature Radio-Frequency (Mini-RF) instrument revealed that while surface and subsurface rock populations associated with lunar impact ejecta are diminished with time due to space weathering processes, the rock content of impact crater interiors increases for the first ∼0.5 Gyr of a crater's lifetime (Fassett, Minton, et al., 2018). A separate study used thermal infrared measurements from the LRO Diviner thermal radiometer to infer that boulders within ejecta deposits associated with

Crater degradation and erosion control the lifetime of craters in the meter‐to‐kilometer diameter range on the lunar surface. A consequence of this crater degradation process is that meter‐scale craters survive for a comparatively short time on the lunar surface in geologic terms. Here, we derive crater lifetimes for craters of <∼200 m in diameter by analyzing existing functional expressions for crater population equilibrium and production. These lifetimes allow us to constrain the topographic degradation needed at different scales to explain when craters become undetectable on equilibrium surfaces. We show how topographic degradation can be treated as a process of anomalous (scale‐dependent) topographic diffusion and find large differences in effective diffusivities at different scales, consistent with a wide range of evidence besides equilibrium behavior. Understanding the range of morphology of meter‐scale craters is particularly relevant for future exploration of the lunar surface with rovers. We illustrate expectations for the d/D distribution of small lunar craters on surfaces with negligible regional‐scale slopes. Our results imply that if volatiles are found in preserved <4 m craters and were delivered after crater formation, the volatiles must have been emplaced in the last ∼50 Ma. Given the rates of surface evolution we infer, the most likely emplacement time for any volatiles discovered at or near the surface in the interior of fresh, small craters may be much younger than this upper limit.

Quantifying the volumes and geologic nature of lunar volcanic eruptions is important for constraining the thermal and geologic evolution of the Moon. Cryptomaria are effusive, basaltic lava flows on the Moon that were subsequently buried, and therefore hidden, by higher-albedo basin and crater ejecta. Radar offers the ability to probe the subsurface for geologic units not otherwise apparent at the surface. We use Arecibo/Green Bank Observatory and Lunar Reconnaissance Orbiter Mini-RF radar data sets to characterize maria and cryptomaria within the Schiller–Schickard region. We find significant variability in the radar backscatter across the region that does not correspond to previously mapped boundaries of maria and cryptomaria in the literature. We use the characteristic low backscatter (due to the attenuating nature in radio waves of some basaltic minerals) to analyze the distribution of cryptomaria. We use the reduction in radar backscatter to estimate burial depths of cryptomaria across the area. We present a new map of Schiller–Schickard cryptomaria and the local variability in the thicknesses of the light plains that bury the basalts. We find burial depths ranging from >100 m in the deepest areas to just a few to tens of meters in areas with shallow cryptomaria (particularly prominent in the southeast). These areas are generally contiguous with maria, allowing us to track mare lava flow units into the subsurface at mare/highland margins. We propose that ∼67% of the region contains surface or buried basaltic volcanism, which represents over twice (2.7× increase) the areal extent of cryptomaria reported in previous studies.