Topographic Degradation Processes of Lunar Crater Walls Inferred From Boulder Falls

Recent explorations by lunar orbiters have shown that boulder falls are distributed over the entire lunar surface. To quantitatively evaluate the effects of moonquakes and meteorite impacts on boulder falls, we performed detailed surveys at two sites: One in the southern part of the Schrödinger basin (Site 1) and the other in Laue crater (Site 2). Using images and topography data from the Lunar Reconnaissance Orbiter and KAGUYA, we measured the detailed distributions of boulder falls, small craters, slope angles, and the optical maturity parameter (OMAT) and modeled maximum ground acceleration due to impacts at these sites. In steeply sloping areas at both sites, we found that the density of small craters was low and areas with high OMAT values corresponded to boulder sources, where many boulders exist. At Site 1, the starting points of boulder falls and ground acceleration due to impacts were correlated. In addition, craters with boulder falls at and around Site 2 were distributed independently of the presumable epicentral distance from a shallow moonquake that occurred in 1975 near Site 2, which was previously inferred to have triggered boulder falls at the site. Our results suggest that boulder falls at these sites were triggered not by moonquakes but by meteorite impacts. We propose a model for the generation and transport of boulders and regolith on slopes by meteorite impacts, which may be directly related to the degradation of crater slopes on the Moon. Plain Language Summary Large rocks or boulders (a few meters to tens of meters in size) with accompanying trails on crater slopes on the Moon are widely visible in high‐resolution images taken by recent lunar orbiters. Proposed boulder fall triggers include seismic events (moonquakes), meteorite impacts, and solar‐driven thermal fatigue. To investigate the cause of boulder falls, we analyzed image data of two sites with boulders and boulder falls on the slopes of large craters. We found that many boulders are located where fresh materials are exposed in steeply sloping areas near the edges of the large craters. In addition, the starting points of boulder falls were within the large areas shaken during meteorite impacts that produced small craters on the slopes. In contrast, we found no clear evidence supporting the triggering of boulder falls by moonquakes. Therefore, we concluded that the boulders were generated in upslope areas and that their falls were triggered by meteorite impacts. Our findings