Topographic Degradation Processes of Lunar Crater Walls Inferred from
Boulder Falls
Abstract
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 fromthe Lunar Reconnaissance Orbiterand
KAGUYA, we estimated the detailed distributions of boulder falls, small
craters, slope angles, the optical maturity parameter (OMAT), and
maximum acceleration due to impacts at these sites. In steeply sloping
areas at both sites, we found that the density of small craters was
small and areas with high OMAT values corresponded to boulder sources,
where many boulders exist. At Site 1, the starting points of boulder
falls and acceleration due to impacts were correlated. In addition,
craters with boulder falls at and around Site 2 were distributed
independently of the 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.