Boulder degradation and exhumation at the rim of lunar kilometer-scale craters

Previous studies have indicated that boulder exhumation results in the longstanding presence of surface boulders at crater rims throughout the lifetime of lunar maria. In our theoretical analysis, we suggest that mass-wasting processes and the negative balance zone effect of surface material movement play major roles in the exposure of shallow surface boulders. Given the numerous craters of various sizes and ages on the Moon, this specific subpopulation constitutes a considerable part of lunar surface boulders. Therefore, we conducted detailed observations of the topography and morphology of these boulder areas, and performed a boulder (> 6 m) statistical analysis to better understand the degradation processes and underlying exhumation mechanisms of these boulders.

Herein, we examined high boulder abundance areas at the rims of 79 km-scale lunar craters, whose ages are uniformly distributed across the estimated survival timescale of most meter-sized boulders (∼300 Ma). Optical image analysis did not reveal distinct topographic features indicative of regolith movement. The observed boulders, if not exogenous, have likely remained in proximity to their original emplacement and experienced minimal subsequent displacement. In these areas, boulder degradation is primarily governed by impact-induced erosion and thermal fatigue, gradually declining both boulder number density and maximum size over time. However, this overall trend is complicated by the continual exhumation of shallow subsurface boulders, which replenishes the surface population. This is supported by the longer estimated boulder half-life in our study areas (∼100 Ma), compared to previously reported values for meter-sized boulders (40–80 Ma), and by the presence of large boulders at some older craters that are comparable in size to those at younger ones. We propose that in older craters, the negative balance zone effect may dominate the exhumation process, and that boulder degradation and exhumation may reach a quasi-equilibrium state, sustaining a stable surface boulder population over the geological history of the lunar maria.

The observations and interpretations of this study provide a unique perspective on the understanding of boulder evolution processes on the lunar surface. Additionally, this study also provides useful insights for future lunar sampling campaigns.