Prolonged Rock Exhumation at the Rims of Kilometer‐Scale Lunar Craters

Abstract

AbstractFresh impact ejecta deposits on the lunar surface can be characterized as heterogeneous mixtures of boulders, cobbles, and fine‐grained regolith that are deposited on the lunar surface during the impact crater formation process. Over time, the surface boulders associated with ejecta deposits break down into fine‐grained regolith due to a combination of later impacts and thermal fatigue. Nonetheless, observations of old (>2.0 Ga) kilometer‐scale (0.8–2.0 km) lunar impact craters in high‐resolution images reveal >1 m boulders along their rims and in their near‐proximal ejecta deposits on the lunar maria. Here, we use a combination of radar and thermal‐infrared data from the Lunar Reconnaissance Orbiter spacecraft to show that the rims of kilometer‐sized impact craters exhibit elevated rock abundances for the lifetime of the lunar maria. We interpret these results as indicating that boulders are continually being uncovered at crater rims due to downslope movement of the overlying regolith. Moreover, rocks found at crater rims that have been exhumed from depth in geologically recent times are locally derived and unlikely to have come from other areas of the Moon. Future collection of lunar samples at crater rims will serve to mitigate the potential for sample contamination from distal sources, helping to ensure accurate geologic interpretations from the collected samples.