Global Drivers and Transport Mechanisms of Lunar Rockfalls

Abstract

The long- and short-term drivers and transport mechanisms of lunar rockfalls are currently not well understood, but could provide valuable information about the geologic processes that still shape the surface of the Moon today. Here, we compare the global distribution of rockfalls with relevant geophysical data, such as seismic, topographic, thermal, gravity anomaly, and tidal displacement data sets. Rockfalls appear to predominantly occur (a) on equator-facing slopes and thus in regions with large thermal amplitudes, (b) on slope angles well above-average (Δ ∼ 10°), and (c) in regions with above-average rock abundance. We do not observe a qualitatively or statistically relevant relation between rockfall abundance, monitored Apollo-era shallow seismic activity, and the distribution of visible tectogenetic features. Informed by our global analysis, we conduct a targeted, in-depth study of 687 rockfall boulders and trajectories in 13 sites across the Moon, including 7 craters, 2 volcanic vents, 2 tectonic structures, and 2 unclassified geomorphic regions. We identify four different source region types, where the type appears to control the occurrence of rockfalls. The source region type in turn is controlled by surface age rather than geomorphic context. We find that rockfall trajectories are mainly controlled by the trigger energy and the geometry of the slope. Our results suggest that erratic small-scale impacts (mainly in old, Imbrian-Nectarian, shallow terranes), aided by solar-induced thermal fatigue of fractured bedrock (mainly in young, Copernican-Eratosthenian steep terranes), were the dominant, global-scale long- and short-term drivers of rockfalls in the Moon's recent geologic past.