The Steepest Slopes on the Moon: Gradual Degradation and Instant Removal by Basin-Forming Impacts

Abstract:

We calculated topographic gradients over the surface of the Moon at a 25 m baseline using data obtained by the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter (LRO) spacecraft. The derived slope-frequency distribution revealed a steep roll-over for slopes close to the angle of repose. Slopes significantly steeper than the angle of repose are almost absent on the Moon due to (1) the general absence of cohesion/strength of the fractured and fragmented megaregolith of the lunar highlands, and (2) the absence of steep-slope producing geological processes in the recent geological past. The majority of slopes steeper than 32 - 35 degrees are associated with relatively young large impact craters. Very rare extremely steep (> 45 degrees) slopes are exclusively associated with large Copernican-age craters. Craters of Early Imbrian age and older are devoid of slopes steeper than ~35 degrees. We interpret these observations in the following way. Every basin-forming impact removes steep slopes by global seismic shaking causing slope collapse. The latest such impact formed Orientale basin and instantly removed all preexisting slopes steeper than ~35 degrees. This makes steep slopes a good global stratigraphic marker at Early/Late Imbrian boundary. After the Orientale impact, craters lose their steepest slopes progressively with time. This makes crater wall steepness an independent proxy for crater age. The global spatial distribution of the proportion of the steepest slopes correlates moderately well with the predicted spatial distribution of impact rate: low latitudes and the leading hemisphere have a higher steep slope proportion than high latitudes and the trailing hemisphere. However, the southern farside has a significant paucity of steep slopes, which remains unexplained. Acknowledgement: Data processing was performed at MIIGAiK by MK and supported by Russian Science Foundation, project 14-22-00197.