THE DEPLETION OF CRATERS LARGER THAN 600-800 M IN DIAMETER ON THE WALLS OF LUNAR COMPLEX CRATERS

Introduction: Impact craters on the Moon are exposed to continuous impact cratering events. We analyze the crater population on the walls of 16 complex craters to analyze their morphologies and compute the relative wall strength of the complex craters. The results show the depletion of craters larger than 600-800 m in diameter (D). We infer that this depletion results from landslides driven by large craters emplacements, while small craters emplacements fail to do so.

Methodology:We used ArcGIS CraterTools [1] to count craters on the walls of 16 complex craters located around the southern pole [2]. For this process, we used the available digital elevation models (DEM) derived by Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter (LOLA/LRO) [3]. Slope overlays granted us visibility of the permanently shadowed regions, and permitted us to further investigate crater morphology.

We used π-scaling impact-crater scaling relationships [4]. By connecting the parameters: energy, crater size, and material strength, we extrapolated conditions for much larger impacts and features.

Results: We found there are two distinctive slope distributions. For craters at D < ~600-800 m, the slope is steep (~-3); otherwise, it is extremely shallow (~-1).

The dominant crater size was 0-700 m, (7251 counts, in contrast to 362 counts for craters in diameter range 700-1500 m, and 289 counts for those > 1500 m), and slopes range from nearly horizontal (5º) to very steep (69º).

Our crater counting analyses show that the observed crater population consists of two modes, divided at D ~ 600-800 m. We consider that this contrast results from the depletion of craters at D > ~600-800 m by mass movement due to the walls’ structural failure during the impact. This implies that the complex crater walls may be weaker than the necessary strength to generate craters at D > 600-800 m.

Discussions: The depletion of craters at D > 600 – 800 m implies that landslides would be large enough to erase their morphologies such as crater cavities. However, the increased number of smaller craters infers that the small impactors do not generate enough energy to produce mass movement.

References: [1] Kneissl et al., (2011), [2] Deutsch et al., (2019), Icarus 336, 113466, [3] Riris et al., (2010), ICSO [4] Holsapple (1993), Annu. Rev. Earth Planet. Sci. 21, 333-337