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

Abstract

Introduction: Impact craters on the Moon are exposed to numerous impact cratering events. They originally have sharp morphologies such as clear rims and cavities but experience gradual degradation and eventual erasure by subsequent bombardment processes [e.g., 1-6]. Among them, topographic diffusion degrades an existing larger crater by emplacements of numerous smaller craters [3]. Complex craters exhibit mass movements at multiple scales on the walls, indicating the exposure to topographic diffusion. Here, 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 that there is a distinctive 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. The present work is complementary to recent geological study that analyzed the crater wall of an unnamed ~3.4 km diameter simple crater in the Schrödinger basin to show dry granular flows [7], but distinguished because we address the surface and impact conditions in complex craters. Methodology: We use the crater counting approach and impact crater scaling relationships. Crater counting: We used the ArcGIS CraterTools [8] to count craters on the walls of 16 complex craters located around the southern pole [9]. For this process, we only used the available digital elevation models (DEM) derived by Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter (LOLA/LRO) with a resolution down to 20 m, and the height uncertainty less than 10 cm [10]. The use of slope overlays granted us visibility of the permanently shadowed regions, and permitted us to further investigate the crater morphology. To reduce bias within our crater counting, we counted craters from 99 m to 9 km. By doing this, we inferred that craters within a sub-km range would be small enough to be placed but not at risk to be rejected based on size and morphology (Figure 1). Impact-crater scaling relationship: To access the impact conditions, we used the π-scaling impact-crater scaling relationship [11]. The impact scaling relationships connects the parameters: energy, crater size, and material strength. By using it, we extrapolate conditions for much larger impacts and features. In this study, we used Richardson’s forms: