TOPOGRAPHIC DEGRADATION BY IMPACT CRATERING ON AIRLESS BODIES IS DOMINATED BY DIFFUSIVE EROSION FROM DISTAL EJECTA (Invited Presentation)

Crater count equilibrium occurs when the rate of crater erasure is equal to the rate of crater production for a given crater size. For crater production populations that have a cumulative power law slope steeper than -3, crater erasure is primarily due to diffusive erosion. Terrains in steep sloped crater equilibrium can help quantify the impact processes involved that give rise to diffusive topographic erosion. We have developed a new analytical model for crater count equilibrium based on topographic diffusion to constrain the cratering processes important for causing topographic erosion on airless bodies like the Moon. We use constraints from crater counts of the Apollo 15 landing site as well as numerical experiments performed using the Monte Carlo Cratered Terrain Evolution Model (CTEM) to quantify the diffusive processes that determine the observed equilibrium cumulative size-frequency distribution (CSFD). We find that the observed equilibrium CSFD requires that each new crater contributes to the degradation of the pre-existing landscape over a region that is both spatially heterogeneous and much larger than the crater's proximal ejecta blanket. Our best fit parameter set has this distal degradation zone being larger than the diameter of the crater's rim. We conclude that it is a distal degradation process like ballistic sedimentation that dominates topographic degradation, and it is this distal degradation which is the dominant process in setting the crater count equilibrium CSFD.