The telltale shapes of small lunar craters at the Apollo 16 and 17 landing sites

Abstract:

The observed shape of an impact crater is a result of multiple physical processes spread over time. Large lunar craters often have distinctive landforms dependent on diameter (e.g. a central peak), and then mass wasting and redistribution processes further alter their morphology over long time periods. In contrast, most small lunar craters (SLCs; 30 to 300 m diameter) have similar bowl shapes at formation, inferred from the shapes of the freshest SLCs. In particular, SLCs formed entirely within regolith are consistently bowl shaped, and changes to bowl shapes typically occur at target strength discontinuities. Starting from a fixed shape, degradation is largely responsible for the difference in the observed shapes of SLCs. SLC impact events are significant contributors to regolith formation and morphologies of SLCs are critical to our understanding of the regolith. Observed shapes of SLCs reflect, at least in part, the target properties where a crater formed and, perhaps, various layers below the surface. Variation in shapes of fresh SLCs can be attributed mostly to the thickness of the regolith layer and the general nature of the target region. Shapes of fresh and older SLCs differ by the amount of change due to degradation and is also dependant on target properties. However, there is currently no comprehensive observation-based comparative standard for the SLC shapes, both fresh and degraded. Shapes of SLCs at the Apollo 16 and 17 landing sites were analyzed from high-resolution topography (2 m/pixel) enabling a new standardized shape model for fresher SLCs (freshness classification via image analysis). We found that most SLCs (> 65%) analyzed were shallow inverted cone shaped and with decreasing freshness, SLCs were shallower and had flatter walls. The influence of degradation rates to the SLC shape is investigated and a larger degradation rate (at least twice) and higher percentage (>10%) of cone shaped craters is observed for Apollo 17 (compared to Apollo 16) SLCs. Finally, possibilities that allow SLC shapes to be influenced by target properties are discussed.