Evaluating Flat-Crater Floor Fill Compositions and Morphologies: Insight Into Formation Processes

Numerous impact craters on Mars have been deeply filled, raising questions about the potential processes that contributed to infilling. Previously proposed processes include ponding of impact melt, aeolian sedimentation, lacustrine sedimentation, or volcanic infilling. Better constraints on the infilling processes of flat floor craters are needed to assess the relative roles of impact, volcanic and sedimentary resurfacing processes throughout Mars history. In this study, we investigate the compositional, thermophysical, and morphological properties of floor materials within a subset of 119 high thermal inertia craters. For each crater, the degree of etching and crater retention of the floor morphologies, as well as the floor mineralogy were characterized. We find evidence for crater infilling by effusive volcanism and by sedimentation (e.g., pyroclastic, volcaniclastic, and/or epiclastic), with weak, clastic rocks being most common. A majority of studied craters also show evidence for olivine enrichments. There is no clear trend between the olivine enrichment and floor morphologies, indicating that olivine is likely associated with both effusive and explosive volcanism. Our observations support the hypothesis that pyroclastic activity may have been widespread on early Mars.