The Rock Abundance of Crater Populations as a Probe of Mare Protolith Properties

摘要

The volcanic parent rock ("protolith") composing the lunar maria is likely to have developed diverse mechanical properties upon emplacement. The abundance of rocks ejected by impacts may be sensitive to protolith properties. We investigate the relationship between the possible diversity of mare protoliths and the abundance of rocks excavated and emplaced by impacts employing cumulative size-frequency distributions (SFDs) of rocky craters in the maria. The SFDs of 15 mare units were calculated through varying degrees of Lunar Reconnaissance Orbiter Diviner-derived rock abundance within crater ejecta blankets. By inspecting the frequency of high rock abundance craters, we find that the age of a surface does not control an impactor's ability to excavate rocks. Through analysis of SFDs, we find that differences in SFD behavior may act as probes for the mechanical strength of the protolith and the thickness of buried lava flows. Generally, regions with SFDs that have shallower slopes with increasing rockiness may be associated with thick, competent flow types, and regions with SFDs that have unchanging slopes with increasing rockiness may be associated with stacked, thin flows that are possibly more friable. The greater frequency of high rock abundance craters and the converging behavior of SFDs at two units within Mare Humorum and Oceanus Procellarum suggest that the surfaces are underlain by thick, competent lava flows. Finally, SFD slope analysis suggests that secondary impact craters are relatively ineffective at excavating rocks and the SFDs of rocky craters may reflect primary cratering populations.Plain Language Summary When ancient lunar lava flows cooled and hardened into rock, they likely developed different properties depending on the conditions of eruption. The variations in these now-buried volcanic rocks can be probed using impacts, which break up and eject material from the subsurface. Ejected pieces of lava flow can be measured in terms of their abundance, which we use to infer lava flow properties in this study. By inspecting the populations of craters with a high abundance of rocks in their ejecta, we determine that age does not control the frequency of highly rocky craters. Further, we determine the size-distribution of craters with rocks in their ejecta and find different relationships between the rock-free crater populations and the rocky crater populations. We find that changes in the rock-free and rocky crater populations may represent variations in lava flow properties. Finally, secondary impact craters that are formed from high-velocity debris made by primary impacts are ordinarily difficult to discern from the primary impact population; however, we determine that by inspecting the slightly rockier crater population, secondaries may be filtered out.