Investigating the relaxation of small craters emplaced on solidified melt sheets on the Moon

Abstract

Larger impact craters and basins on the Moon display a persistent discrepancy in crater size frequency distributions between their interior melt surfaces and exterior ejecta blankets. The population of smaller craters <1 km across used to date the melt and ejecta surfaces often indicate younger surface ages for the melt surface, even though the two should be the same age having formed at the same time from the initial impact. This work investigates the possibility of removal or deformation of these small craters by topographic relaxation via the solid-state creep of lunar mare and highlands materials that have formed on a warm but recently solidified melt sheet on the Moon. We first determine the thermal state of a solidified volume of melt as it cools from the surface to a melt interface at depth that is thick enough to accommodate the emplacement of craters 200 m or 500 m in diameter. We then examine the extent of topographic relaxation under a favorable steady state temperature profile using material rheologies for the applicable lunar materials under lunar gravity. Ultimately, our simulations show that despite elevated thermal conditions that are favorable to relaxation, the low stress state and stiffness of the material rheologies inhibit any significant crater deformation, with displacements <1 m over 10 kyr. This finding suggests that other mechanisms such as self-secondary cratering or target material contrasts, among others and possibly in combination, are likely responsible for the discrepancy in CSFD ages between the melt and ejecta surfaces.