Regolith mixing by impacts: Lateral diffusion of basin melt

Abstract

Impact cratering has been the primary process to alter the distribution of lunar highland material since the formation of a crust. This impact history is recorded in the radiogenic clocks of impact melts which are accessible for study on lunar samples and meteorites. However, primary impact melt is exposed to a long-time gardening process (i.e. re-melting, excavation, burial, and re-excavation) by subsequent impacts resulting in a complex spatial distribution of materials representing specific impact events. To investigate the diffusion behavior of impact melt, a model tracing the evolving distribution of melt laterally and with depth was built using a Monte Carlo approach. Given scaling laws concerning melt production and ejecta distribution, the size-frequency distribution of impact craters, and the rate function for crater formation, we examine the evolution of melt component occurrence of different ages. Three mid- to late-forming basins (Serenitatis, Crisium, and Imbrium) are chosen as a case study for the diffusion of melt from major basin-forming events. The survival probability of basin melt occurrence at the Apollo and Luna sampling spots is derived. It is expected to find abundant Imbrium and Crisium melt at the Apollo and Luna sampling sites, consistent with the K-Ar radiometric dates of highland samples; whereas the older Serenitatis melt was subjected to the later long-term gardening, strongly influenced by later local impacts, and thus is less abundant. Understanding the diffusion of impact melt is helpful for interpretation of radiometric ages of lunar samples and can be used to predict the distribution of differently-aged melts at future landing/sampling sites such as the Chinese Chang'E-4 (CE-4).