Heterogeneous impact transport on the Moon

摘要

Impact cratering is the dominant process for transporting material on the Moon's surface. An impact transports both proximal material (continuous ejecta) locally and distal ejecta (crater rays) to much larger distances. Quantifying the relative importance of locally derived material versus distal material requires understandings of lunar regolith evolution and the mixing of materials across the lunar surface. The Moon has distinctive albedo units of darker mare basalt and brighter highland materials, and the contacts between these units are ideal settings to examine this question. Information on the amount of material transported across these contacts comes from both the sample collection and remote sensing data, though earlier interpretations of these observations are contradictory. The relatively narrow (~4-5 km wide) mixing zone at mare/highland contacts had been interpreted as consistent with most material having been locally derived from underneath mare plains. However, even far from these contacts where the mare is thick, highland material is abundant in some soil samples (>20%), requiring transport of highland material over great distances. Any model of impact transport on the Moon needs to be consistent with both the observed width of mare/highland contacts and the commonality of nonmare material in mare soil samples far from any contact. In this study, using a three-dimensional regolith transport model, we match these constraints and demonstrate that both local and distal material transports are important at the lunar surface. Furthermore, the nature of the distal material transport mechanism in discrete crater rays can result in substantial heterogeneity of surface materials.