Provenance of lunar regolith components: Redistribution of material by craters from the heavy bombardment period through the Copernican era.

摘要

Over the last ∼4.5Gyr, repeated impacts into the lunar surface have lead to the fragmentation of the crust and continual communition of larger fragments. This continuing process has created a layer of fine-grained rock, mineral, and glass fragments known as the regolith across the entire lunar surface. During the first ∼600Gyr 43 large impact basins, all larger than 300km in diameter, formed. The formation of these large craters is predicted to have redistributed material across the entire lunar surface and created a large scale reworked zone known as the megaregolith. This redistributed material is found uniformly everywhere on the Moon. Because several large basins are found in close proximity on the nearside, there is a concentration of basin ejecta on the nearside relative to the farside. The differences in basin modification histories between the nearside and farside is likely responsible for observed geochemical terranes. The farside received relatively little basin material that allowed the composition of the surface to largely unmodified. Alternately, the nearside was significantly modified by basins, which likely created a geochemical terrane largely representative of material derived from basins. One nearside basin in particular, Imbrium, distributed large amounts of material across the entire Moon. Imbrium ejecta are enriched in Thorium, which allows the distribution of its ejecta to be measured remotely. Ejecta from Imbrium are predicted to cover most highland surfaces, however the distribution of such material is not symmetric around the basin. Following the formation Imbrium, cratering continued on a much smaller scale. In the last ∼3.85Gyr, 411 craters larger than {dollar}30km in diameter have formed. The formation of these smaller craters, like the formation of basins except on a much smaller scale, distributed material across the entire lunar surface. This distribution of crater material, like basins, is non-uniform and is predicted to result in various locations on the Moon having differing amounts of material derived from greater than 50km in the regolith. Such material is predicted to represent less than 50% of the regolith over most of the Moon. The remaining portion of the regolith is predicted to contain both basin and locally derived material.