Crustal porosity reveals the bombardment history of the Moon

Abstract:

The lunar highlands are among the oldest surfaces on the Moon and contain a record of the impacts that postdate the solidification of the primordial anorthositic crust from the Lunar Magma Ocean. It has proved difficult, however, to estimate the total number of impacts in this crust from the surface. Particularly, this ancient crust may have reached a state of crater equilibrium, such that the formation of every new crater, on average, is accompanied by the erasure of a similar sized, pre-existing crater. While it is challenging to discern the cratering record from a densely cratered surface, craters are also recorded in the porosity of the target and it is possible to recover information from this record. GRAIL data provide a means to estimate the spatial distribution of porosity in the lunar crust and provide insight into the cratering history of the lunar highlands. The lunar crustal porosity exhibits an inverse correlation with crater number density D≥20 km. Terrains surrounding the largest young basins, Orientale and Moscoviense, exhibit the highest crustal porosity values, while the lowest-porosity regions (southern nearside and the north-central farside highland) display high crater number density values. We present a model to account for the relationship between a planet’s crustal porosity and cratering record, in which large basins create significant porosity in a crust and subsequent sub-basin sized impacts reduce crustal porosity. We consider 77 basins of D≥200 km including 30 basins that have Neukum chronology model ages ranging from 3.81 Ga to 4.31 Ga. Their porosities are modeled as constant values interior to basin rims and tapered to pre-impact porosity values at 3.5 radii. We reproduce large-scale porosity variation within a root mean square error in porosity of ~1.3% and provide porosity-derived ages for some basins that are not stratigraphically constrained. While the oldest lunar highland surfaces appear to be at or near saturation, their subsurfaces have continued to record impacts, allowing us to estimate the total impact history of the most ancient surfaces on the Moon. We suggest that D≥200 km basin record is mostly complete, and the total estimate number of D≥20 km craters is consistent with the mass accreted to the Moon constrained by highly siderophile elements abundance in the lunar mantle.