New insights into lunar terrain properties and their effect on derivation of absolute model ages using Apollo landing sites

Abstract

The lunar crater chronology has been built by connecting radiometric ages of sampled terrains with the areal crater densities on those terrains, particularly the density at diameters (D) ≥ 1 km (aka N(>1)). In the past, very few crater chronologies have considered the effect of terrain properties on the crater densities and size-frequency distributions (SFDs) used to build them. This influence is especially important when N(>1) cannot be directly measured. Here we study this influence by using the Model Production Function (MPF) chronology, which incorporates terrain properties into computing N(>1) and absolute model ages (AMAs) through computing crater diameters using standard crater scaling laws. Furthermore, we also gain a better understanding of actual lunar terrain properties by adjusting the MPF to reproduce AMAs close to the radiometric ages of several Apollo sample sites. The properties examined are the consolidation state of the terrain, its effective cratering strength, and density. Overall, we find that the impact melt of Copernicus crater (∼0.8 Ga) is stronger and more consolidated than the mare and highland terrains investigated. The mare become less consolidated and weaker as they age (from 3.1 to 3.8 Ga) – likely due to fracturing and regolith formation by subsequent impacts. The highlands (∼3.8 Ga) are the weakest terrain. The analysis of terrain proprieties allows our MPF computations to reproduce the radiometric ages, and the impact melt and ejecta of Copernicus crater to have the same age, as expected. The new lunar terrain property constraints can be used with the MPF to derive more robust absolute model ages for unsampled terrains. The values presented in this work for impact melt, ejecta, mare, and highlands can serve as references.