Lunar impact history constrained by GRAIL-derived basin relaxation measurements

GRAIL observations have been used to derive an updated catalog of lunar impact basins with diameters ≥ 450 km. In this study, we assess the age and relaxation state of these basins and find that the 13-15 stratigraphically younger basins are significantly less relaxed than the 8-16 identifiable older basins. The change in relaxation state is most likely a signature of the cooling of the base of the lunar crust below 1300-1400 K, which we infer happened between 4.21-4.45 Ga based on geochronological measurements. We compare the predicted number of basins before and after this reference point with several different modeled lunar cratering chronologies. While these models are sensitive to the ratio of 1km:450 km craters, we conclude that a model with a broad period of late heavy bombardment (a “sawtooth”) is more likely to fit both the number of relaxed and unrelaxed basins, and the mass of material delivered as a late veneer.