The Role of Impact Bombardment History in Lunar Evolution

The lunar surface features diverse impact structures originating from its early bombardment; the largest among them are the lunar basins. Basin-forming impacts delivered large amounts of energy to the target and expelled lots of material that deposited as an insulating blanket in the vicinity of the impact. Here, we investigate how such impact processes have altered the early Moon and its subsequent thermochemical evolution by combining proposed lunar basin chronologies with numerical models of basin formation and 3D thermochemical mantle convection. We analyze the effects of single impacts resulting in different basin size and at different times of lunar evolution. The direct effects of a single impact are enhanced melt generation as well as thermal and heat flux anomalies, but these are limited to 100-200 Myr following the impact. Considering numerous impacts and a widespread ejecta blanket make impact-induced effects more substantial. Lunar contraction history may be altered by the impact bombardment in favor of an early extension phase as suggested by recent observations. Moreover, imprints of the early bombardment may be kept in the thermal and compositional state of the Moon's interior until modern times. These can be as large as those induced by uncertainties in bulk lunar heat content, if surface insulation due to ejecta is efficient. In this case, model-predicted present-day thermal profiles match independent constraints better if the bulk Moon is not significantly enriched in refractory elements compared to Earth.