Thermal effects of late accretion to the crust and mantle of Mercury

Abstract Impact bombardment on Mercury in the solar system's late accretion phase (ca. 4.4–3.8 Ga) caused considerable mechanical, chemical and thermal reworking of its silicate reservoirs (crust and mantle). Depending on the frequency, size and velocity of such impactors, effects included regional- and global-scale crustal melting, and thermal perturbations of the mercurian mantle. We use a 3D transient heating model to test the effects of two bombardment scenarios on early (pre-Tolstojan) Mercury's mantle and crust. Results show that rare impacts by the largest (≳100 km diameter) bodies deliver sufficient heat to the shallow mercurian mantle producing high-temperature ultra-magnesian (komatiitic s.s.) melts. Impact heating leading to effusive (flood) volcanism can account for the eponymous “high-magnesium region” (HMR) observed during the MErcury Surface, Space Environment, GEochemistry Ranging (MESSENGER) mission. We find that late accretion to Mercury induced volumetrically significant crustal melting (≤58 vol.%), mantle heating and melt production, which, combined with extensive resurfacing (≤100%), also explains why its oldest cratering record was effectively erased, consistent with crater-counting statistics.