Thermal and rotational effects of giant impacts during terrestrial planet accretion

Abstract

Terrestrial planets likely experienced one or more giant impacts during their formation that inflicted large thermal, chemical, and rotational perturbations. The early states of terrestrial planets are expected to be dominated by the thermal and rotational outcomes of giant impacts, but critical parameters that control internal processes, such as the pressures and temperatures of core formation, are not fully understood. Here we present the results from a representative suite of collisions between Moon- to super-Earth-mass bodies using the SWIFT hydrocode and updated ANEOS equations of state, allowing more robust temperature calculations. Using these results and the HERCULES planetary structure code, we calculated the contributions from thermal energy, gravitational potential energy, and post-impact rotation on the pressure-temperature conditions of the core-mantle boundary (CMB). We derived scaling laws for the efficiency of impact heating, mantle-core heat partitioning, and CMB pressures and temperatures. We find that post-impact CMB pressures are generally lower than previously assumed, due to both thermal and rotational effects. Full mantle melting is common and a substantial fraction of mantle material is heated above the Fe-MgO solvus closure temperature for impacts with modified specific energies $Q_S>10^6$ J/kg, implying that a miscible layer could form close to the CMB for many giant impacts. The comparatively low internal pressures and large regions of metal-silicate miscibility after giant impacts have significant effects on the processes of core formation, and our work indicates that metal-silicate equilibration would occur near the CMB during later post-impact cooling, consistent with Earth's geochemistry.