Retention of primordial atmospheres through giant impacts

Abstract

Rocky planets the size of Earth or larger are expected to undergo one or more giant impacts during the final stages of their formation. Some of these collisions occur after protoplanetary disk dissipation, potentially driving the loss of primordial hydrogen-rich atmospheres through elevated surface temperatures. However, whether giant impacts promote atmospheric loss or retention remains unclear because they both heat planetary interiors and increase atmospheric gravitational binding. Here we model the evolution of rocky planets after their largest post-disk giant impact, accounting self-consistently for the concurrent increase in gravitational binding energy and mantle thermal energy. Whereas the entire accreted mass contributes to the increase in gravitational binding energy, only part of the impact energy raises mantle temperature. Mantle melting consumes energy as latent heat, while the impactor's metallic core transfers little energy to the mantle and remains superheated at depth. Giant impacts therefore increase gravitational binding energy more than mantle thermal energy, shifting planets towards atmospheric retention. Larger, atmosphere-bearing planets emerge preferentially through collisional growth. By contrast, smaller planets without such impacts tend to lose their primordial atmospheres under sustained stellar irradiation. At the population level, these results suggest that sub-Neptunes have, on average, more massive rocky interiors than super-Earths.