Multiple Giant Impacts and Chemical Equilibria: An Integrated Approach to Rocky Planet Formation

Abstract During the formation of rocky planets, the surface environments of growing protoplanets were dramatically different from those of present-day planets. The release of gravitational energy during accretion would have maintained a molten surface layer, forming a magma ocean. Simultaneously, sufficiently massive protoplanets could acquire hydrogen-rich proto-atmospheres by capturing gas from the protoplanetary disk. Chemical equilibration among the atmosphere, magma ocean, and iron core plays a key role in determining the planet’s interior composition. In this study, we investigate terrestrial planet formation under such primitive surface conditions. We conduct N -body simulations to model the collisional growth from protoplanets to planets, coupled with chemical equilibrium calculations at each giant impact event, where surface melting occurs. Our results show that planetary growth proceeds through a series of giant impacts, and the timing of these impacts relative to the dissipation of disk gas significantly influences the volatile budget. In particular, initial impacts, occurring while nebular gas is still present, can lead to excess hydrogen incorporation into the protoplanet’s core. Subsequent impacts with hydrogen-poor bodies, after gas dispersal, can dilute this hydrogen content. This process allows for the formation of a planet with a hydrogen inventory consistent with Earth’s current core. Our findings suggest that late giant impacts, occurring after the depletion of nebular gas, provide a viable mechanism for producing Earth-like interior compositions near 1 au.