Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets

The effect of protoplanetary differentiation on the fate of life-essential volatiles such as nitrogen and carbon and its subsequent effect on the dynamics of planetary growth is unknown. Because the dissolution of nitrogen in magma oceans depends on its partial pressure and oxygen fugacity, it is an ideal proxy to track volatile redistribution in protoplanets as a function of their sizes and growth zones. Using high-pressure/temperature experiments in graphite-undersaturated conditions, here we show that the siderophilic (iron-loving) character of nitrogen is an order of magnitude higher than previous estimates across a wide range of oxygen fugacity. The experimental data combined with metal–silicate–atmosphere fractionation models suggest that asteroid-sized protoplanets, and planetary embryos that grew from them, were nitrogen depleted. However, protoplanets that grew to planetary embryo size before undergoing differentiation had nitrogen-rich cores and nitrogen-poor silicate reservoirs. Bulk silicate reservoirs of large Earth-like planets obtained nitrogen from the cores of the latter type of planetary embryos. Therefore, to satisfy the volatile budgets of Earth-like planets during the main stage of their growth, the timescales of planetary embryo accretion had to be shorter than their differentiation timescales; that is, Moon- to Mars-sized planetary embryos grew rapidly within ~1–2 Myrs of the Solar System’s formation. Rates of protoplanetary accretion and differentiation control the depletion of nitrogen in rocky planets, according to high-pressure/temperature experiments that show that nitrogen is extremely siderophilic.