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

TL;DR

This study investigates how protoplanetary differentiation influences nitrogen distribution in rocky planets, revealing that rapid accretion of planetary embryos is crucial for maintaining Earth's nitrogen budget.

Contribution

It provides new experimental data on nitrogen solubility in magma oceans and models nitrogen redistribution during planetary growth and differentiation.

Findings

Nitrogen is more soluble in magma oceans than previously thought.

Asteroid-sized protoplanets are nitrogen depleted, while larger planetary embryos can retain nitrogen in cores.

Rapid accretion (within 1-2 million years) is necessary for Earth-like nitrogen budgets.

Abstract

The effect of protoplanetary differentiation on the fate of life essential volatiles like 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 and high temperature experiments in graphite undersaturated conditions, here we show that the 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 and 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 attained nitrogen from the cores of 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 to 2 million years of the Solar System formation.