Compositional evolution during rocky protoplanet accretion

TL;DR

This study uses advanced N-body simulations to explore how collisional erosion during rocky protoplanet growth can explain Earth's non-chondritic composition, revealing diverse outcomes in core-mantle ratios.

Contribution

It introduces a detailed collision model in planet formation simulations, demonstrating mantle stripping can produce Earth's observed composition.

Findings

Embryos exhibit a range of core mass fractions.

Mantle stripping during collisions can explain Earth's Fe/Mg ratio.

Remnant planetesimals show diverse compositions.

Abstract

The Earth appears non-chondritic in its abundances of refractory lithophile elements, posing a significant problem for our understanding of its formation and evolution. It has been suggested that this non-chondritic composition may be explained by collisional erosion of differentiated planetesimals of originally chondritic composition. In this work, we present N-body simulations of terrestrial planet formation that track the growth of planetary embryos from planetesimals. We simulate evolution through the runaway and oligarchic growth phases under the Grand Tack model and in the absence of giant planets. These simulations include a state-of-the-art collision model which allows multiple collision outcomes, such as accretion, erosion, and bouncing events, that enables tracking of the evolving core mass fraction of accreting planetesimals. We show that the embryos grown during this intermediate stage of planet formation exhibit a range of core mass fractions, and that with significant dynamical excitation, enough mantle can be stripped from growing embryos to account for the Earth's non-chondritic Fe/Mg ratio. We also find that there is a large diversity in the composition of remnant planetesimals, with both iron-rich and silicate-rich fragments produced via collisions.