Collisions between Gravity-Dominated Bodies: 2. The Diversity of Impact Outcomes during the End Stage of Planet Formation

TL;DR

This study uses advanced collision physics models to analyze impact outcomes during planet formation, revealing diverse collision regimes, their effects on planetary growth, composition, and debris production, contrasting with simpler perfect-merging assumptions.

Contribution

It introduces a new analytic collision physics model to simulate realistic impact outcomes, showing their impact on planetary growth and composition during formation.

Findings

Collision outcomes are diverse, including hit-and-run, partial accretion, erosion, and disruption.

Fewer planets reach >0.7 Earth masses with realistic collision physics compared to perfect merging.

Significant debris (~15%) is produced, mainly from erosion and hit-and-run events.

Abstract

Numerical simulations of the stochastic end stage of planet formation typically begin with a population of embryos and planetesimals that grow into planets by merging. We analyzed the impact parameters of collisions leading to the growth of terrestrial planets from recent $N$-body simulations that assumed perfect merging and calculated more realistic outcomes using a new analytic collision physics model. We find that collision outcomes are diverse and span all possible regimes: hit-and-run, merging, partial accretion, partial erosion, and catastrophic disruption. The primary outcomes of giant impacts between planetary embryos are approximately evenly split between partial accretion, graze-and-merge, and hit-and-run events. To explore the cumulative effects of more realistic collision outcomes, we modeled the growth of individual planets with a Monte Carlo technique using the distribution of impact parameters from $N$-body simulations. We find that fewer planets reached masses $>0.7 M_{\rm Earth}$ using the collision physics model compared to simulations that assumed every collision results in perfect merging. For final planets with masses $>0.7 M_{\rm Earth}$, 60% are enriched in their core-to-mantle mass fraction by >10% compared to the initial embryo composition. Fragmentation during planet formation produces significant debris ($\sim15$% of the final mass) and occurs primarily by erosion of the smaller body in partial accretion and hit-and-run events. In partial accretion events, the target body grows by preferentially accreting the iron core of the projectile and the escaping fragments are derived primarily from the silicate mantles of both bodies. Thus, the bulk composition of a planet can evolve via stochastic giant impacts.