The energy budgets of giant impacts

TL;DR

This paper investigates the energy dynamics of giant impacts that form Earth-like planets, revealing diverse energy budgets and stratification outcomes, challenging the canonical Moon-forming impact as a typical scenario.

Contribution

It provides a comprehensive analysis of the energy exchange and internal heating during giant impacts, highlighting the variability and complexity beyond the traditional Moon-forming impact model.

Findings

Giant impacts exhibit a wide range of energy budgets.

Post-impact bodies are thermally stratified with vaporized mantles.

The canonical Moon-forming impact is relatively low energy, not representative of all impact scenarios.

Abstract

Giant impacts dominate the final stages of terrestrial planet formation and set the configuration and compositions of the final system of planets. A giant impact is believed to be responsible for the formation of Earth's Moon, but the specific impact parameters are under debate. Because the canonical Moon-forming impact is the most intensely studied scenario, it is often considered the archetypal giant impact. However, a wide range of impacts with different outcomes are possible. Here we examine the total energy budgets of giant impacts that form Earth-mass bodies and find that they differ substantially across the wide range of possible Moon-forming events. We show that gravitational potential energy exchange is important, and we determine the regime in which potential energy has a significant effect on the collision outcome. Energy is deposited heterogeneously within the colliding planets, increasing their internal energies, and portions of each body attain sufficient entropy for vaporization. After gravitational re-equilibration, post-impact bodies are strongly thermally stratified, with varying amounts of vaporized and supercritical mantle. The canonical Moon-forming impact is a relatively low energy event and should not be considered the archetype of accretionary giant impacts that form Earth-mass planets. After a giant impact, bodies are significantly inflated in size compared to condensed planets of the same mass, and there are substantial differences in the magnitudes of their potential, kinetic and internal energy components. As a result, the conditions for metal-silicate equilibration and the subsequent evolution of the planet may vary widely between different impact scenarios.