The Energy Budgets of Giant Impacts

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. Plain Language Summary Collisions between large planetary bodies, known as giant impacts,dominate the final stages of the formation of rocky planets like the Earth and set the configuration and compositions of the final planets. A giant impact is believed to have formed Earth's Moon, but the specific configuration of this impact is under debate. Understanding giant impacts is crucial for understanding the formation and evolution of the Earth and the Moon as well as rocky planets around other stars. The traditional Moon-forming impact model is often considered the archetype of a giant impact; however, a wide range of impacts with substantially different outcomes are possible. In this work, we examine the total energies involved in giant impacts that form Earth-like planets and find that there are large differences across the wide range of possible impacts. The internal energy increases cause large portions of each body to vaporize as the result of impacts. Giant impacts produce planetary bodies that are significantly inflated in size compared to condensed planets of the same mass, and there are substantial differences in their potential, kinetic, and internal energies. As a result, how planets and their cores evolve after different impact scenarios may vary widely.

Partial core vaporization during Giant Impacts inferred from the entropy and the critical point of iron

“…Stirring and disruption during the giant impact and inside the protolunar disk can bring large parts of the silicates mantle and of the impactor's core in contact with each other. As our results and previous work (Carter et al, 2020;Nakajima and Stevenson, 2015) show there is a large entropy gain during the impact, it is conceivable that the temperature rise due to this gain is enough to enhance the mixing of lithophile elements with iron metal, which subsequently increase the lithophile element content of the Earth's core. This would boost the mantle-core chemical exchange and equilibration from an early stage and provide the necessary initial state from which chemical unmixing can proceed to fuel the first stages of the dynamo, as was suggested in experiments recording the exsolution of various lithophile components from the liquid core (Badro et al, 2016;Hirose et al, 2017).…”

“…As the predicted pressure thresholds for vaporization can be easily reached, a large amount of iron receives enough entropy to vaporize. The entropy threshold can even be easier exceeded due to the entropy gain after the first and secondary shocks and the conversion of gravitational potential energy to internal energy (Carter et al, 2020;Nakajima and Stevenson, 2015). Once again during this process the confinement of core fragments by the surrounding mantle may prohibit the isentropic expansion and thus the vaporization.…”

The Lithophile Element Budget of Earth's Core

“…The highest pressure and temperature conditions for core formation likely occur during and in the aftermath of giant impacts. As discussed above, in such events much of the core of the impacting body may not equilibrate efficiently with the proto‐Earth's mantle (i.e., k ≪ 1, e.g., Carter et al., 2020; Landeau et al., 2016) and Earth's core likely inherited a strong memory of its earlier accretion. Therefore, the effect of core formation on the composition of Earth's mantle and core cannot be accurately determined using an “average” pressure and temperature of equilibration, as has often been done (e.g., Chidester et al., 2017), and the complicated dynamics of metal‐silicate partitioning in accretionary processes must be considered.…”

Atmosphere loss in planet–planet collisions

“…This study was motivated by the density disparity observed in exoplanet systems. The post-impact bodies in our simulations are hot and inflated, often with a large mass of vaporised silicate, and thus do not represent the structure of planets millions of years after their final giant impacts (Lock & Stewart 2017;Carter et al 2020;Lock et al 2020).…”