Impact-induced Vaporization During Accretion of Planetary Bodies

TL;DR

This paper investigates how giant impacts during planetary formation cause vaporization of rocky bodies, affecting their composition, using molecular dynamics simulations and a new entropy-based criterion for vapor formation.

Contribution

It introduces a novel entropy-based criterion for vapor formation in impact simulations and quantifies impact velocities leading to vaporization during planetary accretion.

Findings

Vaporization occurs at impact velocities above 7.1 km/s for chondritic bodies.

Most impacts in late-stage accretion simulations reach vaporization thresholds.

Vaporization significantly influences the composition and early environment of terrestrial planets.

Abstract

Giant impacts dominate the late stages of accretion of rocky planets. They contribute to the heating, melting, and sometimes vaporizing of the bodies involved in the impacts. Due to fractionation during melting and vaporization, planet-building impacts can significantly change the composition and geochemical signatures of rocky objects. Using first-principles molecular dynamics simulations, we analyze the shock behavior of complex realistic silicate systems, representative of both rocky bodies. We introduce a novel criterion for vapor formation that uses entropy calculations to determine the minimum impact velocity required to pass the threshold for vapor production. We derive impact velocity criteria for vapor formation (7.1 km per s for chondritic bodies) and show that this threshold is reached in 61 and 89 percent of impacts in dynamical simulations of the late stages of accretion with classical and annulus starting configuration (respectively) for analogs of Earth. These outcomes should be nuanced by factors such as the impact angle and the mass of the impacting bodies, which further influence the vaporization dynamics and the resultant material distribution. Our findings indicate that vaporization was common during accretion and likely played a crucial role in shaping the early environments and material properties of terrestrial planets.