Abstract

The final stage of terrestrial planet formation is marked by highly energetic collisions between planetary sized bodies, known as giant impacts. Giant impacts melt and vaporize substantial fractions of the colliding bodies and can leave the post-impact body rapidly rotating. Some fraction of giant impacts have sufficient energy and angular momentum (AM) to produce a previously unrecognized planetary object, called a synestia. It has recently been suggested that our Moon was formed from a synestia created by the last giant impact in Earth’s accretion. I will demonstrate that the internal pressures of Earth-like planets do not increase monotonically during the giant impact stage, but can vary substantially in response to changes in rotation and thermal state. The internal pressures in an impact-generated synestia are much lower than in a condensed, slowly rotating planet of the same mass. For example, the core-mantle boundary (CMB) pressure can be as low as 60 GPa for a synestia with Earth mass and composition, compared to 136 GPa in the present-day Earth. The lower pressures are due to the low density and rapid rotation of the post-impact structure. After the formation of the Moon from a synestia, the internal pressures in the interior of the synestia would have increased to present-day Earth values in two stages: first by vapor condensation and second by removal of AM from the Earth during the tidal evolution of the Moon. The pressure evolution of the Earth has several important implications for its structure and geochemistry, which I will discuss.