Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus

TL;DR

This study investigates the asymmetry in collision outcomes between proto-Earth and proto-Venus during late-stage planetary formation, revealing how hit-and-run collisions influence their divergent evolutionary paths and compositions.

Contribution

It introduces a detailed dynamical model of hit-and-run collisions, highlighting the asymmetrical accretion and loss of planetary runners between Earth and Venus.

Findings

Venus is more likely to accrete runners, acting as a collision sink.

Proto-Earth loses about half of its runners, affecting its growth.

Asymmetries may explain differences in geology and satellite formation.

Abstract

During the late stage of terrestrial planet formation, hit-and-run collisions are about as common as accretionary mergers, for expected velocities and angles of giant impacts. Average hit-and-runs leave two major remnants plus debris: the target and impactor, somewhat modified through erosion, escaping at lower relative velocity. Here we continue our study of the dynamical effects of such collisions. We compare the dynamical fates of intact runners that start from hit-and-runs with proto-Venus at 0.7 AU and proto-Earth at 1.0 AU. We follow the orbital evolutions of the runners, including the other terrestrial planets, Jupiter, and Saturn, in an N-body code. We find that the accretion of these runners can take $\gtrsim$10 Myr (depending on the egress velocity of the first collision) and can involve successive collisions with the original target planet or with other planets. We treat successive collisions that the runner experiences using surrogate models from machine learning, as in previous work, and evolve subsequent hit-and-runs in a similar fashion. We identify asymmetries in the capture, loss, and interchange of runners in the growth of Venus and Earth. Hit-and-run is a more probable outcome at proto-Venus, being smaller and faster orbiting than proto-Earth. But Venus acts as a sink, eventually accreting most of its runners, assuming typical events, whereas proto-Earth loses about half, many of those continuing to Venus. This leads to a disparity in the style of late-stage accretion that could have led to significant differences in geology, composition, and satellite formation at Earth and Venus.