Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion

TL;DR

This study uses simulations to explore how giant impacts affected early Uranus's rotation, internal structure, atmosphere, and debris, providing insights into its current properties and formation history.

Contribution

It offers detailed simulation results on impact effects on Uranus, including rotation, internal layering, atmospheric loss, and debris distribution, advancing understanding of planetary impact outcomes.

Findings

Impacts of at least 2 Earth masses can induce rapid rotation.

Most atmosphere remains bound, but significant ejection occurs at certain angular momenta.

Impact debris and energy deposition explain Uranus's heat flow and magnetic field asymmetry.

Abstract

We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky core falls in to the core of the target. However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer. Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ~3 Earth radii. This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field. We verify the results from the single previous study of lower resolution simulations that an impactor with a mass of at least 2 Earth masses can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta. At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 Earth mass impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (~3*10^36 kg m^2 s^-1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 Earth mass impactors than 3 Earth mass ones, because it requires tidal disruption that is suppressed by the more massive impactors.