Did Uranus' regular moons form via a rocky giant impactor?

TL;DR

This study investigates whether Uranus's regular moons formed from a giant impact by combining impact simulations, disc modeling, and satellite assembly, suggesting a rocky impactor could explain the system's features.

Contribution

It presents a comprehensive model combining impact, disc evolution, and satellite formation to explain Uranus's moons and tilt, proposing a rocky impactor scenario as the most plausible.

Findings

A rocky impactor best reproduces Uranus's satellite system.

Oblique impact explains Uranus's large tilt and low heat flux.

Impact scenario accounts for the mass and composition distribution of moons.

Abstract

The formation of Uranus' regular moons has been suggested to be linked to the origin of its enormous spin axial tilt (~98^o). A giant impact between proto-Uranus and a 2-3 M_Earth impactor could lead to a large tilt and to the formation of an impact generated disc, where prograde and circular satellites are accreted. The most intriguing features of the current regular Uranian satellite system is that it possesses a positive trend in the mass-distance distribution and likely also in the bulk density, implying that viscous spreading of the disc after the giant impact plays a crucial role in shaping the architecture of the final system. In this paper, we investigate the formation of Uranus' satellites by combining results of SPH simulations for the giant impact, a 1D semi-analytic disc model for viscous spreading of the post-impact disc, and N-body simulations for the assembly of satellites from a disc of moonlets. Assuming the condensed rock (i.e., silicate) remains small and available to stick onto the relatively rapid growing condensed water-ice, we find that the best case in reproducing the observed mass and bulk composition of Uranus' satellite system is a pure-rocky impactor with 3 M_Earth colliding with the young Uranus with an impact parameter b = 0.75. Such an oblique collision could also naturally explain Uranus' large tilt and possibly, its low internal heat flux. The giant impact scenario can naturally explain the key features of Uranus and its regular moons. We therefore suggest that the Uranian satellite system formed as a result of an impact rather than from a circumplanetary disc.