Co-accretion + giant impact origin of the Uranus system: Tilting Impact

TL;DR

This paper investigates whether a giant impact combined with co-accretion can explain Uranus's tilted axis and satellite system, finding that current impact models do not produce sufficient debris to reorient the satellites.

Contribution

The study tests the co-accretion plus giant impact hypothesis for Uranus's system and finds that typical impact simulations do not generate enough debris for reorientation, challenging previous models.

Findings

Impacts do not produce enough inner debris disk mass.

Current impact models cannot reorient the outer debris disk.

Alternative scenarios need exploration.

Abstract

The origin of the Uranian satellite system remains uncertain. The four major satellites have nearly circular, co-planar orbits and the ratio of the satellite system and planetary mass resembles Jupiter's satellite system, suggesting the Uranian system was similarly formed within a disk produced by gas co-accretion. However, Uranus is a retrograde rotator with a high obliquity. The satellites orbit in its highly tilted equatorial plane in the same sense as the planet's retrograde rotation, a configuration that cannot be explained by co-accretion alone. In this work we investigate the first stages of the co-accretion + giant impact scenario proposed by Morbidelli et al. (2012) for the origin of the Uranian system. In this model, a satellite system formed by co-accretion is destabilized by a giant impact that tilts the planet. The primordial satellites collide and disrupt, creating an outer debris disk that can re-orient to the planet's new equatorial plane and accrete into Uranus' 4 major satellites. The needed reorientation out to distances comparable to outermost Oberon requires that the impact creates an inner disk with $\ge 1\%$ of Uranus' mass. We here simulate giant impacts that appropriately tilt the planet and leave the system with an angular momentum comparable to that of the current system. We find that such impacts do not produce inner debris disks massive enough to realign the outer debris disk to the post-impact equatorial plane. Although our results are inconsistent with the apparent requirements of a co-accretion + giant impact model, we suggest alternatives that merit further exploration.