Co-accretion + giant impact origin of the Uranus system

The four largest Uranian satellites have near circular and co-planar orbits. Similar to the other giant planets, the total mass of the satellite system is 10^-4 times the planetary mass, and the composition of these four satellites is ~50/50 rock/ice. But unlike the other gas giants, Uranus is a retrograde rotator (obliquity of 98°), and its satellites orbit in its highly tilted equatorial plane. Co-accretion alone cannot explain the Uranian architecture because gas inflow would produce a prograde disk with respect to Uranus’ orbit, in the opposite sense to that observed. Additionally, formation by a giant impact does not appear able to produce the current Uranian satellite system as it produces disks which are typically rock-poor and radially compact.

In this study, we are exploring whether a combination of co-accretion and a giant impact could explain the observed properties of the Uranian system (Morbidelli et al. 2012). In this scenario, a prograde satellite system is formed by co-accretion around a moderately oblique Uranus. A giant impact further tilts the planet to its current obliquity and impulsively perturbs the primordial satellite system into crossing orbits, creating an outer debris disk, which is initially tilted to the new planetary equatorial plane. An inner compact disk, produced by the Uranus tipping impact, enhances the effect of Uranus’ J₂ and realigns the outer disk to the new equatorial plane. In this way, the reaccreted satellites preserve the mass ratio and composition of the satellite system produced by Uranus’s earlier gas accretion.

We performed impact simulations onto Uranus in order to test this scenario. Previous studies have found that only large, grazing impacts could produce a massive inner disk (>0.01 Uranus mass). In this work, the impact parameters are constrained such that the post-impact angular momentum is comparable to the present-day value, and its orientation is in the same sense as the collisionally-relaxed outer debris disk. This constraint limits the impact phase-space compared to previous studies. We will present impact simulations that will determine whether such impacts can tilt Uranus and generate an inner debris massive enough to reorient the primordial satellite system.