How Uranus Fell Over: Consequences of Giant Impacts with High Resolution Simulations

We have performed a suite of smoothed particle hydrodynamics (SPH) simulations to investigate in detail the results of a giant impact on the young Uranus. This cataclysmic event could explain Uranus' remarkable 98° obliquity and we also study the internal structure, atmospheric retention, and orbiting debris of the post-impact planet.

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 R. 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 M_⊕ 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 nal planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 M impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (3 × 10³⁶ kg m² s^-1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 M_⊕ impactors than 3 M_⊕ ones, because it requires tidal disruption that is suppressed by the more massive impactors.

We also present here results from new simulations with two to three orders of magnitude better resolution than our previous set and the literature norm, with 10⁷ to over 10⁸ SPH particles. The figure shows mid-collision snapshots from one of these simulations with the particles coloured by their internal energy, showing some of the details that can now be resolved.