Planetary Giant Impacts

Abstract

Giant impacts dominate many planets' late accretion and evolution, but the detailed consequences of these violent events are still poorly understood. In this thesis I use 3D smoothed particle hydrodynamics simulations of giant impacts to study three primary topics: the impact origin of Uranus' obliquity; numerical convergence with increasing resolution; and atmospheric erosion by giant impacts. To these ends, the SWIFT code is developed to model planetary impacts with 1000 times more simulation particles than the current standard, and an efficient method for creating relaxed initial conditions is presented. A suite of simulations of giant impacts onto the young Uranus confirms that the planet's high obliquity can be explained by a wide range of impact scenarios. For some grazing collisions, most of the impactor's ice and energy is deposited in a thin shell in the target's outer ice layer, which might help explain Uranus' observed lack of heat flow from the interior. Follow-up simulations with just over $10^8$ simulation particles reveal that standard simulations with fewer than $10^7$ particles fail to converge on even bulk properties like the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these large-scale results reliably, but even $10^8$ particles may not be sufficient to study the detailed composition of scattered debris. This improvement in resolution then enables the first full, 3D simulations of atmospheric erosion on terrestrial planets by giant impacts. For head-on collisions, there is a rapid change with increasing impact speed from very little erosion to total loss. However, for grazing impacts there is a more gradual change with speed and a non-monotonic dependence on the impact angle. These projects highlight the necessity of high-resolution simulations in three dimensions to capture the complexity of giant impacts. In the final chapter, a model of the Moon's argon exosphere is used to test competing explanations for the strange features observed by the recent LADEE mission. The persistent overdensity of argon over the maria can only be reproduced by a localised endogenic source. This offers a novel probe of the lunar interior late after its origin in a giant impact.