Breaking the symmetry by breaking the ice shell: An impact origin for the south polar terrain of Enceladus

Abstract The origin of the unique south polar terrain on Enceladus is not well understood. Gravity measurements made by Cassini suggest that the ice shell is thinner at the south pole than in the north. Tides are recognized to be the ultimate energy source for the observed thermal anomaly, and are believed to regulate the activity of jets, and movement along the tiger-stripes. However, tides alone are insufficient to explain the observed pattern of heat flow and activity, because the tidal potential is symmetric about the equator and no corresponding thermal anomaly or activity is observed in the north. Tidal heating is highly sensitive to the thickness and the mechanical properties of the ice shell, and the presence of significant lateral variations in the mechanical properties of the ice shell, and in general is stronger when the ice is thinner and weaker. Although the tidal heating might be able to sustain these variations once in place, a non-tidal, physical mechanism is therefore needed to break the tidal symmetry. Here we explore the possibility that a large impact into what is now the SPT could break through the ice shell of Enceladus, and break the hemispheric symmetry as well. We have performed hydrocode simulations of the impacts of icy projectiles into the ice shell of Enceladus, and have modeled the thermal evolution of the ice shell post-impact. We find that an impact capable of forming a crater the size of the present-day south polar terrain could have completely penetrated a 20-km thick ice shell, and would have excavated a cavity extending ~30 km deep in a thicker ice shell. Although the impact does not completely punch through a thicker ice shell, the floor of the crater is extensively fractured, exposing the ocean to space. We find that the impact shock heating will be retained for a few Myr, and that it locally softens the ice to permit more tidal dissipation. The ice shell beneath the impact site thickens over time, but a 1–2 km reduction in topography is retained at the surface.