Repeated Impact-Driven Plume Formation On Enceladus Over Megayear Timescales

Abstract

Water plumes erupting from the `tiger stripe' features on the south pole of Enceladus are thought to connect to a global subsurface ocean. Proposed origins for the initial stress necessary to form the `tiger stripes' include a giant impact, which would require true polar wander to explain the location of the plumes if the impact did not occur at the South Pole, or tensile stresses, which would require volumetric expansion associated with partial freezing of the subsurface ocean. In addition to the requirement of a fine-tuned impact, true polar wander, or partial freezing of the subsurface ocean, a further issue with these hypotheses is that the `tiger stripes' may be short-lived. We show here that impact resurfacing can seal off plumes and mass loss can lead to their compression and closure over $\sim 1 \mathrm{\;Myr}$. Since plumes are observed at present, a mechanism by which new plumes can be generated every $\sim 1 \mathrm{\;Myr}$ and by which such plumes are most likely to form at the south pole is needed. We propose and investigate the possibility that impacts constitute an adequate repeating source for the continual instigation of fractures and plumes. We find that the rate of impacts on Enceladus suggests the formation of $\sim 10^3$ independent plume systems per Gyr, the vast majority on the south pole, and is consistent with the Cassini-derived age of the south pole for a lunar-like bombardment history, our estimates of fracture lifetimes, and with the needed parameters for parallel fracture propagation. The model favors a bombardment history similar to that of Triton over one more similar to that of the Galilean satellites.