Probing the Interior of Enceladus from Eruption Activity

Abstract:

Saturn’s moon Enceladus has a very active province at its south pole, where geysers of water vapor and ice emanate from warm tectonic ridges. This intense activity is probably the result of tidal stresses experienced by Enceladus as it moves around Saturn on a slightly eccentric orbit, and it suggests the presence of an internal water reservoir. Infra-red (VIMS, Hedman et al. Nature, 2013) and visible (ISS, Nimmo et al., Astronom. J 2014) observations showed an orbital modulation of the eruption activity, consistent with time varying stresses. However, the activity appears to be delayed by several hours with respect to the diurnal tides predicted for a global ocean and elastic response (Nimmo et al. Astronom. J. 2014). Here, by modeling the viscoelastic tidal response of Enceladus with a full three-dimensional model (Behounkova et al. Icarus 2012), we show that the delay in eruption activity is mostly controlled by the ocean size and the viscosity structure in the south polar region. Comparisons between modeled stress variations along faults and ISS and VIMS plume brightness data indicate that the observed activity is consistent with a regional sea, extending about 40-60^◦ from the south pole at depth of 40 to 60 kilometers, consistent with Cassini gravity data (Iess et al. Science 2014), although viscoelastic solutions with a global ocean cannot be ruled out at the moment. Our calculations show that the tidally-controlled eruption activity requires a thin lithosphere (<5 km) in the south polar region and a warm ice mantle having a viscosity of about 10¹³-10¹⁴ Pa.s above the ocean area. Future observations will permit us to refine the interior models compatible with the data, thus providing a powerful tool to probe the internal structure and dynamics of Enceladus.