Heating of Enceladus due to the dissipation of ocean tides

Abstract

Given Cassini observations of a large and unexplained endogenic heat source, the subsurface ocean on Enceladus is an important test bed for modeling icy-satellite ocean tides and the dissipative heat generated. Here we revisit and expand estimates of the ocean tidal power (heating rate) to describe how the power depends on input parameter and process assumptions. The approach taken in this (and previous) work by this author is that the dissipation process in Enceladus' ocean is unknown and cannot be reliably modeled from first principles. Considered instead are generic dissipation forms that cover specific physical dissipation processes as special cases (specific cases described here include linear/nonlinear drag, eddy viscosity, viscoelastic ice coupling, and vertical wave damping). The justification for basing conclusions primarily on the generic (rather than specific) dissipation forms is that whatever the ocean dissipation process, it must draw from the stored energy (kinetic or potential) in the ocean tidal response, and this process must have an associated time scale for the dissipation to take place. By treating this time scale (as well as the Lamb number/wave speed) as free parameters, millions of tidal solutions are calculated to sample the full domain of tidal scenarios that arrive from different combinations of these two parameters, as well as the assumed dissipation form. Conclusions drawn from the general behavior in this larger solution domain are then considered to be more robust than conclusions drawn from the subdomain that assumes the validity of a specific physical dissipation process. The results show plausible states whereby the ocean tides (even if Enceladus is covered with thick ice) generate the observed heat flux. While the same results show that low-power, non-resonant states are also allowed, an argument is presented that these low-power states may not be stable in the presence of secular trends (e.g. cooling, stratification). Rather, self-tuned resonance stabilizes the tidal configuration in a near-resonant forced state with high power and dissipative heat.