The Thermal and Orbital Evolution of Enceladus: Observational Constraints and Models

Enceladus possesses a global subsurface ocean beneath an ice shell a few tens of kilometers thick, and is observed to be losing heat at a rate of 10 GW from its south polar region. Two major puzzles are the source of the observed heat, and how the ocean could have been maintained. Tidal dissipation in Enceladus is ultimately controlled by the rate of dissipation within Saturn, parameterized by the factor Qp. A Qp of about 2000 is indicated by astrometric measurements and generates an equilibrium heating rate at Enceladus sufficient to explain the observed heat and maintain an ocean indefinitely if the ice shell is conductive. If constant, this Qp would indicate an age for Enceladus much less than that of the solar system. An alternative, however, termed the "resonance-locking" scenario, is that the effective Qp is time-variable, such that the heating rate is almost constant over geological time. This scenario can explain the long-term survival of the ocean and the present-day heat flux without requiring Enceladus to have formed recently.