A Past Episode of Rapid Tidal Evolution of Enceladus?

Abstract

Saturn possesses a dynamically rich system containing numerous moons and impressive rings. Whether the rings of Saturn are much younger than the planet itself has been a long-standing open question; more recently a young age has been proposed for some moons. The recent detection of the fast orbital evolution of Rhea and Titan strongly suggests a highly frequency-dependent tidal response of Saturn, possibly through the excitation of inertial waves within the planet’s convective envelope. Here we show that resonance locking to inertial waves cannot explain the dynamical structure of the Saturnian system or the current tidal heating of Enceladus. On the other hand, both the observations and our modeling results indicate that the system is not consistent with evolution under equilibrium tides. We propose that the system’s architecture can best be explained by a relatively high “background” tidal response coupled with discrete resonant modes. In this view, only Titan may be in a true long-term resonance lock with a tidal mode of Saturn. Rhea is most likely currently experiencing a transient period of fast tidal evolution as it passes through a mode, rather than being locked into it. Assuming that Enceladus went through a temporary period of fast tidal evolution, we can reproduce its present resonance with Dione and satisfy other dynamical constraints. Additionally, we conclude that the long-term tidal response of Saturn to Tethys must be weaker than expected from frequency-independent tides, as already found by observations.