Past Orbital Evolution of Enceladus through Saturn’s Tidal Modes

Tidal evolution of Saturnian moons is a topic of great interest, as it holds keys to determining the age of the system. Direct measurements of tidal acceleration of Rhea and Titan (Lainey et al. 2017, 2020) have shown tidal evolution to be very frequency dependent, possibly involving the moons’ orbits being locked to resonant modes within the planet (Fuller et al. 2016). We argue that locking to resonant modes does not match the observed tidal heating of Enceladus, and that both resonant tides and strong equilibrium (frequency-independent) tides are needed to explain the system. While the current Mimas-Tethys resonance and the tidal heating of Enceladus can be explained by equilibrium tides, past evolution of Enceladus requires episodes of resonant tidal response. We will show numerical simulations that demonstrate that the current Enceladus-Dione resonance combined with the low inclination of Enceladus can only be explained by a a past episode of rapid tidal evolution of Enceladus. This result strongly implies that the inner moons can pass and “outrun”, rather than being locked to, resonant tidal modes. We will discuss the implications of this result for the evolution of the system.