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The formation and orbital evolution of Saturn's inner mid-sized moons arestill debated. The most puzzling aspects are 1) how the Tethys-Dione pair andthe Mimas-Enceladus pair passed through their strong 3:2 mean-motion resonancesduring the tidal orbital evolution, and 2) the current strong heat flow fromEnceladus, which is a few orders of magnitude higher than the tidal energydissipation caused by the present orbital eccentricity of Enceladus. Here weperform N-body simulations of the moons' orbital evolution from various initialconditions -- assuming that the moons were formed from Saturn's hypotheticalmassive ring -- and investigate possible paths to solve the above difficulties.If the influence of the rings is neglected, we find that the Tethys-Dione paircannot avoid becoming trapped in the mean-motion resonances as they recede fromSaturn, and that the Tethys-Enceladus pair cannot avoid collisions after theresonance trapping, in case Saturn's quality factor is smaller than 15,000.These findings are inconsistent with the current orbital configuration.However, taking into account both the eccentricity excitation and the orbitalexpansion caused by the ring torque, we find that these resonance captures areavoided. With the high eccentricity pumped up by the torque, Enceladus passesthrough the resonances with Tethys, and the Dione-Tethys pair passes throughtheir 2:1 and possibly the 3:2 resonance as well. After Enceladus residesbeyond the 2:1 resonance with the outer ring edge, the eccentricity can betidally damped. While this is a promising path of evolution, in most runs,Enceladus collides with Tethys by the excited eccentricity. There is a hintthat a ring mass decrease could avoid the collision. The tidal heat due to theeccentricity excitation by the ring torque is stored in the moons and slowlyradiated away. It may account for the current high heat flow.

Orbital evolution of Saturn's mid-sized moons and the tidal heating of Enceladus