Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

Saturn is orbited by dozens of moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Tidal friction within Saturn causes its moons to migrate outwards, driving them into orbital resonances that pump their eccentricities or inclinations, which in turn leads to tidal heating of the moons. However, in giant planets, the dissipative processes that determine the tidal migration timescale remain poorly understood. Standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance(1), implying negligible migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titan's orbital expansion rate. We find that Titan rapidly migrates away from Saturn on a timescale of roughly ten billion years, corresponding to a tidal quality factor of Saturn of Q similar or equal to 100, which is more than a hundred times smaller than most expectations. Our results for Titan and five other moons agree with the predictions of a resonance-locking tidal theory(2), sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn's moon system. In particular, it suggests that Titan formed much closer to Saturn and has migrated outward to its current position. Titan is migrating away from Saturn on a much shorter timescale than expected, lending support to the resonance-locking tidal theory. This result motivates a revision of the evolutionary history of Saturn's moon system and may be relevant to other giant planets.