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

TL;DR

This study measures Titan's rapid orbital expansion, supporting a resonance locking tidal theory that suggests significant outward migration and revised evolutionary history for Saturn's moon system, with implications for other celestial systems.

Contribution

It provides the first direct measurements of Titan's orbital expansion rate, confirming resonance locking as a key tidal dissipation mechanism in giant planets.

Findings

Titan's orbital expansion rate is 11.3 ± 2.0 cm/year.

Resonance locking explains the observed migration and tidal dissipation.

Implications for the evolution of Saturn's moon system and other celestial systems.

Abstract

Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance, implying little 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 Titan migrates away from Saturn at 11.3 $\pm$ 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q $\simeq$ 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory, 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. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed.