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 distance¹, 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 ≃ 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 theory², 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.

土星由数十颗卫星环绕运行，这个复杂系统的复杂动力为其形成和演化提供了线索。土星内的潮汐摩擦使卫星向外迁移，使它们进入轨道共振，从而推动其离心率或倾斜度，进而导致卫星潮汐加热。然而，在巨型行星中，决定潮汐迁移时间尺度的耗散过程仍然知之甚少。标准理论表明，轨道膨胀率与距离^1中的功率11/2成反比，这意味着土星最大的卫星泰坦等外卫星的迁移微不足道。在这里，我们使用卡西尼号飞船获得的两次独立测量值来测量泰坦的轨道膨胀率。我们发现，泰坦迅速迁移远离土星上的大约10十亿年的时间尺度，相当于土星的潮汐品质因数Q ≃ 100，这比大多数的预期更小的超过一百倍。我们对土卫六和其他五个卫星的研究结果与共振锁定潮汐理论的预测吻合²，由行星内部的惯性波激发而维持。相关的潮汐膨胀仅对轨道距离敏感，从而促使对土星卫星系统演化历史的修正。特别是，它暗示土卫六的形成距离土星更近，并已向外迁移到当前位置。