Librational response of Enceladus

Physical librations could significantly contribute to Enceladus' geophysics through their influence on tidal stress. Therefore it is important to determine their behavior and the present paper is devoted to estimating Enceladus' libration in longitude. In a rotational model of Enceladus with no global ocean, we introduce the main perturbative terms of its orbital longitude and the tidal coupling. The main librations of Enceladus are related to indirect perturbations of the orbit of Enceladus by Dione (11 years and 3.7 years periods) with amplitudes of 933.4″ (1.14 km) and 676.6″ (827 m), respectively. These amplitudes are almost independent of the body's triaxiality. The third main libration is due to the direct gravitational attraction of Saturn and its period is equal to that of the mean anomaly of Enceladus with an amplitude between 93.1″ and 113.5″ (i.e., 112 and 139 m), depending on triaxiality. These amplitudes are consistent with the upper bound of 1.5° (6.6 km) inferred from observations with the Cassini‐Huygens spacecraft. The nonrigid body libration amplitudes due to tidal coupling are negligible. Nevertheless, tidal dissipation induces a small phase shift up to 0.57° corresponding to a displacement of Enceladus' figure of 1 m along the moon's equator at the mean anomaly period.

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