A new spin on librational dynamics and an appraisal of tidal-librational dissipation in Enceladus

Synchronously rotating satellites on eccentric orbits undergo a variable gravitational pull and torque from their parent planet, resulting in periodic global-scale deformations (tides) and spin rate variations (librations). Observations of tides and librations have been insightful to constrain the interior structure of ocean-bearing icy moons. We revisit the subtle coupling between tides and librations, which must really be recognized as the two sides of the same coin. Previous models of librational dynamics based on the formalism of Love numbers misrepresented this coupling, entailing non-physical cavitation and interpenetration at the ocean boundaries. From an extensive benchmark between a set of numerical and semi-analytical models, and with numerous illustrative animations, we pinpoint a new coupling mechanism between tides and librations, which we coin librational loading. We also show that, for Enceladus, librational loading makes tidal-librational dissipation at least 25% larger than traditional tidal dissipation.