Librations of Enceladus and Titan

A moon in synchronous rotation has longitudinal librations (small deviations from the average rotation) because of its nonspherical mass distribution and its elliptical orbit around the planet. We study the long-period librations of Enceladus and Titan and include deformation effects and the existence of a subsurface ocean. We take into account the fact that the orbit is not Keplerian and has other periodicities than the main period of orbital motion around Saturn due to perturbations by the Sun, other planets and moons. An orbital theory is used to compute the orbital perturbations due to these other bodies. We numerically evaluate the amplitude of the long-period librations for many interior structure models of the moon constrained by the mass, radius and gravity field. Measurements of the librations may give constraints on the interior structure of the icy satellites.