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Most of the main planetary satellites of our Solar System are expected to bein synchronous rotation, the departures from the strict synchronicity being asignature of the interior. Librations have been measured for the Moon, Phobos,and some satellites of Saturn. I here revisit the theory of the longitudinallibrations in considering that part of the interior is not hydrostatic, i.e.has not been shaped by the rotational and tidal deformations, but is fossil.This consideration affects the rotational behavior. For that, I derive thetensor of inertia of the satellite in splitting these two parts, beforeproposing an analytical solution that I validate with numerical simulations. Iapply this new theory on Mimas and Epimetheus, for which librations have beenmeasured from Cassini data. I show that the large measured libration amplitudeof these bodies can be explained by an excess of triaxiality that would notresult from the hydrostatic theory. This theory cannot explain the phase shiftwhich has been measured in the diurnal librations of Mimas. This speaks againsta solid structure for Mimas, i.e. Mimas could have a global internal ocean.

Interpreting the librations of a synchronous satellite -- How their phase assesses Mimas' global ocean