Several planetary satellites apparently have subsurface seas that are ofgreat interest for, among other reasons, their possible habitability. Thegeologically diverse Saturnian satellite Enceladus vigorously vents liquidwater and vapor from fractures within a south polar depression and thus musthave a liquid reservoir or active melting. However, the extent and location ofany subsurface liquid region is not directly observable. We use measurements ofcontrol points across the surface of Enceladus accumulated over seven years ofspacecraft observations to determine the satellite's precise rotation state,finding a forced physical libration of 0.120 $\pm$ 0.014{\deg} (2{\sigma}).This value is too large to be consistent with Enceladus's core being rigidlyconnected to its surface, and thus implies the presence of a global oceanrather than a localized polar sea. The maintenance of a global ocean withinEnceladus is problematic according to many thermal models and so may constrainsatellite properties or require a surprisingly dissipative Saturn.
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