Enceladus’s subsurface sea is part of a global ocean as shown by measured physical libration

Abstract:

The Saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression. The source appears to be a liquid reservoir rather than active heating, but the extent and location of any subsurface liquid region is not directly observable. We use measurements of control points across the surface of Enceladus from seven years of spacecraft observations to determine the satellite’s precise rotation state; we find a forced physical libration of 0.120 ± 0.014° (2σ). This value is too large to be consistent with Enceladus’s core being rigidly connected to its ice shell, and thus implies the presence of a global ocean rather than a solitary polar sea. Together with other constraints, our results imply that the global ocean is thin and connected to a thicker sea under the south polar terrain. We present models of the range of shell, ocean, and core sizes consistent with the libration and other geophysical data. The maintenance of a global ocean within Enceladus is problematic according to some thermal models and so may require particular satellite properties or a surprisingly dissipative Saturn.