The shape of Enceladus as explained by an irregular core: Implications for gravity, libration, and survival of its subsurface ocean

Enceladus' global shape is not consistent with a simultaneously hydrostatic and fully differentiated body, but its geophysical hyperactivity strongly implies that it is differentiated. Enceladus' surface conforms to a triaxial shape, consistent with relaxation to a global geoid, but its rocky core need not be hydrostatic. A modestly irregular core, either in terms of topography or density, and dynamically aligned, would act to enhance the global geoid. Explaining the full global shape of Enceladus requires ~10 km of excess core polar flattening and ~4 km of excess core equatorial distortion, for a uniform density core. The stresses associated with this excess topography can be sustained indefinitely, but because the rocky core is not hydrostatic, Enceladus' degree‐2 gravity coefficients J 2 and C 22 will not conform to the hydrostatic ratio of 10/3 (testable by Cassini gravity). Notably, core polar axes could be depressed by up to 4–6 km with respect to internal isobars. Such a topographic variation could help preserve polar ocean remnants. For example, if Enceladus' ocean undergoes secular freezing, it will first ground near the tidal axes. Polar seas would be the last to freeze, and survival of a south polar sea may be one reason why Enceladus' activity today is concentrated at high southern latitudes. Isostatic variations in the thickness or density of a floating ice shell may also contribute to Enceladus' nonhydrostatic shape, but a strength‐supported, nonisostatic icy lithosphere is argued here to be unlikely, given Enceladus' record of regional high heat flows, faulting, and viscous relaxation.