The Shape of Enceladus' Core: Predictions for Degree-2 Nonhydrostatic Gravity, and Role in Survival of the Subsurface Ocean

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The global shape of Enceladus is not consistent with a simultaneously hydrostatic and fully differentiated body, but hypotheses that Enceladus is either undifferentiated or preserves a globally unrelaxed figure from an earlier position closer to Saturn are implausible. Enceladus' geophysical activity (and surface) is best understood in the context of a differentiated (rock separated from ice) interior. Topographic profiles indicate that Enceladus' surface conforms to a triaxial shape, consistent with relaxation to a global geoid. Enceladus' rocky core need not be hydrostatic, however. A modestly "lumpy" core, either in terms of topography or density, and dynamically aligned, will act to enhance the global geoid. Explaining the global shape of Enceladus requires ~12 km of excess core polar ellipticity and ~5 km of excess core equatorial ellipticity, for a uniform density core. The stresses in Enceladus' core associated with this modest level of dynamically excess topography can be sustained indefinitely. Enceladus' icy shell should be isostatic with respect to the satellite's degree-2 gravity, but because the rocky core is not hydrostatic, Enceladus' degree-2 gravity coefficients J2 and C22 should not conform to the hydrostatic ratio of 10/3. The moments-of-inertia implied also indicate that Enceladus could be near a low-order spin-orbit librational resonance, and thus tidal heating associated with this resonance type could have contributed to the moon's phenomenal heat flow. Finally, the core c-axis will be depressed by some 8 km with respect to a hydrostatic shape. This true topographic variation can help preserve polar ocean remnants against freezing (and grounding elsewhere) during epochs of low tidal heating.

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