ISOSTASY ON A SMALL ICY MOON: IMPLICATIONS FOR ENCELADUS'S ICE SHELL STRUCTURE (Invited Presentation)

The interior of Enceladus, the extent of its subsurface ocean, and the structure of its floating ice shell, are of primary importance in understanding the nature of the ongoing geologic activity, its concentration at the south pole, and the thermal state and evolution of this tiny Saturnian moon.

Measurements of Enceladus’s shape, gravity field, and physical libration amplitude have permitted the development of interior models that confirm the presence of a global subsurface liquid ocean; the thickness and structure of the overlying ice shell, however, has been more difficult to determine with precision. Part of the difficulty is that, due to its fast rotation and substantial departure from hydrostatic equilibrium, standard approaches to interior modeling are not well suited to Enceladus, and can lead to significant errors. Because of the small radius, results are also sensitive to the details of the isostatic compensation model. Converging on a picture of the ice shell structure that simultaneously satisfies all the theoretical and observational constraints has thus been a challenge.

Here we resolve some of the apparent discrepancies by employing a new compensation model that accommodates the spherical geometry in a manner that is consistent with the principle of isostatic equilibrium. We also employ a high fidelity numerical approach to modeling the hydrostatic equilibrium figure. The resulting shell thickness estimates are smaller than in early gravity-based interior models, suggesting the possibility of an extremely thin ice shell (perhaps only a few km thick) at the south pole. Finally, we discuss implications of the inferred shell structure for the heat budget and thermal evolution of Enceladus.