Geodynamic modeling of the ice-ocean system on Enceladus

Kurzfassung

The Saturnian moon Enceladus is one of the most geologically active bodies in the solar system. Ridged terrains dominated by intense tectonism are observed on both hemispheres while plume jets emanating from geological surface cracks are con?ned to the South Polar Terrain, thereby suggesting lateral heterogeneity of the satellite's internal structure. While the latter is not directly accessible, thermodynamic modeling provides important insights on its composition and physical properties. This study aims at building a consistent thermal model of the ice-ocean system to derive the layering of the ice shell and its lateral variations. Comparison with current ice shell thickness estimations and key observables such as mean density, moment of inertia factor or heat flux allows to further constrain the satellite's core density. We start from building internal structural reference models composed of four spherically symmetric homogeneous layers: a core, a salty liquid water layer, a lower warm ductile ice layer and an upper cold brittle ice layer. Since the total ice shell thickness is small compared to the satellite's mean radius (less than 10%), we apply and validate the thin shell approximation. It allows us to further compute a three dimensional thermodynamic model of the ice-ocean system accounting for the dissipation of tidal energy induced by diurnal tides. We show that the higher the core density, the thinner the ice shell to keep the salty ocean density within a realistic range (typically below 1250 kg m-3). We show that tidal dissipation solely occurs within the viscous ductile ice layer, and increases towards the poles. The resulting tidal polar surface heat flow is roughly ?ve times higher compared to the equatorial one. We finally propose a method to derive lateral variations of the brittle-to-ductile boundary that is consistent with the computed temperature distribution. Comparison with output power measurements and current estimations of lateral variations of the ice shell thickness allows us to constrain the core density around 3000 kg m-3, corresponding to a partly de-hydrated core.