Differentiation of Enceladus and Retention of a Porous Core

Abstract

The Cassini mission revealed gas plumes associated with surface features called “tiger stripes” at the south pole of Saturn’s moon Enceladus. The composition of plume particles and local cryovolcanism suggested as a possible cause for the activity are typically considered in the context of hydrothermal circulation in the rocky core within a differentiated core–ocean–ice crust structure. We model the internal evolution and differentiation of Enceladus heated by radioactive nuclides and tidal dissipation. Calculating the core formation, we investigate its compaction by modeling the evolution of porosity, thereby varying the rock rheology based on different assumptions on the composition, such as grain size, creep activation energy, degree of hydration, and oxygen fugacity. We present final structures with a core radius of 185–205 km, a porous core layer of 4–70 km, an ocean of ≈10–27 km, and an ice crust layer of ≈30–40 km, that are largely consistent with the current estimates for Enceladus. By fitting the model results to these observations, we determine an accretion time of 1.3–2.3 Ma after calcium–aluminum-rich inclusions for Enceladus. Our models produce a porous outer core for wet and dry olivine rock rheologies supporting the hypothesis of hydrothermal circulation of oceanic water in the core. No porosity is retained for an antigorite rheology, implying that the core of Enceladus is not dominated by this mineral.