Tidal pumping and heating of Enceladus’s porous core

Dr. Noah G Randolph-Flagg, PhD, University of California Berkeley, Berkeley, CA, United States, Tushar Mittal, Penn State, Earth and Planetary Science, University Park, United States and Douglas Hemingway, Carnegie Institution for Science Washington, Department of Terrestrial Magnetism, Washington, DC, United States
Saturn’s small (∼ 500 km diameter) moon Enceladus is a high priority target for future exploration and a key to our developing understanding of icy ocean worlds. Gravity observations from the Cassini mission have found evidence of a low density, porous core (e.g., Hemingway and Mittal, 2019) and observations of plume composition (e.g., Waite et al., 2017) suggest active hydrothermal chemical reactions. To understand the role of different physical and chemical processes in Enceladus's (and other ocean worlds’) core(s), we quantify the energy fluxes into the rocky core and the efficiency of the hydrothermal system to transport that energy to the overlying ocean. Dissipation due to body tides is the primary sources of heat on small ocean worlds. We use results from previous studies (e.g., Matsuyama et al. 2018, Beuthe 2019) as well as new numerical calculations to constrain the range of tidal energy dissipation in Enceladus's core. Using order of magnitude and scaling results, we find an additional mechanism for tidal energy dissipation in the rocky core: viscous energy dissipation due to groundwater. We find that liquid saturation is the most dissipative saturated phase and most consistent with geophysical observations. Our results hence suggest that most of the tidal energy dissipation can occur in the core without requiring exotic rheological properties. Finally, we build upon the published global models of ocean worlds with thermal convection in porous media and icy slurries (e.g., Choblet et al., 2017). We systematically compare the relative importance of topographically, thermally, and mechanically driven flow on Enceladus. We find that the most efficient transport mechanism is tidally driven 'pumping' due to the interaction of body tides with the hydrothermal system. We also present numerical models of the hydrothermal fluid fluxes across the ocean-core interface to understand potential spatial and temporal heterogeneities of chemical fluxes into the ocean. Our results help to provide a better understanding of the chemical interactions between cores and oceans for ocean worlds and available energy for biological activity.