Geophysical Investigations of Habitability in Ice-Covered Ocean Worlds

Geophysical measurements can reveal the structures and thermal states of icy ocean worlds. The interior density, temperature, sound speed, and electrical conductivity thus characterize their habitability. We explore the variability and correlation of these parameters using 1-D internal structure models. We invoke thermodynamic consistency using available thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3; pure water ice phases I, II, III, V, and VI; silicates; and any metallic core that may be present. Model results suggest, for Europa, that combinations of geophysical parameters might be used to distinguish an oxidized ocean dominated by MgSO4 from a more reduced ocean dominated by NaCl. In contrast with Jupiter's icy ocean moons, Titan and Enceladus have low-density rocky interiors, with minimal or no metallic core. The low-density rocky core of Enceladus may comprise hydrated minerals or anhydrous minerals with high porosity. Cassini gravity data for Titan indicate a high tidal potential Love number (k(2)>0.6), which requires a dense internal ocean ((ocean)>1,200kgm(-3)) and icy lithosphere thinner than 100km. In that case, Titan may have little or no high-pressure ice, or a surprisingly deep water-rock interface more than 500km below the surface, covered only by ice VI. Ganymede's water-rock interface is the deepest among known ocean worlds, at around 800km. Its ocean may contain multiple phases of high-pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's interior structure may be intermediate to those of Titan and Europa, with a water-rock interface 250km below the surface covered by ice V but not ice VI. Plain Language Summary Seismometers, magnetometers, and other tools may be used in the future to glimpse the insides of ocean worlds-moons of Jupiter and Saturn that have lots of liquid water under their icy surfaces. These measurements could reveal whether water and rock interact to produce chemical conditions that on Earth support life, how much life such chemical activity might support, and how long that activity has persisted through time. The pressures and temperatures in these extraterrestrial oceans differ from those in Earth's oceans, so only just now are the needed tools and data becoming available to predict what future measurements might reveal. In this work, we investigated the interior structures of icy ocean worlds based on available informationmainly NASA's Galileo and Cassini missionsand used chemical data to test what ocean and rock compositions are possible. Our calculations make predictions for Saturn's moons: Titan should not have an iron core, and its ocean may contain little or no high-pressure ice. Fluids may flow through the whole of the rock core of Enceladus because Cassini gravity measurements seem to point to a porous interior. Geophysical investigations could test whether the ocean in Jupiter's moon Europa has a composition like Earth's or may instead by very acidic if water and rock have not interacted much. Our calculations predict that these two scenarios can create unique combinations of measurable properties that can be probed by future missions using seismology, magnetic field measurements, and other means.