Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Abstract

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 (k2>0.6), which requires a dense internal ocean (ρocean>1,200 kg m−3) and icy lithosphere thinner than 100 km. In that case, Titan may have little or no high‐pressure ice, or a surprisingly deep water‐rock interface more than 500 km below the surface, covered only by ice VI. Ganymede's water‐rock interface is the deepest among known ocean worlds, at around 800 km. 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 250 km below the surface covered by ice V but not ice VI.

Holistic Approach for Studying Planetary Hydrospheres: Gibbs Representation of Ices Thermodynamics, Elasticity, and the Water Phase Diagram to 2,300 MPa

“…Figure depicts the behavior with pressure of major thermodynamic properties (density, thermal expansivity, specific heat, and isothermal bulk modulus) computed using the LBF representations and seismic velocities for ices Ih, III, V, and VI. These are provided up to 2,300 MPa, on isotherms relevant to the interiors of icy ocean worlds, at 250, 260, 270, and 300 K (Lunine, ; Sohl et al, ; Vance, Planning, et al, ). Temperature has a notable effect only on specific heat and thermal expansivity which shows 20% to 30% increases with increasing temperature in this 50 K range.…”

“…Isotherms at 250, 260, 270, and 300 K are shown. The estimated pressure range found in the hydrosphere of Europa (E), Titan (T), and Ganymede (G) are reported as blue, yellow, and green bars, respectively (Vance, Planning, et al, ). Black dots on the 250 K profile represent the equilibrium pressure for the solid‐solid phase transitions.…”

“…A uniform numerical environment for the thermodynamic properties of water and phases in equilibrium with water from ambient to high pressures supports research in a broad range of fields including astrobiology, planetary sciences, oceanic science, geochemistry, and condensed‐matter physics. Planetary interior studies have illustrated the importance of using thermodynamically consistent properties of hydrosphere materials in icy ocean worlds (Mitri et al, ; S. Vance et al, ; Vance, Panning, et al, ) while also noting inconsistencies in available parameterizations. SeaFreeze offers clarity in how measurements inform the representations and consistency in how properties relate to energy and entropy reference states.…”

“…In order to better constrain the structure, evolution, and habitability of the interiors of water‐rich planetary bodies, a new generation of accurate measurements and internally consistent thermodynamic representations of aqueous solutions and ice polymorphs is required. Furthermore, since next‐generation planetary exploration missions are likely to investigate the seismic structure of icy worlds (Vance, Kedar, et al, ; Vance, Panning, et al, ; Panning et al, ; R. D. Lorenz et al, ; Stähler et al, ), data on seismic wave speeds in ices and aqueous solutions as a function of pressure and temperature are also needed.…”

Internal Energy Dissipation in Enceladus's Subsurface Ocean From Tides and Libration and the Role of Inertial Waves

“…Lin and Ogilvie () have shown that ohmic dissipation dominates over viscous dissipation only when , where Ek m is the magnetic Ekman number which are, respectively, defined as where B 0 is the background magnetic field, μ is the magnetic permeability, and ν m is the magnetic diffusivity. Recent estimates of the electrical conductivity inside the ocean (Vance et al, ) give Ek m ∼1. The ohmic dissipation is therefore unlikely to contribute significantly to the total.…”