Thermodynamics and Interior Structure Measurements of Ocean Worlds

Abstract:

Extending thermodynamics of aqueous solutions to elevated salinity and pressure is key to understanding icy moon oceans. Such a project is intrinsically linked to future seismic investigations, which would offer the most comprehensive views into the deep interiors of planetary bodies. The InSight Mars mission and Europa Lander mission concept both identify seismology as a critical measurement to constrain interior structure and thermal state of astrobiological targets.

By pinpointing the radial depths of compositional interfaces, seismology in a broad frequency range can address uncertainty in interior structures inferred from gravity and magnetometry studies, such as those planned for the NASA’s Europa and ESA’s JUICE missions. Seismology also offers information about fluid motions within or beneath ice, which complement magnetic studies; and can record the dynamics of ice layers, which would reveal mechanisms and spatiotemporal occurrence of crack formation and propagation. Investigating these in the future calls for detailed modeling of seismic sources and signatures.

Thermodynamic measurements of sound velocities and elastic moduli can provide needed seismic model inputs, as well as associated densities, thermal expansion, heat capacities, and chemical potentials for modeling interior structure. Sound speed profiles built from our internal structure models for Ganymede (Vance et al. 2014), using thermodynamically consistent data (Shaw 1986, Vance and Brown 2013) point to needed research, both in developing thermodynamic data from new laboratory measurements and applying them to problems in planetary geophysics. Comparing these profiles with simulated electrical conductivities of MgSO4 along the geotherms illustrates how dual seismology-magnetometry investigations can infer ocean density structure and salinity. Electrical conductivities are based on measurements by Larionov (1984) and Huang and Papangelakis (2006), and extrapolation to 273 K by Hand and Chyba (2007). Accurate interpretation of future sounding measurements for ocean worlds would benefit from extending the theory of electrical conductivity for multi-component systems through measurements at salinities exceeding 0.01 moles/kgH2O at elevated pressures from 0.03-1.6 GPa relevant to ocean worlds.