Prior analyses of oceanic magnetic induction within Jupiter’s large icy moons have assumed uniform electrical conductivity. However, the phase and amplitude responses of the induced fields will be influenced by the natural depth-dependence of the electrical conductivity. Here, we examine the amplitudes and phase delays for magnetic diffusion in modeled oceans of Europa, Ganymede, and Callisto. For spherically symmetric configurations, we consider thermodynamically consistent interior structures that include realistic electrical conductivity along the oceans’ adiabatic temperature profiles. Conductances depend strongly on salinity, especially in the large moons. The induction responses of the adiabatic profiles differ from those of oceans with uniform conductivity set to values at the ice–ocean interface, or to the mean values of the adiabatic profile, by more than 10\% for some signals. We also consider motionally induced magnetic fields generated by convective fluid motions within the oceans, which might optimistically be used to infer ocean flows or, pessimistically, act to bias the ocean conductivity inversions. Our upper-bound scaling estimates suggest this effect may be important at Europa and Ganymede, with a negligible contribution at Callisto. Based on end-member ocean compositions, we quantify the magnetic induction signals that might be used to infer the oxidation state of Europa’s ocean and to investigate stable liquids under high-pressure ices in Ganymede and Callisto. Fully exploring this parameter space for the sake of planned missions requires thermodynamic and electrical conductivity measurements in fluids at low temperature and to high salinity and pressure as well as modeling of motional induction responses.

Modeling of the electrical conductivity (EC) of icy moon oceans has previously assumed that chloride, sulfate, and other ions released from rock leaching are the main solutes and carriers of electrical conductivity. Here, we show that accreted volatiles, such as carbon dioxide and ammonia, can add a significant fraction of solutes in bodies whose volatile content was in part supplied from cometary materials. These volatiles can increase the EC of aqueous solutions above 1 S/m. Our salinity and EC estimates can serve as a basis for planning future magnetometer investigations at icy moons and dwarf planets. In particular, oceans expected in some of the Uranian satellites and Neptune’s satellite Triton could have EC above 3 S/m as a result of accretion of large abundances of carbon dioxide and ammonia, even if rock leaching during water-rock separation was limited, and chlorine and sulfur abundances may be at CI carbonaceous chondritic levels.

Brinicles are self-assembling tubular ice membrane structures, centimeters to meters in length, formed by the downward migration of supercooled brine rejected from ice sheets, and found beneath sea ice in the polar regions of Earth.  They provide a plausible setting for geochemical gradients amenable to life at the ice-ocean interface, in some ways analogous to hydrothermal vents at the seafloor-ocean interface. Their occurrence in icy ocean worlds like Europa and Enceladus remains hypothetical. The context of brinicles on Earth includes influences from oceanic flow, which will differ in other worlds, and surficial inputs from the atmosphere that do not exist in oceans with kilometers-thick global coverings of ice formed from the underlying ocean. Thus, it is difficult to project the likely occurrence and role of brinicles based on field observations of their earthly analogues. We discuss brinicles as they are currently understood, including their electrochemical properties in connection with potential habitats at the ice-ocean interface on Europa and Enceladus. We employ a fluid mechanical model (Cardoso and Cartwright, 2017) to assess the properties of brinicles on other worlds and consider their longevity relative to potential brine outflows from the overlying ice. We demonstrate how brinicles may grow by thermal diffusion, and provide simple scaling for their growth and outflow rates. The specifics of the composition and dynamics of both the ice and the ocean in these worlds remain poorly constrained. We demonstrate through calculations using FREZCHEM that sulfate likely fractionates out of accreting ice in Europa and Enceladus, and thus that an exogenous origin of sulfate observed on Europa’s surface need not preclude additional endogenous sulfate in Europa’s ocean. We suggest that, like hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal places where ecosystems of organisms might be found.

Detecting a seismic event from Europa’s silicate interior would provide information about the geologic and tectonic setting of the moon’s rocky interior. Reflections off a metallic core would indicate the presence, size, and state of the hypothesized core. However, the subsurface ocean will attenuate the signal, possibly preventing the waveforms from being detected by a surface seismometer. Here, we investigate the minimum magnitude of a detectable event originating from Europa’s silicate interior. We analyze likely signal-to-noise ratios and compare the predicted signal strengths to current instrument sensitivities. We show that a magnitude M_w >3.5 would be sufficient to overcome the predicted background noise. However, a minimum magnitude of M_w > 5.5 would be required for current instrumentation to be able detect the event. A thinner ice shell transmits greater ground acceleration amplitudes than a thicker ice shell, which might allow for M_w > 4.5 to be detectable.

Europa likely contains an iron-rich metal core. For it to have formed, temperatures within Europa reached ≳1250 K. At that temperature, accreted chondritic minerals - e.g., carbonates and phyllosilicates - would partially devolatilize. Here, we compute the amounts and compositions of exsolved volatiles. We find that volatiles released from the interior would have carried solutes, redox-sensitive species, and could have generated a carbonic ocean in excess of Europa’s present-day hydrosphere, and potentially an early CO2 atmosphere. No late delivery of cometary water was necessary. Contrasting with prior work, CO2 could be the most abundant solute in the ocean, followed by Ca2+, SO42-, and HCO3-. However, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved S/Cl ratio, such that Cl>S at the shallowest depths, consistent with recently inferred endogenous chlorides at Europa’s surface. Gypsum would form a 3 - 10 km thick sedimentary layer at the seafloor.

We explore the possibility that Callisto’s ocean sits beneath its high-pressure ice, rather than above it. Oceans perched between ice phases are considered to be stable configurations for Ganymede, Callisto, and Titan. High-pressure ices under the liquid water ocean will transport heat and solutes into the ocean as long as the convective adiabat for the ices remains close to the melting temperature (Choblet et al. 2017, Kalousova and Sotin 2018). However, this configuration may become unstable when the perched ocean is close to freezing and its salinity increases, if the ocean becomes denser than the underlying ice. Among the oceans in the solar system, Callisto’s must be among the coldest and most saline because the internal heat appears to be low in the absence of tidal dissipation. Surface geology indicates its lithosphere is fully stagnant (Moore et al. 2004). Solid-state convection may continue beneath less than 100 km or dirty non-convecting ice (McKinnon 2006). And just below this layer may reside a liquid water ocean that is the lag deposit of Callisto’s thicker primordial ocean, the concentrated result of 4 Gyr of freezing. Using representative interior structures based on the current constraints from the Galileo mission (Anderson et al. 2001) coupled with recently obtained thermodynamic data (Vance et al. 2018), we demonstrate the possibility for using magnetic induction to identify where the ocean currently resides in Callisto. Anderson, J. D. et al. (2001). Shape, mean radius, gravity field, and interior structure of Callisto. Icarus, 153(1):157–161. Choblet, G. et al. (2017). Heat transport in the high-pressure ice mantle of large icy moons. Icarus, 285:252–262. Kalousovà, K. and Sotin, C. (2018). Melting in high-pressure ice layers of large ocean worlds - implications for volatiles transport. Geophysical Research Letters. McKinnon, W. (2006). On convection in ice I shells of outer solar system bodies, with detailed application to Callisto. Icarus, 183(2):435–450. Moore, et al. (2004). Callisto. Jupiter. The Planet, Satellites and Magnetosphere, 1:397–426. Moore, J. and Pappalardo, R. (2011). Titan: An exogenic world? Icarus, 212:790–806. Vance, S. D. et al. (2018). Geophysical investigations of habitability in ice-covered ocean worlds. Journal of Geophysical Research: Planets, 123, 180–205.