Ocean Circulations in the Outer Solar System

A growing number of satellites in the outer solar system likely have global oceans beneath their outer icy shells. While the presence of liquid water makes these ocean worlds compelling astrobiological targets, their ocean circulations are also an essential component of the ice-ocean-rock exchange process that is critical for promoting habitable environments. However, no direct observations of ocean dynamics are presently available, leaving open critical questions such as: How are heat, chemical constituents, and potential biomaterials distributed and transported in the ocean? What is the degree of coupling with the underlying mantle/high pressure ice layer and overlying ice shell? Are there signatures of oceanographic processes that may be remotely observable?

Fluid motions within icy satellite oceans may be driven by convection due to thermo-compositional density gradients (e.g., temperature and salinity contrasts between the mantle and ice shell), mechanical forcing (e.g., tides, libration, and orbital precession), and electromagnetic pumping by the host planet’s magnetic field. Numerical modeling and laboratory experiments, in combination with theoretical arguments and complemented by study of analogous terrestrial environments, have shown that strong currents may be expected in many of these icy ocean worlds. Moreover, these flows will promote mixing within the bulk ocean, which will influence the distribution of thermo-compositional gradients, especially along the seafloor and ice-ocean interface, and potentially have important implications for the thermophysical structure of the ice shell.

Future missions will better constrain the physical and chemical processes occurring within exo-oceans. Looking specifically to the Jovian system, the Europa Clipper and JUICE missions will determine the ocean thickness and composition of Europa and Ganymede. Ice penetrating radar will provide information on ice shell structure and constrain ice-ocean exchange processes, while magnetometer measurements may allow probing of ocean currents through their induction of magnetic fields. Lander mission concepts with in situ sampling of the ice shell and ocean would further revolutionize our understanding of their properties and dynamics.