Ocean Circulation on Icy Moons

Icy moons are satellites orbiting the giant planets in the outer solar system. Some of them are believed to harbor global liquid oceans beneath ice shells, and are therefore prime targets in the search for life beyond Earth. Direct observations of these oceans are nearly impossible because the oceans are hidden beneath ice shells tens of kilometers thick. One of the few observational constraints is the ice shell thickness distribution inferred from shape and gravity measurements, which is closely linked to ocean circulation and heat transport. Understanding the dynamical processes that govern ocean heat transport is therefore essential for interpreting observations and assessing the habitability of icy moon oceans.

Ocean heat transport in icy moon oceans is primarily controlled by two processes: baroclinic eddies driven by buoyancy forcing at the ice-ocean interface associated with ice thickness variations, and slantwise convection driven by heating from the underlying silicate core. In this dissertation, we first study how salinity influences ocean stratification and circulation, as well as the meridional heat transport by baroclinic eddies on Enceladus. Then, we turn to slantwise convection in rapidly rotating icy moon oceans. Unlike the upright convection commonly assumed in terrestrial ocean models, slantwise convection in icy moon oceans is strongly influenced by planetary rotation and develops preferentially along the rotation axis. Traditional convection parameterizations cannot adequately represent convection in rapidly rotating icy moon oceans. We therefore use theoretical analysis and numerical simulations to investigate the instability criterion and heat transport scaling laws for slantwise convection in icy moon oceans. Based on these results, we develop a new slantwise convection parameterization for global ocean simulations that captures the poleward heat transport neglected by traditional convection schemes. We apply this parameterization to study the interaction between baroclinic eddies, which transport heat equatorward on Enceladus, and slantwise convection, which transports heat poleward.

Scaling analysis suggests that on Europa, Titan, and Ganymede, baroclinic eddies dominate the meridional heat transport when the equator-to-pole ice thickness contrast exceeds about 10 m, whereas slantwise convection dominates when the contrast is smaller. On Enceladus, the entire ocean can convect when the salinity is sufficiently high, whereas a global stably stratified layer forms near the ocean surface when the salinity is sufficiently low. Regardless of salinity, baroclinic eddies are expected to dominate the meridional heat transport on Enceladus due to its large ice shell thickness gradient inferred from shape and gravity measurements. The resulting equatorward heat transport acts to smooth the poleward-thinning ice shell geometry, indicating that ocean heat transport alone is unlikely to maintain the present-day ice shell topography on Enceladus.