Ocean Dynamics on Icy Worlds

Abstract:

With a liquid ocean beneath an outer ice shell, icy moons in our solar system are the next target in the search for life. However, ocean circulation regimes in such worlds has yet to be systematically studied, despite their potential impact on nutrient transport and biosignatures, and hence the habitability and detectability of possible life. To fill the gap, we configured an ocean circulation model to include non-hydrostatic effects, full treatment of the Coriolis force and relaxed the shallow fluid approximation, to explore ocean dynamics assuming a range of depths and bottom heat fluxes. Radically different dynamics are found to develop within and outside the tangent cylinder (the cylinder whose axis is aligned with the moon’s rotation vector and whose surface is tangent to the silicate core): latitudes outside the tangent cylinder (low latitudes) are filled by convection rolls which efficiently pump heat upward, whereas the high latitudes are filled by upright convective plumes. These differing dynamics give rise to a temperature gradient between equator and pole which in turn could lead to a heterogeneous ice shell favoring melting of ice at the equator and freezing at the pole. Our findings can be applied to generic icy moons, and we present results using Europa and Enceladus as examples.