Evidence for a weakly stratified Europan ocean sustained by seafloor heat flux

Abstract

We interpret chaos-type features on the surface of Europa as melt-through structures formed by rotationally confined, steady and/or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor. Smaller lenticular features in the vicinity of chaos-type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites. The ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above. Seafloor heating, maintained by tidal dissipation in the rocky interior, generates an estimated global heat flux of 8.7 × 1012 W and limits the mean ice thickness to 2-5 km. For seafloor heat sources with radii r that are less than the ocean's deformation radius, rD = ND/\f\ (N is the Brunt-Väisälä frequency, D is the water depth, and f is the Coriolis parameter), the diameters of chaos-type regions are expected to diminish from O (100 km) within equatorial regions to O(10 km) at high latitudes (assuming spatially uniform water depth and density structure). Where r > rD, the scale of the source region determines the scale of the melt-through features. Provided there is sufficient time before refreezing, ice rafts in large melt-through regions are imbedded in episodes of preferentially anticyclonic circulation, corresponding to clockwise (counterclockwise) motions in the northern (southern) hemisphere. We calculate that 1021 J were required to melt the ice in the ∼100 km diameter Conamara Chaos region and that for a steady, localized heat flux F ≈ 1011 W (∼1% of the global heat flux) it took ∼1000 years for the initial melt-through to occur. Assuming that ice raft displacements in Conamara Chaos occured during a major melt-through event, maximum current speeds in the region were O(10 cm s-1), and refreezing occured within ∼20 hours. A lack of well-defined ice drift in other major melt-through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion.