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

We interpret chaos-type features on the surface of Europa as melt-through s tructures formed by rotationally confined, steady and/or episodic oceanic p lumes that rise to the base of the ice shell from magmatically heated regio ns of the seafloor. Smaller lenticular features in the vicinity of chaos-ty pe regions might be formed by baroclinically unstable vortices that spin of f the main convective plume or by persistent heating from localized hydroth ermal venting sites. The ocean is assumed to be weakly stratified because o f turbulent convection generated by heating from below and cooling from abo ve. Seafloor heating, maintained by tidal dissipation in the rocky interior , generates an estimated global heat flux of 8.7 x 10(12) W and limits the mean ice thickness to 2-5 km. For seafloor heat sources with radii r that a re less than the ocean's deformation radius, r(D) = ND//f/ (N is the Brunt- Vaisala: 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 spatial ly uniform water depth and density structure). Where r > r(D), the scale of the source region determines the scale of the melt-through features. Provi ded there is sufficient time before refreezing, ice rafts in large melt-thr ough regions are imbedded in episodes of preferentially anticyclonic circul ation, corresponding to clockwise (counterclockwise) motions in the norther n (southern) hemisphere. We calculate that 10(21) J were required to melt t he ice in the similar to 100 km diameter Conamara Chaos region and that for a steady, localized heat flux F approximate to 10(11) W (similar to1% of t he global heat flux) it took similar to 1000 years for the initial melt-thr ough to occur. Assuming that ice raft displacements in Conamara Chaos occur red during a major melt-through event, maximum current speeds in the region were O(10 cm s(-1)), and refreezing occurred within similar to 20 hours. A lack of well-defined ice drift in other major melt-through regions suggest s 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.