Numerical simulations of marine hydrothermal plumes for Europa and other icy worlds

The liquid water interiors of Europa and other icy moons of the outer Solar System are likely to be driven by geothermal heating from the sea floor, leading to the development of buoyant hydrothermal plumes. These plumes potentially control icy surface geomorphology, and are of interest to astrobiologists. We have performed a series of simulations of these plumes using the MIT GCM ocean circulation model. We assume here that Europa’s ocean is deep (of order 100 km) and unstratified, and that plume buoyancy is controlled by temperature, not composition. Our experiments explore a limited region of parameter space, with ocean depth H ranging from 50 to 100 km deep, source heat flux Q between 0.1 and 10 GW, and Coriolis parameter f corresponding to Europa latitudes between 9° and 47°. As predicted by earlier work, the plumes in our simulations form narrow cylindrical chimneys (a few km across) under the influence of the Coriolis effect. These plumes broaden over time until they become baroclinically unstable, breaking up into cone-shaped eddies when they become 10–35 km in diameter; the shed eddies are of a similar size. Large-scale currents in the region of the plume range between 1 and 5 cm/s; temperature anomalies in the plume far from the seafloor are tiny, varying between 10 and 180 μK. Variations in plume size, shape, speed, and temperature are in excellent agreement with previous laboratory tank experiments, and in rough agreement with theoretical predictions. Plume dynamics and geometry are controlled by a “natural Rossby number” which depends strongly on depth H and Coriolis parameter f, but only weakly on source heat flux Q. However, some specific theoretical predictions are not borne out by these simulations: this may occur because the plumes are “reingesting” their own emissions, a process not considered in our earlier theory.