Europa's Ocean Can Be Sustained By Hydrothermal Plumes and Salt Transport

Data returned by the Galileo spacecraft provide considerable evidence that Jupiter's satellite Europa possesses a liquid ocean beneath its solid, icy outer shell. However, it is not known if that ocean has existed throughout Europa's history. Previous thermal evolution models of Europa suggest that without active tidal dissipative heating (TDH), a global liquid ocean layer would eventually freeze long before the present. However, previous models have not coupled all the various thermal and flow processes that may be operating in Europa. Recently, we have developed a whole-moon numerical model for Europa. This model couples radiogenic heating, thermal diffusion, hydrothermal convection and salt transport in mantle pore water, hydrothermal flow and transport in an ocean layer, parameterized convection in the ice shell, and change of phase between ice and liquid water. Application of our model suggests that, even without TDH active until recently, hydrothermal convection in a salty, rocky mantle can sustain flow in an ocean layer throughout Europa's post-differentiation history. The model thermal history covers three phases: (i) an initial, roughly 0.5 Gyr-long period of radiogenic heating and differentiation, (ii) a long period from 0.5 Gyr to 4 Gyr with continuing radiogenic heating but no TDH (following Yoder, Nature 279: 767-770, 1979), and (iii) a final period covering the last 0.5 Gyr until present day, during which TDH is active. In our model, hydrothermal plumes develop throughout phases II and III, transporting heat and salt from Europa's silicate mantle to its ocean. The outer ice shell thickens over time, growing to about 75 km in depth. When TDH becomes active, the ice shell melts quickly to a thickness of about 10 km, and then stabilizes at roughly 20 to 25 km thickness, leaving an ocean 80 km deep. Parameterized convection in the ice shell is spatially non-uniform and changes over time, reflecting its ties to the evolving deeper ocean-mantle dynamics. A salt-free ocean/salty mantle pore water profile retards hydrothermal plume penetration into the ocean initially, but is homogenized over time, in roughly 50 Myrs, by turbulent diffusion in the ocean and time-dependent flow driven by initial thermal gradients. After homogenization, the uniformly distributed salt concentration is no longer a major factor in controlling plume transport, but does suppress the freezing point of the ocean layer. Salt transport leads to the formation of salt inclusions at the bottom of the ice shell. The presence of salt in the ice shell could strongly influence convection in that layer.