Variations in Heat Transport and Salinity Drive Ocean Circulation and Ice Shell Thickness on Icy Satellites

Enceladus, a small icy satellite of Saturn. Credit: NASA.

Icy satellites such as Enceladus and Europa may have saline subsurface oceans, which means that they have potentially habitable environments and are targets of missions aimed at detecting possible life. Understanding processes like ocean circulation is critical to determining the spatial and temporal distribution of potential biosignatures that will be targeted by in-situ missions to icy satellites.

Ocean circulation dynamics are driven by variations in density, which stem from differences in ocean temperature and salinity. Although Cassini returned data that revealed the chemical composition of the ocean on Enceladus, there is a lack of detailed knowledge of how plausible variations in salinity and the distribution of heat could affect circulation in this ocean.

Wanying Kang and colleagues from the Massachusetts Institute of Technology addressed this question, applying an ocean circulation model to Enceladus with inputs including a range of salinity values and various distributions of tidal heating between the core and the ice shell. The results of the model indicate that in a freshwater ocean scenario, circulation is driven at the poles by variations in temperature; however, if the ocean water is salty, salinity-induced density drives water to sink at the equator. In both scenarios, heat and freshwater converge at the equator when the warm polar water mixes with cold equatorial water. Vigorous circulation of either type would result in the complete removal of observed ice shell thickness variations via excessive heat flux through the shell, so the authors concluded that the ocean is of intermediate salinity and that tidal heating contributions from the core are small. This kind of model could also explain ice shell geometry for other icy satellites. For example, because Europa has much higher gravity than Enceladus due to its size, conditions would be ideal for driving ice flow such that all substantial ice shell thickness variations are removed, regardless of salinity. READ MORE