How does salinity shape ocean circulation and ice geometry on Enceladus and other icy satellites?

Abstract

Of profound astrobiological interest, Enceladus appears to have a global subsurface ocean that is salty, indicating water-rock reaction at present or in the past, important for its habitability. Here, we investigate how salinity and the partition of heat production between the silicate core and the ice shell affect ocean dynamics and the associated heat transport -- a key factor that determines the equilibrium ice shell geometry. Assuming steady state conditions, we show that the meridional overturning circulation of the ocean, driven by heat and salt exchange with the ice, has opposing signs at very low and very high salinities. Regardless of these differing circulations, heat and freshwater converge towards the equator, where the ice is thick, acting to homogenize thickness variations. In order to maintain the observed ice thickness variation, the polar-amplified ice dissipation needs to be strong enough and ocean heat convergence cannot overwhelm well-constrained heat loss rates through the thick equatorial ice sheet. This requirement is found violated if the main heat source is in the core rather than the ice shell, or if the ocean is very fresh or very salty. Instead, with a salinity of intermediate range, the temperature- and salinity-induced density gradient largely cancel one another, leading to much reduced overturning and equatorial heat convergence rates and consistent budgets in appearance of a significant ice dissipation.