In the absence of abyssal heat sources, the following processes contribute: (1) tidal heating in the ice shell, (2) heat diffusion within the ice, (3) heat loss to space, (4) ocean heat transport, and (5) water-ice heat exchange. In (A), the temperature difference between the water-ice interface and the very top of the ice induces outward diffusion of heat, which is ultimately lost to space. Ocean circulation is induced by freezing point temperature variations at the ice-ocean interface, which make the poles warm because the ice is thin there, relative to the equator where it is thick. The ocean therefore carries heat from the poles to the equator. The resulting heat exchange between the ice and the ocean tends to smooth out ice shell geometry variations.
In (B), the ocean circulation is driven by a prescribed top temperature variation. Black lines represent temperature surfaces, here synonymous with isotherms because salinity plays no role. The top temperature is higher over the poles and lower at the equator. This pole-to-equator temperature gradient is diffused down vertically into the ocean [process (6)], supporting zonal currents that spawn baroclinic eddies. The eddies flux heat down the temperature gradient [process (7), eddy heat transport]. This eddy transport can be equivalently viewed as an overturning circulation sinking at the equator and rising near the poles [marked as (8), eddy-driven overturning circulation, shown only in the Northern Hemisphere]. There is a balance between eddy transport of heat (7) [or equivalently, the overturning circulation (8)] and vertical diffusion of heat (6). Note that here we have assumed that the buoyancy of seawater depends only on temperature and that the thermal expansion coefficient is positive. The temperature and circulation patterns in (B) are inspired by previous works (14, 15). — Science Advances via PubMed
We explore ocean circulation on a rotating icy moon driven by temperature gradients imposed at its upper surface due to the suppression of the freezing point of water with pressure, as might be induced by ice thickness variations on Enceladus.
Using high-resolution simulations, we find that eddies dominate the circulation and arise from baroclinic instability, analogous to Earth’s weather systems. Multiple alternating jets, resembling those of Jupiter’s atmosphere, are sustained by these baroclinic eddies.
We establish a theoretical model of the stratification and circulation and present scaling laws for the magnitude of the meridional heat transport.
These are tested against numerical simulations. Through identification of key nondimensional numbers, our simplified model is applied to other icy moons. We conclude that baroclinic instability is central to the general circulation of icy moons.
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻
