Nonsynchronous rotation of icy moon ice shells: The thermal wind perspective

The ice shells of icy satellites have been hypothesized to undergo nonsynchronous rotation (NSR) under the influence of tidal torques and/or ocean currents. In this work, the author proposes that the thermal wind relationship can be combined with geostrophic turbulence theory to predict ocean stress onto the ice shell inside the tangent cylinder. High-resolution numerical simulations validate the prediction within a factor of 2. For the prediction to be valid, the rotation effect must dominate (Rossby number < 1), and the upper ocean should be stratified. The latter can be achieved with sufficiently large ice thickness variations [the threshold for Europa is O(100) m]. Using this framework, once the ice rheology, thickness variations and NSR rate are determined, one may be able to estimate the ocean overturn timescale and put constraints on the ocean vertical diffusivity or the heat flux originating from the silicate core.