Salt partitioning between water and high-pressure ices. Implication for the dynamics and habitability of icy moons and water-rich planetary bodies

Water-rich planetary bodies including large icy moons and ocean exoplanets may host a deep liquid water ocean underlying a high-pressure icy mantle. The latter is often considered as a limitation to the habitability of the uppermost ocean because it would limit the availability of nutrients resulting from the hydrothermal alteration of the silicate mantle located beneath the deep ice layer. To assess the effects of salts on the physical properties of high-pressure ices and therefore the possible chemical exchanges and habitability inside H 2 O-rich planetary bodies, we measured partitioning coefficients and densities in the H 2 O–RbI system up to 450 K and 4 GPa; RbI standing as an experimentally amenable analog of NaCl in the H 2 O-salt solutions. We measured the partitioning coefficient of RbI between the aqueous fluid and ices VI and VII, using in-situ Synchrotron X-ray Fluorescence (XRF). With in-situ X-ray diffraction, we measured the unit-cell parameters and the densities of the high-pressure ice phases in equilibrium with the aqueous fluid, at pressures and temperatures relevant to the interior of planetary bodies. We conclude that RbI is strongly incompatible towards ice VI with a partitioning coefficient K d (VI-L) = 5.0 ( ± 2.1 ) ⋅ 10 − 3 and moderately incompatible towards ice VII, K d (VII-L) = 0.12 ( ± 0.05 ) . RbI significantly increases the unit-cell volume of ice VI and VII by ca. 1%. This implies that RbI-poor ice VI is buoyant compared to H 2 O ice VI while RbI-enriched ice VII is denser than H 2 O ice VII. These new experimental results might profoundly impact the internal dynamics of water-rich planetary bodies. For instance, an icy mantle at moderate conditions of pressure and temperature will consist of buoyant ice VI with low concentration of salt, and would likely induce an upwelling current of solutes towards the above liquid ocean. In contrast, a deep and/or thick icy mantle of ice VII will be enriched in salt and hence would form a stable chemical boundary layer on top of the silicate mantle. Such a contrasted dynamics in the aqueous–ice VI–ice VII system would greatly influence the migration of nutrients towards the uppermost liquid ocean, thus controlling the habitability of moderate to large H 2 O-rich planetary bodies in our solar system (e.g., Ganymede, Titan, Calisto) and beyond.