Convection in Extraterrestrial Oceans: Fluid Dynamics and Astrobiological Implications

The main reservoirs of liquid water in the Solar System are hidden beneath the icy shells of some of the “icy moons” orbiting the gas giants Jupiter and Saturn. Although these moons lie well outside the traditional habitable zone, tidal forces exerted by their parent planet and internal radiogenic heating can sustain subsurface oceans of liquid water. These environments may offer the necessary conditions for life, making icy moons key targets in the search for extraterrestrial biospheres. Unfortunately, direct exploration of these oceans remains out of reach by current space mission technology, which is limited to surface observations. However, surface past or present activity observed on several of these moons suggests that internal processes may be coupled with surface dynamics, potentially enabling surface-subsurface interactions. In particular, a crucial role might be played by ocean dynamics. Previous global models have shown that large-scale fluid motions within the oceans may lead to latitude-dependent variations in heat flux at the ice-ocean boundary. Using this as a starting point, in this PhD thesis I focused on intermediate-scale, localized, convective dynamics within the subsurface oceans of the icy moons of Jupiter (Europa, Ganymede and Callisto), showing that these oceans can be dominated by intense thermal convection characterized by the presence of localized turbulent plumes, which can generate differential heat fluxes and local interactions at the ice-water interface. To explore this issue, I numerically integrated a simplified turbulent convective fluid model (RBSolve), coupled with a linear approximation for the freeze-melt processes of the overtopping ice layer. The simulations revealed pronounced spatial variability in basal melting and freezing, which in turn can induce heterogeneities in ice-shell thickness and create surface “hotspots,” i.e., regions where the surface lies closer to the hidden ocean, thereby promoting transient connections and upwelling of endogenous material. These effects are especially prominent at equatorial latitudes consistently with past equatorial imaging of Ganymede and Europa that revealed peculiar geological features associated with past or present activity, as well as with spectral detections of compounds plausibly sourced from subsurface oceans. Quantitatively, this work provides a conservative lower bound on the maximum melting achievable by thermal convection alone, acting in concert with processes such as ice convection and tidal stressing, of up to ~4 km over geological timescales (10^6-10^7 years). These predictions can be tested by upcoming missions such as ESA’s JUpiter ICy moons Explorer (JUICE) through gravity and altimetry measurements, offering new insights into the physical coupling between surface and interior also at small spatial scales. Beyond heat transport and mechanical coupling, convection also bears directly on potential habitability. If seafloor hydrothermal activity supplies chemical energy, then a chemosynthetic ecosystem could exist in the depths of these extraterrestrial oceans. In this perspective, convective motions can distribute nutrients and passively advected organisms throughout the water column. Extending the fluid model with advection–reaction equations for nutrient and biomass evolution, I found that convection efficiently homogenizes passive scalars over the water column and allows to delineate the key dynamical parameters and ecosystem characteristics required to sustain detectable biomass for forthcoming missions such as JUICE. All the results of this work contribute to JUICE mission Phase E.