Ocean Circulation as a Control on Chemical Energy Availability and Habitability in Icy Moon Subsurface Oceans

Introduction. The habitability of icy ocean worlds is commonly assessed in terms of the availability of chemical disequilibria capable of sustaining metabolic activity. In this perspective, an efficient transport mechanism may represent a key bottleneck in determining whether such disequilibria can be established and maintained. In the case of Europa and other icy moons, oxidants produced by surface radiolysis may be transported downward through the ice shell and reach the ocean, while reductants may be supplied from the rocky seafloor through hydrothermal or low-temperature water-rock reactions. Previous studies have shown that both oxidant delivery and reductant production may reach astrobiologically relevant fluxes [1], but the actual power available to life depends on the efficiency with which oxidants and reductants are transported, mixed, and consumed within the ocean.

In this work, we investigate the role of oceanic transport in controlling the spatial and temporal distribution of redox energy in icy moon-like subsurface oceans. We focus on a simplified H₂-O₂ system, where H₂ is supplied from the bottom boundary and O₂ from the upper boundary, representing reduced inputs from the seafloor and oxidized material delivered from the ice shell, respectively. The aim is to determine whether ocean circulation combined with chemical reactivity allows the persistence of redox disequilibria, and to identify where the resulting chemical power is concentrated.

 

Methods. We solve advection-diffusion-reaction equations for H₂ and O₂ in idealized ocean domains, starting from 2D benchmarks and extending to full 3D configurations. The reaction is parameterized through a third-order kinetic term proportional to [H₂]²[O₂], consistent with the elementary reaction stoichiometry (2H₂+O₂--> 2H₂O), and to a dimensionless parameter K that can be seen as the ratio of the diffusive timescale to the reaction timescale. High K values correspond to reaction-dominated regimes, whereas low K values represent transport-dominated regimes in which chemical disequilibrium can persist over longer timescales. Kinetic parameters are constrained by previous analyses associated with terrestrial hydrothermal system environments. The problem is solved using the pseudo-spectral Rayleigh-Bénard convection solver RBSolve [2], which provides H₂ and O₂ concentrations at each grid point and at each time step.

 

Results. Preliminary simulations show that the spatial structure of redox power is strongly controlled by the competition between transport and reaction. Using kinetic parameters consistent with low-temperature aqueous H₂-O₂ abiotic reaction rates [3] and putative biotically influenced kinetics parameters [4], the system operates in a transport-dominated regime. In this regime, oxidants and reductants coexist over large volumes, allowing redox power to be distributed over oceanically relevant length scales. This suggests that abiotic and minimal biotic reactions are sufficiently slow for ocean circulation to maintain chemical disequilibria over the whole water column. The 3D simulations further indicate that the spatial distribution of redox power is non-uniform, with hotspots whose location is controlled by the interplay between convective circulation and chemical consumption. These are preliminary results, and further simulations are needed to better characterize redox power hotspots and provide astrobiological constraints relevant for future missions such as JUICE and Europa Clipper. Our results already indicate that ocean circulation plays a key role in determining and maintaining the habitability of icy moons' oceans and must be further investigated to derive appropriate astrobiological constraints.

 

References

[1] Vance S. D. et al. (2016). Geophysical Controls of Chemical Disequilibria in Europa. Geophysical Research Letters, Vol. 43, Iss. 10, pp. 4871–4879. https://doi.org/10.1002/2016GL068547

[2] Parodi A. et al. (2004). Clustering of Plumes in Turbulent Convection. Physical Review Letters, Vol. 92, Iss. 19, 194503. https://doi.org/10.1103/PhysRevLett.92.194503

[3] Foustoukos D. I. et al. (2011). Kinetics of H₂–O₂–H₂O Redox Equilibria and Formation of Metastable H₂O₂ under Low Temperature Hydrothermal Conditions. Geochimica et Cosmochimica Acta, Vol. 75, Iss. 6, pp. 1594–1607. https://doi.org/10.1016/j.gca.2010.12.020

[4] Wulff P. et al. (2014). How Oxygen Reacts with Oxygen-Tolerant Respiratory [NiFe]-Hydrogenases. Proceedings of the National Academy of Sciences, Vol. 111, Iss. 18, pp. 6606–6611. https://doi.org/10.1073/pnas.1322393111

 

Additional Information. This work is part of the JUICE Phase E project, and it was funded by ASI under agreement n. 2023-6-HH.0, CUPF83C23000070005