A Terrestrial Perspective on Seawater-Rock Reactions in the Subsurface Oceans of Icy Satellites

On Earth, reactions between oceanic crust and seawater occur over a range of pressures and temperatures. The attending flux of reacted fluids to the ocean creates chemical/redox gradients near the seafloor, providing chemosynthetic energy for primitive microbes that subsist in the absence of sunlight. Diverse tectonic activity governs the heat and rock lithology available for reaction and therefore the rate, chemistry and magnitude of these fluxes. In volcanic spreading centers reactions with new mafic crust result in the venting of hot (~350°C) acidic fluids, rich in reduced chemical species (such as H₂, H₂S, CH₄ and Fe²⁺). Alteration of ultramafic crust (serpentinization) is kinetically less prohibitive at lower temperatures, and yields high pH fluids enriched in H₂ and CH₄. Similar processes may occur in subsurface oceans of ice covered satellites such as Europa and Enceladus, facilitated by tidal dissipative heating, with the implication that these icy worlds could support past or present habitable environments (assuming a concomitant oxidant flux from the ice shell). However, core temperatures may be too low for sustained partial melting and the recycling of ocean crust that occurs on Earth. Questions that should be addressed concern the hydration/alteration state of core lithologies spanning the (uncertain) thermal history of icy satellites, and the extent that water-rock reactions might presently be affecting the geochemical evolution of their oceans (what are the rates of this process?). For example, despite the small size of Enceladus, the chemistry of plumes jetting from the outer ice shell (measured by Cassini) is consistent with contemporary hydrothermal processes. Similar active or recent expressions of subsurface chemistry are important targets for future study, and measurements of stable isotope fractionation can provide clues to the temperature structure of liquid oceans. Elucidating the present ocean chemistry of these icy worlds is paramount because it will reflect a culmination of geochemical cycles as it does on Earth. Unlike on Earth, where ocean chemistry is predominantly affected by continental weathering, the geochemical evolution of these oceans should be dominated by seawater-rock reactions, processes we are only beginning to understand through deep ocean exploration.