The Role of Clathrate Formation in Europa’s Ocean Composition

We use a thermodynamic statistical model to evaluate how the composition of Europa’s internal ocean may have been affected by clathrate hydrate formation. Assuming an input of the observed O 2 and CO 2 from the surface into a mildly acidic ocean (pH < 6), and considering the possibility of contributions by reduced (with CH 4 and H 2 S) or oxidized (CO 2 -bearing) hydrothermal fluids, we calculate the fractional occupancies in clathrate and deduce the effect on the ocean’s composition. The structure of the clathrate formed, and therefore its density and composition is influenced by the relative amount of O 2 compared to the other compounds present. We also include a mixture of noble gases—argon, krypton, and xenon—based on cometary abundances measured at comet 67P and find that the Ar/Kr ratio can be affected by almost two orders of magnitude. In most cases, the formed clathrate is likely to become part of the icy crust, with guest molecules possibly accessible to future in situ measurements by the Europa Clipper and JUICE missions.