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Project Progress

Co-I M. Zolotov has evaluated the composition of fluids formed through aqueous alteration of CI-type carbonaceous chondritic materials at possible environments of the ocean-rock interface on Europa. Chemical equilibria in water-chondrite type systems were calculated for ranges of initial fluid compositions, water to rock ratios, temperatures, and pressures. In open system models, fugacity of hydrogen was fixed as well. In the closed system models, calculated aqueous solutions have alkaline pH and are rich in alkali, chloride, and carbonate species (Na⁺, Cl^-, HCO₃^-, CO₃^2-, OH^-, NaCl, K⁺, etc.). Abundances of these species depend on conditions of alteration. We conclude that Na-, Cl-, and carbonate-bearing species could be abundant in the subsurface ocean of Europa.

This work also demonstrates that sulfate species detected on the surface of Europa have negligible concentrations in closed system models, at which hydrogen does not leave the ocean. To explore formation of sulfates, we performed open-system runs and vary fugacity of H₂ to explore effects of H₂ escape. The results show that elevated temperature and low hydrogen fugacity favor formation of sulfate-bearing fluids rich in Mg, Na, K, and Cl. These solutions form through oxidation of sulfides and increasing solubility of magnesium phyllosilicates (serpentine and saponite) that supply cations. The results show that formation of sulfate ions increases salinity and ionic strength of solutions. Therefore, oxidized sulfate-bearing ocean on Europa would be more saline than reduced NaCl-NaHCO₃ type ocean. This work shows that formation of MgSO₄-rich solutions suggested in the ocean requires elevated temperatures, H₂ escape, or a presence of strong oxidants. These results are consistent with our previous evaluations that were based on less sophisticated approaches.

Previous works demonstrate that the Na/K ratio in the atmosphere of Europa could reflect the composition of the ocean. In turn, our models show that the Na/K ratio in rock alteration fluids depends on temperature, as illustrated in the figure. We argue that the low atmospheric Na/K ratio compared to modeled fluids is consistent with hydrothermal processes at the bottom of the ocean.