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Project Progress

Sulfates are present in the non-ice material of Europa’s surface and in carbonaceous chondrites. Although the occurrence of sulfates in endogenic features on Europa’s may indicate oceanic sources of sulfates, their origin in the interior remains unknown. Zolotov has performed thermodynamic calculations to show that both low H₂ pressure and elevated temperature favor aqueous oxidation of chondritic sulfides (e.g., FeS) to sulfates (SO₄^2-) by water. However, kinetic evaluations of sulfide oxidation show that sulfates do not form at low-temperature (< ~150oC) aqueous processes. Negligible amounts of sulfides could be oxidized within millions of years even at appropriate (very low) pressure of H₂. It was shown that low H₂ pressures cannot be achieved through H2 escape from Europa or asteroids. However, the presence of sulfates in carbonaceous chondrites could be explained by accretion of irradiated water ices which contain strong oxidants such as O₂ and H₂O₂. Zolotov argued that irradiation of ices in the solar nebula could have led to formation of O₂ and H₂O₂. Much oxidation of water ices is expected at the “show line” which could have situated close to the current position of Jupiter. Melting of ices after accretion could have released O₂ and H₂O₂ followed by their rapid interaction with sulfides. This mechanism explains the formation of sulfates in CI/CM carbonaceous chondrites, which were altered at temperature < 150oC. Carbonaceous bodies (asteroids) and irradiated ices with oxidants were likely building blocks of Europa. Europa’s sulfates could be low-temperature products of sulfide oxidation inside the moon and/or products of leaching during the formation of the ocean. Oxidation of sulfides by trapped O₂, H₂O₂ and O₃ explains a presence of oceanic sulfates on Europa without invoking hydrothermal reactions and/or H₂ escape. Although both oxidants and sulfates currently form at the surface of Europa, these species may not have affected oceanic chemistry within the last 50-100 Myr.