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W.B. McKinnon (Washington Univ., Saint Louis)
A prominent model for the formation of Europa's water+ice layer argues for a hypersaline sulfate composition (Kargel et al. 2001). The model is based on a plausible compositional affinity between the bulk of Europa and the most volatile rich meteorite class, CI carbonaceous chondrites. These meteorites are extensively aqueously altered, with abundant Mg and Ca sulfates. The central tenet is that a Europa accreted from similar materials essentially matches Europa's bulk density, and whether the sulfates form in precursor satellitesimals or by aqueous alteration within Europa, early heating causes sulfate (principally epsomite) to melt incongruently to from a buoyant hypersaline brine that erupts to the surface. Later additions of sulfate-poor waters from the breakdown of, e.g., gypsum and serpentine, lead to a variety of compositional and structural paths for the icy layer, but the emphasis is on hypersaline compositions.
This scenario needs to be reconsidered: (1) sulfates, especially magnesium sulfates, require very oxidizing conditions to form, and such conditions have not been shown to arise naturally in models of meteorite aqueous alteration; (2) magnesium sulfate veins in CI chondrites (e.g., Orgueil) have been shown to most likely be the result of terrestrial exposure, e.g., from corrosion of meteoritic sulfides (Gounelle and Zolensky 2001); 3) Tagish Lake, possibly the most primitive carbonaceous chondrite, has been hypothesized to be a sample of a D-type asteroid. D asteroids predominate in the Trojan groups, and are arguably a better compositional analogue for the volatile-rich Galilean satellites; sulfides but no sulfates have been reported for this meteorite. The implication is that the early melting of ices and rock-ice differentiation on Europa is unlikely to have released highly oxidized, sulfate-rich water to the satellite's surface. If anhydrous minerals and free metal were originally incorporated (a possibility in recent satellite formation models), initial oxidation and hydration of these materials would have produced complementary reduced aqueous fluids (Rosenberg et al. 2001). So where do the observed sulfates in Europa come from? This will be discussed, as will implications for ocean and shell composition.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.