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T. McCord, F. Fanale, G. Hansen (HIGP/SOEST, U. of Hawaii), T. Orlando, M. Sieger, N. Petrik (EMSL, Pacific NW Nat'l Lab.), J. Crowley (U.S. Geological Survey/Reston)
Galileo NIMS observations of Europa indicate the presence of hydrated non-ice materials on the surface. McCord et al. (Science, 280, 1242, 1998; JGR, 104, 11827, 1999) suggested hydrated sulfates and/or carbonates, with MgSO4*xH2O being one candidate. Carlson et al. (submitted to JGR) suggests sulfuric acid (H2SO4*xH2O). MgSO4*xH2O and H2SO4*xH2O have similar spectral and chemical characteristics that are dominated by the solvated sulfate species. In fact, the indicative distorted water bands in the NIMS spectra are most likely associated with solvation shell and water molecules that interact most strongly with the sulfate and carbonate anions (McCord et al. ibid.). The cations present (e.g., Mg, Na, H) affect these bound water molecules less, making it difficult to identify them in the NIMS spectra. A number of hydrated SO4- and CO3-bearing materials are predicted by models of interior evolution (Fanale et al., Science, 186, 922, 1977; Kargel, Icarus, 94, 368, 1991). McCord et al. (ibid.) suggested that the Europa material is probably a mixture of these materials with possible modification by radiation-induced chemistry. We report further analysis of the NIMS spectra and recent laboratory work on the radiation and thermal stability of these materials under Europa conditions. Our results indicate that the hydrated magnesium sulfate epsomite (x=7) remains hydrated under high vacuum (<109 Torr) at temperatures less than about 250 K and are much more stable with respect to electron irradiation than is water ice. Thus, deposits of at least hydrated sulfates can become concentrated on Europa due to electronic sputtering of ice and may account for the apparently high concentrations of these materials in some places. These results, with the likelihood that ice sputtering provides sufficient water molecules at the surface to maintain fully solvated sulfate ions, makes these materials reasonable for Europa’s surface.
The author(s) of this abstract have provided an email address for comments about the abstract: tom@pgd.hawaii.edu