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T. B. McCord, G. B. Hansen, F. P. Fanale (HIGP/SOEST, Univ. Hawaii)
We reported evidence for hydrated salt minerals on Europa using the Galileo NIMS spectra (McCord {\it et al.}, {\it Science, 280}, 1242, 1998), based on distorted H2O absorptions. We showed that the H2O molecules present are active spectrally, closely bound to the host molecule and in numerous slightly different bonding configurations, and that spectra of heavily hydrated salts such as magnesium sulfates [e.g., epsomite: MgSO4 \cdot 7H2O] are good candidate materials. We mapped the salts spatial distributions and noted the heaviest concentrations in the recently disrupted chaotic terrain and dark linea. Salts are reasonable minerals to be associated with Europa from models (Kargel, {\it Icarus, 94}, 368, 1991) and laboratory studies of meteorites (Fanale, {\it LPS XXIX}, #1248, 1998). It has been pointed out (R. Clark, personal comm.) that distorted water absorption features similar in shape to those found on Europa are found for pure water ice with very long optical pathlength. In this case, the core of the absorptions are totally saturated and the albedo is zero. For Europa, however, the reflectance in the band centers is near 15%, suggesting that the absorption is due to many slightly different H2O energy transitions, each fairly weak, rather than a single energy transition of great strength, such as in pure ice. Also, the distorted Europa water features are found in close association with the recently endogenically disrupted regions such as the mottled terrain and the linea. If a special form of ice causes the Europa H2O bands, the mechanism forming it must operate only and always in these regions. Also, Clark points out that the Europa 1.2 \mum water feature position does not match the salt spectra presented in McCord {\it et al.} (1998) Recent laboratory measurements by us and by Grundy and Schmitt ({\it JGR}, in press, 1998) show that this band is especially sensitive to temperature and moves toward Europa's position at low temperature. The band position also shifts toward the Europa position as the number of H2O molecules in the MgSO4 molecule increases from \cdot 4H2O to \cdot 7H2O and probably to the \cdot 12H2O Kargel suggests for Europa.
The author(s) of this abstract have provided an email address for comments about the abstract: tom@pgd.hawaii.edu