Hydrated salt minerals on Europa's surface from the Galileo near-infrared mapping spectrometer (NIMS) investigation

We reported evidence of heavily hydrated salt minerals present over large a reas of Europa's surface from analysis of reflectance spectra returned by t he Galileo mission near infrared mapping spectrometer (NIMS) [McCord et al. , 1997a, b, 1998a, b]. Here we elaborate on this earlier evidence, present spatial distributions of these minerals, examine alternate water-ice interp retations, expand on our hydrated-salts interpretation, consider salt miner al stability on Europa, and discuss the implications. Extensive well-define d areas on Europa show distinct, asymmetric water-related absorption bands in the 1 to 2.5-mu m region. Radiative transfer modeling of water ice invol ving different particle sizes and layers at Europa temperatures does not re produce the distinctive Europa water bands. However, ice near its melting t emperature, such as in terrestrial environments, does have some characteris tics of the Europa spectrum. Alternatively, some classes of heavily hydrate d minerals do exhibit such water bands. Among plausible materials, heavily hydrated salt minerals, such as magnesium and sodium sulfates, sodium carbo nate and their mixtures, are preferred. All Europa spectral features are pr esent in some salt minerals and a very good match to the Europa spectrum ca n be achieved by mixing several salt spectra. However, no single or mix of salt mineral spectra from the limited library available has so far been fou nd to perfectly match the Europa spectrum in every detail. The material is concentrated at the lineaments and in chaotic terrain, which are tectonical ly disrupted areas on the trailing side. Since the spectrum of the material on Europa is nearly the same everywhere so-far studied, the salt or salt-m ixture composition may be nearly uniform. This suggests similar sources and processes over at least a near-hemispheric scale. This would suggest that an extensive subsurface ocean containing dissolved salts is the source, and several possible mechanisms for deposit emplacement are considered. The hy drogen bonds associated with hydration of these salts are similar or greate r in strength and energy to those in pure water ice. Thus, once on the surf ace, the salt minerals should be as stable to disruption as water ice at th e Europa temperatures, and mechanisms are suggested to enhance the stabilit y of both materials. Spectra obtained of MgSO4. 6H(2)O at 77 K show only sm all differences from room temperature spectra. The main difference is the a ppearance of the individual absorptions composing the broad, composite wate r features and associated with the several different H2O sites in the salt hydrate molecule, This suggests that the Europa absorption bands are also c omposites. Thus higher spectral resolution may reveal these diagnostic feat ures in Europa's spectrum. The specific salts present and their relative ab undances would be indicators of the chemistry and conditions of an ocean en vironment, and areas of fresh, heavy concentration of these minerals should make ideal lander mission sampling sites.