Evidence for Hydrated Salt Minerals on Europa's Surface

We reported evidence of heavily hydrated salt minerals composing large areas of Europa’s surface from analysis of reflectance spectra returned by the Galileo Mission Near Infrared Mapping Spectrometer (NIMS) [1, 2, 3, 4]. We elaborated on this earlier evidence, presented spatial distributions of these minerals, examined alternate water-ice interpretations, expanded on our hydrated-salts interpretation, considered salt mineral stability on Europa, and discussed the implications [5, 6]. Here we review this evidence and discuss several advancements in our understanding. Extensive well-defined areas on Europa show distinct, asymmetric water-related absorption bands in the 1-2.5 μm spectral region (Fig. 1). We were unable to reproduce the distinctive Europa water bands using radiative transfer modeling of water ice involving different particle sizes and layers at Europa temperatures. However, ice near its melting temperature, such as in terrestrial environments, does have some but not all characteristics of the Europa spectrum. Alternatively, some classes of heavily hydrated minerals do exhibit such water bands. Among plausible materials, heavily hydrated salt minerals, such as magnesium and sodium sulfates, sodium carbonate and their mixtures, are preferred (Fig. 2). All Europa spectral features, including the 1.25-μm band, are present in some salt minerals and a very good match to the Europa spectrum can be achieved by mixing several salt spectra. However, no mix of salt mineral spectra from the limited library available has so far been found to perfectly match the Europa spectrum in every detail. The salt minerals are concentrated at the lineaments and in chaotic terrain, which are tectonically disrupted areas on the trailing side. Since the spectrum of the salts on Europa is nearly the same everywhere so far studied the salt or salt-mixture composition may be nearly uniform. This suggests similar sources and processes over at least a hemispheric scale, implying that a subsurface ocean containing dissolved salts is the source. Several possible mechanisms for deposit emplacement are considered. The hydrogen bonds associated with hydration of these salts are similar or a bit greater in strength and energy than those in pure water ice [7]. Thus, once on the surface, the salt minerals should be as stable to disruption as is water ice at the Europa temperatures, and mechanisms are suggested to enhance the stability of both materials. Spectra obtained of MgSO4•6H2O near 77 K show only small differences from roomtemperature spectra. The main differences are the movement to slightly longer wavelengths of the 1.25μm water band and the appearance of individual absorptions within each water absorption band, which are associated with the several different H2O sites in the salt hydrate molecule. This suggests that the Europa absorption bands are also composites. Thus, higher spectral resolution may reveal these diagnostic features in Europa’s spectrum. The specific salts present and their relative abundances would be indicators of the chemistry and conditions of an ocean environment, and areas of fresh, heavy concentration of these minerals should make ideal lander-mission sampling sites on Europa.