Analysis of the Europan surface composition based on Galileo NIMS spectra.
Reflectance spectra of the surface of the Jovian satellite Europa exhibit absorption bands in the 1.0 to 2.5 micron range which have been attributed to water ice (Pilcher et al., 1972; Calvin et al., 1995). However, significant departures from the spectral behavior of ordinary water ice have been observed. The Galileo NIMS spectral data vary in absorption band strength, shape, and position over the surface of the satellite. Several mechanisms have been proposed to explain this departure, including the presence of additional surface components, scattering effects due to small bubbles or pits in the ice, grain size effects, thermal and mechanical stresses, radiation damage to the ice, and the presence of water in a bound state such as water of hydration. The postulation of additional surface components suggest minerals and salts of varying hydration states, which could contribute to the observed spectral effects. Hydrated salts, especially the magnesium sulfates epsomite, hexahydrite, and bloedite have received a great deal of attention. Besides salts, hydrates such as zeolites and clays are under consideration, along with simple organics and acids. Previous work (Dalton and Clark, 1999; McCord et al., 1998, 1999) has relied on linear mixture analysis to model the Europa spectrum with mixtures of ices and hydrates, yielding relatively high hydrate abundances ranging from tens of percent to 100%. In the case where the albedo of the additional component closely matches that of the ice, this is a reasonable first approximation. Given that the surface is likely to be very fine-grained, intimate mixtures need to be considered. Nonlinear mixing and spectral effects can provide an additional bound on estimates of surface composition. Laboratory and numerical simulations permit further evaluation of the various hypotheses, placing upper limits on a number of materials. A synthesis of these results indicates that high abundances of the materials proposed thus far are not consistent with the observed Europa spectra. Specifically, at concentrations sufficient to produce the observed asymmetry in the 2.0 micron band, all of the hydrated salts and zeolites exhibit other bands not observed in the Europa spectrum. Although a few percent, and in some cases up to a few tens of percent, could still be present at or near the surface, these materials cannot satisfactorily explain the Europa spectrum from 1.0 to 3.0 microns. Other materials, such as simple organics (Delitsky and Lane, 1998) and acids such as sulfuric (Carlson et al., 1999) should not be overlooked at this stage; and other spectral effects such as radiation damage, thermal and mechanical effects, and scattering have not been ruled out and must still be examined in order to fully account for the observations.