Abstract
We reported evidence of heavily hydrated salt minerals present over
large areas of Europa's surface from analysis of reflectance spectra
returned by the 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 interpretations, expand on our hydrated-salts
interpretation, consider salt mineral stability on Europa, and discuss
the implications. Extensive well-defined 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 involving different particle
sizes and layers at Europa temperatures does not reproduce the distinctive
Europa water bands. However, ice near its melting temperature, such
as in terrestrial environments, does have some 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. All Europa spectral
features 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 single or 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 material is concentrated at the lineaments
and in chaotic terrain, which are tectonically 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-mixture
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 hydrogen bonds associated with hydration of these
salts are similar or greater in strength and energy to those in pure
water ice. Thus, once on the surface, the salt minerals should be
as stable to disruption as water ice at the Europa temperatures,
and mechanisms are suggested to enhance the stability of both materials.
Spectra obtained of MgSO4. 6H(2)O at 77 K show only small differences
from room temperature spectra. The main difference is the appearance
of the individual absorptions composing the broad, composite water
features and 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.
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