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J.B. Dalton (University of Colorado, Boulder), R. N. Clark (U.S. Geological Survey, Denver)
Infrared spectra of Europa collected by the NIMS instrument on the Galileo spacecraft in the 1 to 3 micron range exhibit strong absorptions indicative of water, but the state of the water has been brought into question because spectra of some regions show asymmetries and minima thought uncharacteristic of ice. McCord et al. (Science, v280, p. 1242, 1998) has interpreted these shapes as matching hydrated salts.
The interpretations published so far have relied predominantly on room temperature and atmospheric pressure spectra of salts and other materials. As Europa's surface temperature is closer to 120 Kelvin, many of these measurements have limited applicability to the Galilean satellites. Another key disagreement is that positions of the 1.04 and 1.25 micron absorptions in the hydrated minerals are shortward of those observed in the Europa spectra.
We have constructed a cryogenic environment chamber for spectral measurements of candidate outer solar system ices. Preliminary results indicate that many of the water-bearing materials (mineral+nH2O) display nontrivial temperature responses, including changes in absorption strengths, shapes and positions. Some hydrated salts, such as natron (Na2CO3+10H2O) and epsomite (MgSO4+7H2O) appear to promptly dehydrate when placed under vacuum at cryogenic temperatures, significantly altering their spectral behavior. While some candidates remain viable,the results of these investigations cast doubt on the suitability of several materials as Europan surface constituents.
In contrast, in situ studies of the spectral properties of lake ice (high elevation lakes in the Colorado Rockies) show key aspects of Europa's spectral properties. Subsequent studies of ice containing small bubbles and frost inclusions show Europa-like spectra. It appears that the asymmetric Europa spectral features may be explained by relatively pure ice with a glazed surface and bubbles or other microstructure deeper down.
The author(s) of this abstract have provided an email address for comments about the abstract: dalton@speclab.cr.usgs.gov