R. W. Carlson and the NIMS Team
The Galileo Near Infrared Mapping Spectrometer (NIMS) is currently obtaining spectral maps of Jupiter's moons in the wavelength range 0.7 to 5.2 microns. This spectral region contains the fundamental stretch transitions of the water molecule and several stretching, bending, librational, and translational combination bands, all of which are useful in investigating the surface properties of the three icy satellites. The inner Galilean satellite - Io - is known to contain a different surface ice, sulfur dioxide, which is being studied by NIMS using the multitude of absorption bands.
Among the icy satellites, Callisto shows the least amount of water ice, covering about 10 % of the surface in patchy concentrations. Most of the surface is covered with unidentified (as yet) dark non-ice minerals. The exposed ice is often associated with impact craters, implying that the darker material exists as a blanket over more pure ice. Spectral features at 3.88, 4.03, 4.25, and 4.57 microns are present in Callisto's spectra, with different spatial distributions for each. Possible materials causing the 4.25 micron feature include carbon dioxide and hydroxylated silicates. Sulfur dioxide, tholins, S-H, Si-H, and deuterated constituents are candidates for the other features. There is evidence for the presence of hydrated minerals, based on water band shifts and shapes.Ganymede's surface exhibits water absorption bands, largely due to ice but hydrates are also apparent. A dark component is present, but with a smaller proportion compared to Callisto. Some of the non-ice features seen in Callisto's spectra are also present on Ganymede. The purest ice is found to be associated with craters; the Osiris ejecta blanket shows ice grain sizes of approximately 100 microns. Band depth maps at 1.04 and 1.25 microns show that relatively large ice grains are equatorially distributed while band-depth maps for the stronger bands indicate that the total amount of ice cover is greater in the polar regions, suggesting poleward transport.
Water is the dominant molecule covering Europa's surface, occuring as ice but also as a hydrate. The trailing side shows high concentrations of this hydrous material, whose identification is not yet established. The depths of water bands are greatest at high latitudes, likely due to higher purity ice. Grain sizes there are of order one millimeter. Spectra of the leading side also show more pure ice, with grain sizes of approximately 250 microns.
Sulfur dioxide frost or ice covers Io's surface, except in hot volcanic areas. Large grains occur with partial coverage in the equatorial region of Colchis Regio, while greater concentrations of smaller grains are found at high latitudes, existing as a frost covering or intimately mixed with darker material. Atmospheric transport and redistribution of sulfur dioxide must be quite rapid given the large volcanic resurfacing rate.
Contact Information:
Robert W. Carlson
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive, MS 183-601
Pasadena, California 91109
Telephone: (818) 354-2648
email: rcarlson@issac.jpl.nasa.gov