SURFACE COMPOSITION OF THE GALILEAN SATELLITES THROUGH THE EYE OF THE GALILEO NEAR INFRARED MAPPING SPECTROMETER

The Galileo spacecraft completed its twenty-ninth perijove pass in its tour of the Jovian system on the December 29, 2000. The spacecraft has obtained multiple observations of the Galilean satellites (Io, Europa, Ganymede, and Callisto) at distances ranging from about 200 to 1,400,000 kilometers. The Near Infrared Mapping Spectrometer (NIMS), a unique new class of instrument at the inception of the project in 1977, has been mapping the Galilean satellites in 408 wavelengths ranging from 0.7 to 5.2 microns at a spatial angular resolution of 0.5 milliradian. The instrument has spectrally mapped most of the surface of the three outer satellites at a spatial resolution of about 60 kilometers/pixel, and has mapped small areas at spatial resolutions approaching 100 meters/pixel. Close approach mapping of the inner satellite, Io, complicated by the intense radiation environment at Jupiter, has been achieved in latter orbits of the mission.

The outer satellites have a spectral signature dominated by H₂O and H₂O-related features — which include nearly saturated bands between 2.8 and 5 microns. For the Callisto and Ganymede, additional absorptions have been detected, principally in areas having less water, that are consistent with the presence of SO₂ (4.05 microns) CO₂ (4.25 microns), CN (4.5 microns), CH (3.4 microns), and possibly SH (3.8 microns). H₂O bands on Europa are highly distorted relative to water ice, and have been identified as water of hydration on H₂SO₄ or anhydrites (such as CaSO₄). These materials suggest either an exogenic process (radiolysis) or an endogenic process (freezing brine). The presence of H₂O₂ (clearly a product of radiolysis) has also been detected. Io has a spectral signature dominated by SO₂, a weak band around one micron, and endogenic thermal radiation. The distribution of SO₂ is non-uniform and even absent in warm areas (which are also dark). The thermal signature has been used to measure areas of active and recent volcanism and to estimate the depth at which tidal energy is deposited.

Abstract co-authored by the Galileo NIMS Team