Thermal infrared spectroscopy of Europa and Callisto

The trailing hemispheres of Europa and Callisto were observed at 9 - 13 mu m, and a spectrum of Europa with better spectral resolution and a better si gnal-to-noise ratio than was previously possible has been derived. The rati o spectrum of the two satellites has a signal-to-noise ratio of approximate ly 30 for a spectral resolving power of approximately 50. The disk-integrat ed, effective color temperature ratio for the two satellites is consistent with broadband, thermal infrared photometry from previous ground-based stud ies and from the Galileo photopolarimeter radiometer. The ratio spectrum wa s combined with the average Voyager 1 spectrum of Callisto to obtain a 9 - 13 mu m spectrum of Europa with a signal-to-noise ratio that is a factor of 10 better than that in the average Voyager spectrum of Europa. After convo lving the measured spectrum to the expected width of water ice emissivity f eatures, similar to 1 mu m, no spectral features that could be attributed t o water ice on the surface of Europa are apparent at the 0.6 - 0.7% level. The absence of spectral features attributable to water ice is consistent wi th the proposal that the equatorial region of Europa that was observed may be composed primarily of a heavily hydrated mineral. The absence of water i ce features may also be the result of a large fractional abundance of fine particles, such as that found on the surface of the Moon.