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Published June 25, 2000 | Published
Journal Article Open

Thermal infrared spectroscopy of Europa and Callisto


The trailing hemispheres of Europa and Callisto were observed at 9–13 μm, and a spectrum of Europa with better spectral resolution and a better signal-to-noise ratio than was previously possible has been derived. The ratio spectrum of the two satellites has a signal-to-noise ratio of approximately 30 for a spectral resolving power of approximately 50. The disk-integrated, effective color temperature ratio for the two satellites is consistent with broadband, thermal infrared photometry from previous ground-based studies and from the Galileo photopolarimeter radiometer. The ratio spectrum was combined with the average Voyager 1 spectrum of Callisto to obtain a 9–13 μm 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 convolving the measured spectrum to the expected width of water ice emissivity features, ∼1 μm, no spectral features that could be attributed to 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 with the proposal that the equatorial region of Europa that was observed may be composed primarily of a heavily hydrated mineral. The absence of water ice features may also be the result of a large fractional abundance of fine particles, such as that found on the surface of the Moon.

Additional Information

© 2000 American Geophysical Union. Received 17 August 1999; accepted 8 March 2000; published 1 June 2000. The observations described in this paper would not have been possible without the assistance of the Palomar Observatory staff and Tom Hayward. Helpful reviews of the manuscript were provided by Eric Gaidos, Gary Hansen, Jack Salisbury, John Spencer, and Albert Yen. Michael E. Brown is an Alfred P. Sloan Research Fellow. This is contribution 5766 from the Division of Geological and Planetary Sciences of the California Institute of Technology.

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