Thermal infrared spectroscopy of Europa and Callisto

Abstract

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.