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J. C. Pearl (NASA GSFC), J. R. Spencer (SwRI), M. Segura (NASA GSFC), CIRS Team
The Cassini CIRS instrument records spectra over a broad range of thermal wavelengths. Its three focal planes provide coverage from 7 micrometers to 1 mm, with selectable spectral resolutions. We are investigating the thermal properties of the Saturnian satellites. As limiting cases, we compare the long wavelength behavior of daytime spectra of Enceladus and Iapetus. Enceladus has the lowest, and Iapetus the highest daytime temperature of the satellites. In addition, the spectra show sharp rolloffs toward long wavelengths; for example, afternoon spectra of Cassini Regio show a brightness temperature drop of 20K between 25 and 250 micrometers. We are attempting to explain this behavior. The effect can arise from any or all of several causes: thermal inhomogeneities in the field of view due to albedo, shadowing, latitudinal or local time variations, or to "thermal sounding" of the vertical temperature gradient in the near surface regolith made possible by spectral variability of the regolith material. However, an explanation due to thermal sounding seems to fail, both because the decrease in the ice absorption coefficient toward long wavelengths appears inadequate, and because an expected reversal or attenuation of the brightness temperature rolloff at night is not observed. The situation for Enceladus may be complicated by the presence of significant spatial variations in thermal inertia, as seen in CIRS darkside observations. As an additional comparison, we also contrast the rolloff of the satellite spectra with the rolloff observed by CIRS for spectra of Saturn's rings.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.