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L. J. Spilker, S. H. Pilorz (JPL/Caltech), C. Ferrari (CEA Saclay/Univ. Paris 7), B. D. Wallis (JPL/Caltech), F. M. Flasar, J. C. Pearl (GSFC), M. R. Showalter (Stanford), S. M. Brooks, S. G. Edgington (JPL/Caltech), R. K. Achterberg (SSAI), C. A. Nixon (Univ. of Maryland), Cassini CIRS Team
The Cassini Composite Infrared Spectrometer (CIRS) obtained thermal spectra of the rings during Saturn orbit insertion (SOI). CIRS is a Fourier-transform spectrometer that measures radiation in the thermal infrared from 7 microns to 1 millimeter (1400 to 10 cm-1). For 65 minutes immediately following the SOI burn, the spacecraft flew over the unilluminated side of the main rings and was nearly an order of magnitude closer to the rings than it will be at any other time in the mission. CIRS obtained a scan of segments of the main rings at an average spatial resolution of 200 km with a spectral resolution of 15.5 cm-1. Following the outbound ring plane crossing, CIRS obtained a single scan of a portion of the lit A ring.
CIRS retrieved temperatures for the unlit side of Saturn's rings from the SOI scan. Temperatures varied from approximately 70 K to 110 K. The data show that opaque sections of the rings, like the A and B rings, are cooler than the more transparent sections, like the Cassini Division and C ring. Distinct cool and warm regions are also present in the C ring and Cassini Division, with the cool regions corresponding to the more optically thick ringlets. CIRS observed the lit A ring at a phase angle of ~130 degrees and the unlit A ring at ~60 degrees phase. In this case, for very different phase angles, the retrieved lit and unlit A ring temperatures appear comparable, providing new evidence for possible slowly rotating particles. Preliminary information on retrieved temperatures and ring characteristics will be reported.
This work was performed at JPL under contract with NASA and at CEA Saclay supported by the “Programme National de Planetologie”.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.