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B.D. Wallis, L.J. Spilker, S.H. Pilorz (JPL/CalTech), J.C. Pearl (GSFC), N. Altobelli, S.G. Edgington (JPL/CalTech), F.M. Flasar (GSFC), CIRS Team
Cassini's Composite Infrared Spectrometer (CIRS) observations of Saturn's main rings during the first set of orbits designed for prime ring viewing geometry have shown the main rings to be thermally complex. The thermal appearance of the rings changes significantly with viewing geometry, ring local time and phase angle. During one observation near the zero phase on the rings, the CIRS data showed an unexpected thermal enhancement in the A Ring at a phase angle of 0.3 degrees and a local time of 6 hours. A similar radial scan taken one orbit later at a phase angle of 1.5 degrees showed a similar thermal enhancement.
For comparison, comparable data taken in the same region, at the same ring opening and local time, but at a phase angle of ~ 10.5 degrees, showed a lower effective temperature by about 2 degrees than the zero phase measurement. Afternoon scans of this region at phase angles of 14 to 32 degrees show derived temperatures that are comparable to those seen in the morning near zero phase. In general, the A ring appears to be 4 to 5 degrees warmer in the afternoon than in the morning, except at zero phase, where the A ring temperature is warmer than the afternoon temperature.
Future observations are planned to help us understand what has been observed near zero phase. One possible explanation for this enhancement in ring temperature is the clumping of particles into larger irregular shaped structures where the inability to see into warmer chasms and craterlike features would be impaired except for viewing conditions near the zero phase point much like the Thermal Beaming effect observed on asteroids and other solar system bodies without atmospheres. This work was performed at JPL/Caltech under contract with NASA.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.