Interpreting Temperature Contrasts across Saturn's Rings with Cassini CIRS

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Cassini's Composite Infrared Spectrometer has returned spectra of Saturn's rings for nearly one full season. We have used these spectra to infer ring temperatures. The thermal contrast between the illuminated and unilluminated sides of the rings correlates with optical depth. Unlit and lit temperatures are nearly equal in the C ring and Cassini Division, whereas the A and B rings show significant temperature contrasts (Spilker et al. 2006). Moreover, these thermal contrasts exhibit seasonal variability. This is consistent with the Sun dominating the thermal budget across a static layer of ring particles.
However, other data suggest the situation is not so simple. Unlit ring temperatures have decreased as the Sun drops lower in Saturn's sky. The unlit side of the A and B rings have cooled significantly. Between late 2004 and early 2008, temperatures in the B ring core dropped by 10K. Marouf et al. (2006) place a lower limit of 5.5 on the optical depth of the thickest features in the B ring from Cassini Radio Science Subsystem data. Temperature decreases have been modest across the C ring and Cassini Division.
Explaining how the unlit temperatures of such an optically thick structure could vary by 10K is difficult. Despite the intra-wake gaps present in the B ring (Colwell et al. 2007), little sunlight should filter across such an optically thick ring to heat its unlit side, even when the solar illumination is at its most direct. We believe CIRS data can provide crucial clues about the structure of Saturn's rings. We will discuss these data and present suggestions as to how thermal energy transport may be more efficient than previously thought.
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2009 California Institute of Technology. Government sponsorship acknowledged.

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