Temperature Contrasts across Saturn's Rings through Equinox with Cassini CIRS

Details Temperature Contrasts across Saturn's Rings through Equinox with Cassini CIRS Temperature Contrasts across Saturn's Rings through Equinox with Cassini CIRS

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51b1133b&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P51B-1133

Other

[5759] Planetary Sciences: Fluid Planets / Rings And Dust, [6265] Planetary Sciences: Solar System Objects / Planetary Rings, [6275] Planetary Sciences: Solar System Objects / Saturn

Scientific paper

Cassini has now been in orbit around Saturn for nearly one full Saturnian season. We have used spectra returned by the Composite Infrared Spectrometer to infer ring temperatures. Recent data from the equinox period of August 2009 have revealed that the previously observed thermal contrast between the illuminated and unilluminated sides of the rings has all but disappeared. Earlier in the mission, with the Sun at a higher elevation angle, this thermal contrast was observed to correlate with optical depth. Unlit and lit temperatures are nearly equal in the C ring and Cassini Division, whereas the A and B rings showed significant temperature contrasts (Spilker et al. 2006). This seasonally variable contrast is consistent with the Sun dominating the thermal budget across a static layer of ring particles. Other data suggest the situation is not so simple. Unlit ring temperatures decreased as the Sun dropped lower in Saturn's sky. The unlit side of the A and B rings have cooled significantly. Between late 2004 and the 2009 equinox, temperatures in the B ring core dropped by 20K. 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 more modest across the C ring and Cassini Division. Explaining how the unlit temperatures of such an optically thick structure could vary by 20K 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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