37th DPS Meeting, 4-9 September 2005
Session 62 Planetary Rings I
Oral, Friday, September 9, 2005, 9:00-10:30am, Music Concert Hall

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[62.06] Cassini CIRS: Thermal Changes In Saturn's Main Rings With Increasing Phase Angle

L.J. Spilker, S. H. Pilorz, B.D. Wallis (JPL/CalTech), C. Ferrari (CEA Saclay/Univ. Paris 7), N. Altobelli, S.M. Brooks, S.G. Edgington (JPL/CalTech), J.C. Pearl, F.M. Flasar (GSFC), B.J. Pollard (Univ. Idaho), CIRS Team

The Cassini Composite Infrared Spectrometer (CIRS) recently obtained spatially resolved scans of Saturn's main rings, measuring the spectrum over a broad range of wavelengths in the infrared from 7 microns to 1 millimeter. Thermal spectra of the rings were obtained at a variety of phase angles, ring local times and ring opening angles. For each ring, fitting a blackbody curve to the CIRS thermal spectrum between 25 and 100 microns (400 and 100 cm-1), and accounting for the ring opacities, we derived average ring temperatures that also accounted for the ring filling factor.

The thermal characteristics of each main ring vary noticeably with phase angle. Radial scans of the A, B and C rings show a decrease in temperature with increasing phase angle for both the lit and unlit sides of the rings; the C ring exhibits the largest change. The temperature of the lit C ring decreases by about 10 K as the phase angle changes from 20 degrees to 135 degrees for afternoon local times; a similar contrast is present for the unlit side. Smaller temperature contrasts are present in the A and B rings. The A ring is particularly interesting because the magnitude of the thermal contrast decreases with increasing radial distance from Saturn.

These changes in temperature with increasing phase angle suggest slowly rotating ring particles. Fast rotating, isothermal ring particles would not produce a pronounced variation in temperature with phase angle. The modeling of these and future observations at additional phase angles may provide particle rotation rates, spin direction and spin axis within each ring.

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA and at CEA Saclay supported by the Programme National de Planetologie.

The author(s) of this abstract have provided an email address for comments about the abstract: Linda.J.Spilker@jpl.nasa.gov

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