[19.14] Saturn's A Ring Now and Then: a Comparison of Voyager IRIS and Cassini CIRS Observations

36th DPS Meeting, 8-12 November 2004
Session 19 Rings
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

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[19.14] Saturn's A Ring Now and Then: a Comparison of Voyager IRIS and Cassini CIRS Observations

S. M. Brooks, L. J. Spilker, S. H. Pilorz, S. G. Edgington, B. D. Wallis (Jet Propulsion Laboratory), J. C. Pearl, F. M. Flasar (NASA Goddard Space Flight Center), C. Ferrari (CEA Saclay), M. R. Showalter (Stanford University), R. K. Achterberg (SSAI), C. A. Nixon (University of Maryland), A. A. Simon-Miller, P. N. Romani (NASA Goddard Space Flight Center), CIRS Investigation Team

During Cassini's successful orbital insertion manuever at Saturn earlier this summer its Composite Infrared Spectrometer (CIRS) obtained thermal spectra of Saturn's ring system at range of geometries not seen since the Voyager flybys. CIRS is a Fourier-transform spectrometer that measures infrared radiation from 7 microns out to 1 millimeter (1400 to 10 cm^-1 ). Among the main rings, the A Ring was observed at the greatest range of phase angles and emission angles during Saturn orbit insertion. Scans of the lit and unlit sides of the A Ring were obtained at a spectral resolution of 15.5 cm^-1 and at low ( ~60^\circ ) and intermediate ( ~130^\circ -- 140^\circ ) phase angles.

The infrared interferometer spectrometer (IRIS) experiments aboard Voyagers 1 and 2 also obtained thermal spectra of the rings. IRIS, the predecessor of the CIRS instrument, is a Michelson interferometer that records spectra between 4 and 55.5 \mu m (2500 to 180 cm^-1 ). Useful spectra of the lit and unlit A Ring with a resolution of 4.3 cm^-1 were obtained during the Voyager flybys at phase angles somewhat lower than CIRS SOI scans ( ~30^\circ ) and at intermediate phase angles comparable to those observed by CIRS.

We will present and interpret Cassini observations of the A Ring in the context of the earlier Voyager observations. The range in viewing geometries provided by the two sets of observations yields leverage in determining physical properties of the ring as a whole as well as its constituent particles. Additionally, the separation in time between the two sets of observations will allow us to identify any changes in ring temperatures between the Voyager and Cassini epochs as might be expected from, for example, differences in solar elevation angle.

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