37th DPS Meeting, 4-9 September 2005
Session 61 Planetary Rings
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Lecture Room 5

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[61.04] Saturn's A Ring As Seen By Voyager IRIS: a Preview for Cassini CIRS?

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

Having arrived at Saturn just past the northern winter solstice, Cassini is observing the Saturnian system at a rather different phase in its seasonal cycle than did the two Voyager spacecraft. When Cassini arrived at Saturn the Sun's inclination angle was 24\circ. In 1980 Voyager 1 passed through the system shortly after the northern vernal equinox, when the solar inclination angle was just 4\circ. This angle was 8\circ when Voyager 2 encountered Saturn. As a result, the thermal structure of Saturn's rings as derived from Cassini's Composite Infrared Spectrometer (CIRS) and Voyager's Infrared Radiometer and Spectrometer (IRIS) show significant differences.

An interesting result from Cassini's first observations of Saturn's A ring was the lack of a temperature contrast between the ring's illuminated and unilluminated sides [1]. Although phase angle coverage was limited in observations taken at Saturn orbit insertion, subsequent observations during 2004 failed to turn up any large difference between lit and unlit A ring temperatures. Observations from Voyager 1, however, yield a temperature difference of ~ 5 - 10, K. Such a difference would easily have been revealed by CIRS.

We will interpret Cassini A ring observations in the context of the Voyager observations. Observations from April 2005 onward have begun to show significant contrasts in A ring temperature. Changes in solar inclination may drive these differences. But, factors such as phase angle, emission angle and local hour angle within the rings must also be considered. By the end of the nominal tour, Cassini will see the Sun drop to an elevation of 5.8\circ from the ring plane. Thus, Voyager 1 and 2 data may provide a preview of what is to come later in the Cassini mission.

\noindent [1] F.M. Flasar, \textit{et al}.\ 2005.\ ``Science'', vol. 307, pgs. 1247-1251.

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