HR: 10:35h
AN: P52A-02    [Abstracts]
TI: Initial Cassini Ultraviolet Observations of Saturn's Rings
AU: * Colwell, J E
EM: josh.colwell@lasp.colorado.edu
AF: LASP, University of Colorado, 392 UCB University of Colorado, Boulder, CO 80309-0392
AB: The first ring occultation to be observed by Cassini is of the star Xi Ceti on October 6-7, 2004, from a distance of 6.22 million km. The Ultraviolet Imaging Spectrograph (UVIS) includes a High Speed Photometer with a bandpass of $\sim$110-190 nm that has an integration period of 8 msec for this observation. The Fresnel zone at this distance is $\sim$40 m, and the speed of the occultation gives a radial sampling interval of 5-8 m. The occultation covers the C ring, Cassini Division, A ring, and F ring. The occultation is particularly well-suited for the low to intermediate optical depth regions of the C and F rings and the Cassini Division, and will have lower effective resolution in the higher optical depth regions of the A ring. The Xi Ceti occultation is by far the most distant that Cassini will observe. Ultraviolet spectral reflectance observations were also made at Saturn Orbit Insertion on July 1, 2004, providing the highest spatial resolution UV reflectance of the rings, with a 150 km effective resolution. Spectral variations are seen between the C, B, and A rings and the Cassini Division indicating varying amounts of dark material mixed in with water ice in the rings. The A ring is significantly brighter than the B and C rings, and its brightness increases with increasing radial distance. Meteoroid impacts into the rings darken the rings through the addition of silicates and carbonaceous material. Observed fluctuations in the UV brightness at length scales of less than 3000 km can be explained by impact fragmentation of moonlets or large ring particles in the rings. These fragmentation events expose relatively pure ice from the interior of the moonlet, leading to a local brightening of the ring which is gradually reduced and mixed with the surrounding ring material as the meteoroid bombardment continues. We will present our initial findings on ring structure, UV brightness, and particle sizes.
DE: 5757 Remote sensing
DE: 5759 Rings and dust
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting