HR: 08:35h
AN: P51C-04 INVITED     [Abstracts]
TI: Cassini-VIMS Observations of Saturn's Rings at SOI.
AU: * Nicholson, P D
EM: nicholso@astro.cornell.edu
AF: Cornell University, Space Sciences Building, Ithaca, NY 14853 United States
AU: Brown, R H
AF: University of Arizona, Lunar and Planetary Lab, Tucson, AZ 85721 United States
AU: Clark., R N
AF: USGS, MS 964 Box 25046 Federal Center, Denver, CO 80225 United States
AU: Cruikshank, D P
AF: NASA Ames Research Center, MS 245 Moffett Field, Mountain View, CA 94035 United States
AU: Showalter, M R
AF: NASA Ames Research Center, MS 245 Moffett Field, Mountain View, CA 94035 United States
AU: Sicardy, B
AF: Observatoire de Paris, DESPA, Bat. 10, Meudon, 92195 France
AB: Following the Cassini spacecraft's Saturn Orbit Insertion (SOI) burn on 1 July 2004, the Visual and Infrared Mapping Spectrometer (VIMS) obtained near-infrared spectra from 0.9 to 5.1~$\mu$m in two continuous radial scans across the unlit side of the rings, at ranges of $\sim30,000$~km. The first scan covers the outer C and inner B rings at a phase angle, $\alpha = 82^\circ$ and an emission angle, $e = 47^\circ$, while the second covers the Cassini Division and entire A ring at $\alpha = 59^\circ$ and $e = 63^\circ$. The solar incidence angle was $114^\circ$ and the radial resolution of both scans is $15-20$~km, with sampling intervals of $2-3$~km. Structurally, the rings appear to have changed little, if at all, since the Voyager observations in 1980/81 and the 28~Sgr occultations in 1989. This similarity extends even to the quasi-irregular structure which characterizes the inner B ring on scales of $\sim100$~km. Spectrally, all regions of the rings scanned are dominated by water ice, with prominent absorption bands at 1.55, 2.0 and 3.0~$\mu$m, as well as weaker bands at 1.04 and 1.25~$\mu$m seen primarily in the A and B rings. The ice bands are strongest in the middle A ring, somewhat weaker in the B ring, and much weaker in the C ring and Cassini Division. However, the transitions between the C and B rings and between the Cassini Division and A ring are marked by gradual changes in band depth over radial distances of a few thousand km, perhaps indicative of ballistic redistribution of material. Besides water ice, the most noteworthy spectral feature is a broad, shallow absorption in the $0.9-1.8~\mu$m region which we tentatively attribute to Fe-bearing minerals, most likely silicates. This feature is seen primarily in the outer C ring and the Cassini Division, but like the ice band depths it pays scant attention to structural boundaries. This work was supported by NASA and ESA under contracts with the Cassini-Huygens Project.
DE: 5759 Rings and dust
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting