36th DPS Meeting, 8-12 November 2004
Session 7 Rings
Oral, Monday, November 8, 2004, 3:30-6:00pm, Lewis

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[7.05] Cassini-VIMS Observations of Saturn's Rings at SOI

P. D. Nicholson (Cornell), R. N. Clark (USGS), D. P. Cruikshank, M. R. Showalter (NASA Ames), B. Sicardy (Obs. de Paris), Cassini VIMS Team

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~\mum in two continuous radial scans across the unlit side of the rings, at ranges of ~0,000~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 ~00~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~\mum, as well as weaker bands at 1.04 and 1.25~\mum 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. Locally, however, the fractional band depths appear to be independent of optical depth, suggesting that the light diffusely transmitted through the rings at moderate phase angles is dominated by single scattering. Regionally, 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. A broad reflectance maximum at 3.6~\mum, characteristic of ice grain sizes less than 100~\mum, is prominent everywhere but particularly strong in the outermost parts of the A ring, exterior to the Encke Gap. Besides water ice, the most noteworthy spectral feature is a broad, shallow absorption in the 0.9-1.8~\mum 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 only modest attention to structural boundaries. This work was supported by NASA and ESA under contracts with the Cassini-Huygens Project.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.