37th DPS Meeting, 4-9 September 2005
Session 62 Planetary Rings I
Oral, Friday, September 9, 2005, 9:00-10:30am, Music Concert Hall

[Previous] | [Session 62] | [Next]

[62.05] Cassini-VIMS Observations of Stellar Occultations by Saturn's Rings.

P. D. Nicholson, M. M. Hedman (Cornell), B. Wallis (JPL), Cassini VIMS Team

On May 24 and June 11, 2005 the Cassini Visual and Infrared Mapping Spectrometer (VIMS) observed grazing occultations of the long-period variable star, o Ceti (spectral type M7 III, mK = -2.60). The first occultation track penetrated the rings to a minimum radius of 115,000~km, providing 2 complete profiles of the F and A rings as well as the Cassini Division, while the second track penetrated to a radius of 126,000~km, in the inner A ring. The sampling interval was 80~ms, but the radial resolution is limited by the projected stellar diameter which ranges from 0.25 -- 2.7~km, depending on ring longitude. Due to the very small ring inclination to the line of sight of 3.45\circ, the B ring and much of the A ring appear almost opaque, while extremely high S/N ratio lightcurves were obtained of low optical depth regions such as the F ring and Cassini Division. All four cuts across the eccentric F ring reveal a dense central strand of FWHM 25 -- 45~km flanked by inner and outer strands which are variable both in normal optical depth (0.005<\tau<0.035) and radial separation (250 -- 465~km). All three strands are embedded in a broad `skirt' which is at least 1200~km wide. Within the 325-km wide Encke Gap in the outer A ring are three quasi-circular narrow ringlets, as seen in Cassini images (Porco et al., 2005), with variable optical depths in the range 0.001<\tau<0.007. In the Cassini Division, density waves associated with the weak Atlas 5:4 and Janus 7:5 resonances (ILRs) are clearly seen, with the former providing an estimate of the surface mass density of 1.4±.2~g/cm2, identical to that obtained by Porco et al. (2005). Systematic variations in the transmission of the A ring with longitude are qualitatively consistent with gravitational wake models of Salo et al. (2004). This work was supported by NASA under a contract with the Cassini-Huygens Project.

[Previous] | [Session 62] | [Next]

Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.