HR: 08:15h
AN: P31D-02 INVITED     [Abstracts]
TI: Cassini-VIMS Observations of Stellar Occultations by Saturn's Rings
AU: * Hedman, M M
EM: mmhedman@astro.cornell.edu
AF: Cornell University, Space Sciences Building Cornell University, Ithaca, NY 14850 United States
AU: Nicholson, P D
EM: nicholso@astro.cornell.edu
AF: Cornell University, Space Sciences Building Cornell University, Ithaca, NY 14850 United States
AU: Wallis, B D
EM: bwallis@frazmtn.com
AF: JPL/Caltech, Jet Propulsion Lab 4800 Oak Grove Drive, Pasadena, CA 91109 United States
AB: Between May 24 and August 9, 2005 the Cassini Visual and Infrared Mapping Spectrometer (VIMS) observed four grazing occultations of the long-period variable star, o Ceti (spectral type M7 III, m_K = -2.60). The first occultation track penetrated the rings to a minimum radius of 115,000~km in the B ring, while the second and third tracks penetrated to 126,000~km and 132,000~km in the A Ring, and the last track reached 139,500~km, just interior to the F Ring. The sampling interval was 80~ms,but the radial resolution is limited by the projected stellar diameter which ranges from 0.4 -- 4~km, depending on ring longitude. Due to the very small ring inclination to the line of sight of 3.45°, 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 cuts across the eccentric F ring reveal a dense central strand of FWHM 10 -- 50~km flanked by inner and outer strands which are variable both in normal optical depth and radial separation. 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 normal optical depths in the range 0.001<τ<0.25. Systematic variations in the transmission of the A ring with longitude are qualitatively consistent with gravitational wake models of Salo et al. (2004). Subsequently, on August 20, 2005, VIMS observed a single occultation of the star α Scorpii (spectral type M1 Iab, m_K = -3.78). The occultation track penetrated to a radius of 101,200~km in the B-Ring. The sample interval was 40~ms, which determined the radial resolution of about 0.4~km. Since the inclination angle for this observation was much higher, 32.1°, 25 -- 50% of the light was transmitted through the A ring. For all parts of the ring with significant optical depth, the signal varies erratically from sample to sample. These fluctuations are larger than the noise observed with the unocculted star and they are strongly correlated across all wavelength channels, indicating that they are due to fine scale structure in the rings as described in Showalter and Nicholson (1990). This work was supported by NASA under a contract with the Cassini-Huygens Project.
DE: 5465 Rings and dust
DE: 6275 Saturn
SC: Planetary Sciences [P]
MN: Fall Meeting 2005