Cassini-VIMS Observations Of Self-gravity Wakes In Saturn's Rings

Details Cassini-VIMS Observations Of Self-gravity Wakes In Saturn's Rings Cassini-VIMS Observations Of Self-gravity Wakes In Saturn's Rings

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dda....37.0803n&link_type=abstract

American Astronomical Society, DDA meeting #37, #8.03; Bulletin of the American Astronomical Society, Vol. 38, p.670

Physics

Scientific paper

Between May 24 and August 9, 2005 Cassini's Visual and Infrared Mapping Spectrometer (VIMS) observed four occultations of the long-period variable star, o Ceti (Mira) by Saturn's rings. Large, systematic variations in the transmission of the A ring with longitude are evident in the data, with a maximum in transmission occurring at a longitude of 250° relative to the observer. This is consistent both with the well-known azimuthal asymmetry in reflected light from the A ring, and with numerical models of spontaneous gravitational wakes (e.g., Salo et al. 2004). The physics underlying such wakes is essentially the same as the "swing amplifier" which was studied in the context of galactic disks by Julian & Toomre (1966) and Toomre & Kalnajs
(1991).
We have developed a simple model of transmission through a set of parallel wakes, modelled as opaque cylinders with elliptical cross-sections. By fitting the predictions of this model to the observed variations of transmission with longitude, we find the following. (1) The orientation of the wakes varies slightly but systematically across the A ring, from 18° to 26∘ relative to the azimuthal direction. (2) The peak transmission reaches a maximum in the middle A ring at a radius of 129,000 km, in good agreement with the radius of minimum reflected brightness seen in the Voyager images by Dones et al. (1993). (3) Both the orientation and peak transmission vary anomalously in the vicinity of strong density waves. (4) The width of the transmission peak implies an average vertical thickness for the wakes, H, which varies from 9-12% of the wake wavelength, λ. On the assumption that λ is given by Toomre's critical wavelength for axisymmetric instability in a self-gravitating disk, Ω2/4π2Gσ, we estimate an effective ring thickness of 5 m.
Supported by NASA and the Cassini-Huygens Project.

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