Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p12b..02f&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P12B-02
Physics
[6265] Planetary Sciences: Solar System Objects / Planetary Rings
Scientific paper
Although Saturn's rings appear at first glance to be axisymmetric, more precise measurements reveal that many of the gap edges and narrow ringlets within the rings are noncircular, a characteristic they share with the narrow uranian rings. A careful study of these features is of interest for several reasons: (i) resonantly-forced perturbations are believed to prevent the rings from spreading under the influence of collisions, (ii) unforced distortions, mostly eccentricities, can lead to estimates of the surface mass density and viscosity of the rings, and (iii) accurately-measured apsidal precession rates provide information on Saturn's zonal gravity harmonics. We present preliminary results from a comprehensive study of noncircular features in the Cassini Division and in the C ring. The data used in this study come from three Cassini experiments, and cover the period from May 2005 to September 2010. Over 120 stellar occultations have been observed by the Ultraviolet Imaging Spectrometer (UVIS) and by the Visual and Infrared Imaging Spectrometer (VIMS). In addition, we include 12 occultations of the spacecraft's radio Radio Science Subsystem (RSS) by the rings observed on Earth in May-September 2005. The simplest noncircular features can be modeled as inclined Keplerian ellipses, freely precessing under the influence of Saturn's oblate gravity field. In agreement with similar fits to the VIMS occultation data alone, we find that the inner edges of 7 of the 8 gaps within the Cassini Division are eccentric, with amplitudes ranging from 0.9 km to 28.3 km. In contrast, most of the outer gap edges are near-circular. We also find a rich assortment of normal modes on the edges of both ringlets and gaps. We have searched for modes with wavenumber m as high as 8, and find convincing evidence for modes with m = 0, 2, 3, 4 and 5, all with amplitudes of 1 km or greater. In some cases, as many as 3 or 4 normal modes coexist at a single edge with comparable amplitudes. Our fits also reveal the pervasive effects of the strong Mimas 2:1 inner Lindblad resonance (ILR), which has long been recognized to define the outer edge of the B ring. We find that almost all sharp-edged features in the Cassini Division exhibit a small but detectable m = 2 variation whose apoapse is locked to Mimas. The amplitudes of these distortions decrease with distance from the resonance, and conform to a simple analytical model for isolated test particles perturbed by the resonance. We confirm that the Colombo (or Titan) ringlet precesses at virtually the same rate as Titan's mean motion, 22.5770 deg d-1, with an apoapse oriented to within 4 deg of Titan's mean longitude. Both edges of this ringlet also exhibit what appear to be free normal modes, with m = 0 on the inner edge and m = 2, 3 and 4 on the outer edge. In contrast, the Maxwell ringlet is a freely precessing ellipse, and we see no evidence for additional normal modes on either edge. We also find clear evidence for normal modes on the edges of the Bond ringlet and Dawes gap.
Colwell Josh
French Richard G.
Hedman Matthew M.
Lonergan K.
Marouf Essam A.
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