Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p54a..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P54A-03
Other
[5759] Planetary Sciences: Fluid Planets / Rings And Dust, [6265] Planetary Sciences: Solar System Objects / Planetary Rings
Scientific paper
The Cassini Division between Saturn's A and B rings is punctuated by eight nearly empty gaps. It was suggested by Lissauer, Shu & Cuzzi (1981) that each gap contains a small moonlet which holds it open, analogous to the Encke and Keeler gaps in the outer A ring, but to date no direct evidence for such moons has been reported. Based on analysis of 41 high-quality stellar occultation cuts through the rings obtained by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft, we propose an alternative explanation for the locations of these gaps. We find that all the gaps in the Cassini Division which do not contain dense ringlets have nearly circular outer edges, while all the other edges exhibit significant variations in their radial locations. As expected based on previous observations (Porco et al. 1984, Spitale et al. 2006), we find that the outer edge of the B ring has a large m=2 component due to the 2:1 inner Lindblad resonance with Mimas. However, detailed studies of this pattern suggest that the orientation of the B-ring edge slowly drifts or librates relative to Mimas. The inner edge of the Barnard gap exhibits an m=5 component that is likely due to the nearby 5:4 ILR with Prometheus. The inner edges of all the remaining gaps are well-fit by simple precessing ellipses, whose pattern speeds form a quasi-regular series with a spacing of approximately 0.05-0.07 degrees/day. We propose that the eccentric inner edges of the Cassini Division gaps are controlled by a type of three-body resonance which involves interacting perturbations from Mimas itself and the B-ring edge. In this model, the periodic variations in the orientation of the B-ring edge relative to Mimas are responsible for the observed quasi-regular spacing of the eccentric gap edges. The m=1 mass anomaly at the inner edge of each gap may then lead to a "shepherding'' affect that repels the nearly circular outer edge and thus may control the width of the gap.
Hedman Matthew M.
Nicholson Philip D.
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