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C. Porco, F. Namouni (SwRI)
The stability of the narrow ring arcs of Neptune has been a puzzle since their discovery. First detected in 1984 from the Earth in stellar occultations and imaged by the Voyager spacecraft in 1989, the 5 arcs spanning approximately 40 deg in longitude are apparently confined against the rapid azimuthal and radial spreading that results from energy dissipation in inter-particle collisions. Voyager data were used to argue in favor of an arc confinement model (Goldreich et al. AJ 1986; Porco, Science 1991) that relies on both the vertical and mean angular motions of the nearby Neptunian moon, Galatea, to produce a pair of Lindblad (LR) and corotation inclination (CIR) resonances capable of trapping ring particles into a sequence of arcs. However, HST and Earth-based observations taken in 1998 (Dumas et al. Nature 1999; Sicardy et al. Nature 1999) indicate a revised arc mean angular motion which displaces the arcs away from the CIR, leaving their stability once again unexplained.
In this presentation, we will discuss the workings of a hitherto neglected resonance which relies on Galatea's orbital eccentricity and which, together with the LR, is likely responsible for the angular confinement of the arcs. The action of this resonance, which operates through the precession of Galatea's eccentric orbit forced by the arcs' inertia, will allow a determination of the arcs' mass from future measurements of Galatea's eccentricity.
We acknowledge the financial support of NASA's Planetary Geology and Geophysics Program and the Southwest Research Institute's Internal Research Grant program.
The author(s) of this abstract have provided an email address for comments about the abstract: carolyn@ciclops.swri.edu
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Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.