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J. A. Burns (Cornell U.)
Using spacecraft and telescopic observations, I review the gross physical and morphological properties of the distinctive ring systems of each of the giant planets. In addition to their intrinsic interest, planetary rings provide proximate, but imperfect, analogs for more distant and massive structures, such as accretion and protoplanetary disks, which are of considerable current astronomical interest. Jupiter's dusty rings, strongly influenced by electromagnetic forces, appear to be derived of debris sloughed off small exterior satellites. Uranus's nine surprisingly narrow, eccentric and inclined rings hint at a role for self-gravitational effects. Neptune's most massive ring contains several clumps, likely due to satellite interactions. Saturn's resplendent rings display aspects of all the other systems. As time allows, I will mention observations planned for Cassini, a spacecraft mission just now starting a 4+-year tour of Saturn.
Structures that develop owing to gravitational forces and collective effects can be destroyed by physical collisions and tidal shear. Satellites --near and far-- play important roles in sculpting planetary rings. Embedded satellites can be sources but also sinks for ring material. Moons generate clumps and kinks in narrow rings, and gravitational wakes and waves in continuous disks. The frequencies contained in the epicyclic motion of nearby satellites can resonate with the epicyclic frequencies of ring particles, thereby initiating spiral density and bending waves at some locations; from such features, one can infer ring surface density and viscosity. The resultant angular momentum transfer can open gaps or terminate rings. Besides moons, non-uniform magnetic or gravity fields can generate noticeable features. I provide simple illustrations of such phenomena, and supplement them with numerical simulations. Several lines of evidence indicate that planetary rings are considerably younger than the solar system. The jury is still out on possible ring origin(s).
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.