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G.R. Stewart (Univ. of Colorado)
Recurrent local gravitational instabilities in the outer portions of Saturn's rings tend to form hundred-meter-scale ``wakes" that are believed to cause the observed azimuthal brightness asymmetry in the rings. Local N-body simulations of planetary rings show that such structures readily form and can greatly enhance the rate of viscous angular momentum transport in the rings. Numerical calculations of the rate of momentum transport closely match an analytic estimate derived by Ward and Cameron (1978) to describe the viscous evolution of the protolunar disk formed by a giant impact on the protoearth. An outstanding puzzle, however, is why the amplitude of the brightness asymmetry peaks in the middle of the A ring and declines sharply in the outer A ring. N-body simulations cannot readily solve this puzzle because the scale of instability in the outer A ring becomes larger than the local region that can be simulated.
One possible explanation is that the gravitationally enhanced viscous stirring actually inhibits the growth of local instabilities in the outer A ring. A self-consistent theory of viscous transport in a marginally stable planetary ring has been derived from kinetic theory, which includes both the enhanced viscous stirring by gravitational instabilities and the inhibition of instabilities by viscous stirring. This theory extends the work of Ward and Cameron by predicting the steady-state velocity dispersion and the amplitude of the ``wakes" in a marginally stable planetary ring. This research was supported by NASA's Planetary Geology and Geophysics Program.
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