G. R. Stewart (Univ. of Colorado), M. C. Lewis (Trinity University)
The conventional explanation for the maintenance of narrow planetary rings against radial spreading by inter-particle collisions is the confining torques exerted by nearby “shepherd satellites.” We offer an alternative explanation that is in better agreement with our N-body simulations of Saturn’s F ring. The key observation is that inter-particle collisions cause particles to migrate up eccentricity gradients. This happens for two reasons: (1) Strong eccentricity gradients and strongly correlated orbital phases in a satellite-perturbed ring cause the leading term in the density evolution to be proportional to the second derivative of the eccentricity with respect to the semi major axis. (2) Since the vast majority of inter-particle collisions occur in the satellite wake peaks where the particle number density is greatly enhanced, the local shear reversal in the wake peaks weakens the collisional diffusion down density gradients that is predicted by conventional accretion disk theory. The resultant evolution of the ring has two phases: In the first phase, the particles migrate toward Lindblad resonances where the forced eccentricities have local maxima. During the second phase, the eccentric narrow ringlets formed at Lindblad resonances collide with their nearest neighbors and merge to form a smaller number of ringlets that are not generally located at Lindblad resonances. We acknowledge the support of NASA's Planetary Geology and Geophysics Program.
The author(s) of this abstract have provided an email address for comments about the abstract: glen.stewart@lasp.colorado.edu
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.