The Encke and Keeler Gaps as Chirikov-Wisdom effects
I. I. Shevchenko+*, S. A. Drobchik+
+Saint Petersburg State University, 199034 Saint Petersburg, Russia
*Institute of Applied Astronomy, Russian Academy of Sciences, 191187 Saint Petersburg, Russia
Abstract
We consider and explain the Encke and Keeler Gaps in the A ring of
Saturn as Chirikov-Wisdom effects, namely, as due to accumulation
and overlap of the first-order mean-motion resonances of the ring
particles with satellites Pan and Daphnis. An analytical
improvement of Wisdom's formula for the radial width of the
"planetary" (here, satellite's) chaotic zone is provided. It
accounts for specific properties of the system: large number of
interacting resonances in the multiplet, and their large (similar
to each other, by the order of magnitude) amplitudes. Using the
improved theory, we show that the width of the Encke Gap is
accurately predicted, while the theoretically estimated Keeler Gap
width is two times greater than the actual one. The disparity in
the latter case is tentatively explained by the relatively small
mass of Daphnis (two orders of magnitude less than that of Pan);
when mass is so small, one has to take into account the central
regular (horseshoe-orbit) part of the Gap.