The Mimas 2:1 Eccentric Corotational Resonance in Saturn's Outer B Ring

Abstract

We present evidence of an unusual and previously unidentified feature in Saturn's outer B ring at a radius of 117,130 km. The Voyager 2 optical occultation experiment shows a step in opacity, dropping from 1.4 to 0.5 across several kilometers and generally remaining low out to the edge of the ring. By contrast, the Voyager 1 radio occultation showed no distinct change at this location, a rare lack of correlation to a major optical feature. The ratio of opacities outside 117,130 km implies a nearly complete depletion of subcentimeter-sized ring particles unique in Saturn's main rings. We suggest this feature is the Mimas 2:1 eccentric corotational resonance (ECR). Unlike inner Lindblad resonances and inner vertical resonances, which produce waves on dynamic time scales (and which affect all sizes of particle equally), an ECR produces no observable effects on a dynamic time scale. However, we show that the torque exerted by the ECR can influence ring structure on the diffusion time scale. Moreover, a simple computation assuming two particle sizes and diffusion rates qualitatively reproduces the differences between the data sets for simulated times of a few million years. The ECR provides a new observational probe of some poorly known parameters of the rings: the physical mechanisms that determine the ring particle velocity dispersion and size distribution. Specifically, the difference between the optical and the radio opacities is the first observational evidence for the dependence of the diffusion coefficient on particle size, which is expected to arise from the size dependence of the velocity dispersion. Implicitly, it also places a lower limit of a few million years on the time scale for particle size redistribution processes. Hence, this is the first observational test of the suggestion of S. J. Weidenschilling et al. (1984, Planetary Rings, pp. 367-415, Univ. of Arizona Press, Tucson) that Saturn's ring particles are dynamical ephemeral bodies (DEBs) in a statistical equilibrium, accreting and fragmenting on a time scale of weeks. Detection of the ECR implies integrity of individual particles over much greater time scales, arguing against the DEBs model. The ECR time scale is consistent with ring formation due to breakup of a massive object with subsequent erosion of the remnant material. An interesting limit can be placed on the parameters of erosion by micrometeoroid bombardment.