Tiscareno, M. S., Hedman, M. M., Burns, J. A., Porco, C. C., Weiss, J. W., Murray, C. D. (2005). "Cassini ISS Observations of the Encke and Keeler Gaps in Saturn's Rings." Eos Trans. AGU 86(52), Fall Meet. Suppl.


Abstract

Near the outer edge of Saturn's main rings are two narrow gaps, each of which is inhabited and cleared out by a moon: the Encke Gap maintained by Pan, and the Keeler Gap maintained by newly-discovered S/2005~S1. Despite their gross similarity, these two gaps have many dissimilarities. The edges of the Encke Gap are wavy over all 360° of longitude, with most of that extent characterized by a wavelength connected with Pan. However, far downstream from Pan that wavelength is modified in unexpected ways that may be due to inter-particle effects. Sharing the gap with Pan are 4 ringlets, three of which are narrowly confined. These exhibit radial variation ("kinks") and bright patches, which are often spatially coincident with each other. At a given longitude, any of the 3 narrow ringlets may be brightest, due to the presence of bright patches. In the central ringlet, coincident with Pan's orbit, most bright patches occur between Pan and its leading (L4) Lagrange point, suggesting a connection with horseshoe, or even tadpole, orbits. Bright patches in the central ringlet move with respect to Pan on a timescale of months -- several appear to have been dispersed after approaching Pan, while at least one has survived and now moves away from Pan again. At the inner ringlet's conjunction with Pan, smooth sinusoidal perturbations in the inner ringlet are observed, though these damp quickly (unlike the wavy edges). No positive signs have yet been observed to indicate moonlets in the gap other than Pan, although the very existence of multiple stable ringlets is highly suggestive. Although the Keeler Gap is 8 times narrower than the Encke Gap (due to the perturbing moon being much smaller than Pan), variations in the gap edges are the same size or larger, making it a much less genteel place overall. Variations in the inner edge, with a characteristic wavelength of 12° consistent with the expected influence of the 32:31 resonance (ILR) with Prometheus, have an amplitude of 16 km. This causes the overall width ofthe Keeler Gap to vary by 40%, a situation perhaps unique in known ring systems. Furthermore, these variations are highly distorted from a simple sinusoid, and there are at least two different frequencies at which an individual feature may orbit. The wavy edges raised directly by S/2005~S1 are only seen within a few degrees of longitude of the moon. Although they are similar in amplitude to those of the Encke Gap (the closer proximity of the gap edge compensating for the smaller size of the moon), the Keeler wavy edges quickly take on a chaotic appearance and then damp out completely. The outer edge is instead characterized by sharp-edged structures dubbed ``wisps'', about 1~km in amplitude, which orbit at the mean motion of the outer edge. Examples of these phenomena will be presented, and their origins and mechanics will be discussed.