Dust dynamics in the Encke Gap: Hill's equations and ion interactions.

The Encke Gap is a 320-km-wide opening in Saturn's outer A ring centered on the orbit of the small moon Pan. In addition to Pan itself, this gap contains an array of dusty features composed primarily of particles less than 100 microns across. In particular, there are three narrow ringlets in this region that are not longitudinally homogeneous, but instead contain series of bright clumps. Using images obtained by the Cassini spacecraft, we have tracked the motions of these clumps and found that they do not follow the predicted trajectories of isolated ring particles moving under the influence of Saturn's and Pan's gravitational fields. We have also examined the orbital properties of the ringlets by comparing images taken at different longitudes and times. We find evidence that the particles' orbits have forced eccentricities induced by solar radiation pressure. In addition, the mean orbital radii of the particles in these ringlets appear to vary with the local co-rotating longitude. We suggest that these trends may be due to a combination of collisions among the ring particles and drag forces that may be due to interactions with the ambient magnetosphere plasma. Furthermore, we speculate that a combination of these same drag forces and Pan's gravitational perturbations may be responsible for maintaining at least two of the narrow ringlets in this gap.