Particle dynamics in the central ringlet of Saturn's Encke gap

Abstract

A kinky and clumpy ringlet shares orbit with the moon Pan in the center of the 320-km wide Encke gap in Saturn's rings (Porco et al., 2005). The ringlet is mainly composed of micron-sized particles (Showalter, 1991, Hedman et al., 2011), implying that these particles may be significantly perturbed by non-gravitational forces, which can limit their lifetimes. We establish a kinetic model considering the birth, evolution, and death of dust in the Encke central ringlet allowing to evaluate the ringlet optical depth. First, we investigate the generation of dust by micrometeorite impacts (the `impact-ejecta' process) on putative, yet undetected embedded moonlets. Taking into account the orbital evolution under the influence of the relevant perturbation forces, the dominant loss mechanisms are collisions with ring particles in the gap edges, the putative moonlets in the gap, or erosion by sputtering in Saturn's plasma environment. However, our results show that this impact-ejecta process alone can only sustain a ringlet of optical depth 3-4 orders of magnitude smaller than the observed values. Consequently, other processes must be taken into account. For example, mutual collisions among putative moonlets should produce dust at about the same rate, while further disruption of ejecta, as proposed by Dikarev (1999), should increase the total amount of particles. Furthermore, observations show an azimuthal asymmetry of the material in the Encke gap ringlets (Ferrari and Brahic 1997, M. Srem\v{c}evi\'c, private communication). We investigate the scenario proposed by Hedman et al. (2013) that for the Encke central ringlet, the observed asymmetry is mainly due to the combined action of plasma drag and Pan's gravity causing a focusing of dust in the region leading Pan's orbit (Hedman et al. 2013).