A curious ringlet that shares Prometheus’ orbit but precesses like the F ring

Abstract

Images obtained by the Cassini spacecraft of the region just beyond Saturn's main rings reveal a previously unreported narrow and dusty ringlet that has dynamical connections with both Saturn's small satellite Prometheus and the F ring. The radial position of this ringlet is observed to vary with time and longitude, indicating that it is eccentric with an eccentricity of 0.0012 and that its mean orbital radius varies between 139,300 km and 139,400 km. These mean radii are consistent with material trapped in a co-orbital 1:1 resonance with Prometheus. However, the apsidal precession rate of this ringlet is not that expected for material close to Prometheus’ orbit (2.76°/day). Instead, the ringlet appears to be precessing at the same rate as the F ring (2.70°/day). This ringlet therefore appears to consist of material co-rotating with Prometheus whose apsidal precession rates have been modified by interactions with F-ring material. This ringlet may therefore provide new insights into how rings can maintain organized eccentric structures over a range of semi-major axes.