The Dynamics of the Neptunian Adams Ring's Arcs

Abstract

We examine the resonant forcing of a narrow ringlet by a nearby satellite on an inclined, but circular, orbit. The general techniques that we develop are used to study the dynamics of Neptune's ring arcs, near the 43:42 mean motion resonances with the satellite Galatea. More specifically, the averaged equations of motions are used to analyze the coupling between the various resonances at work, while a mapping integrator allows us to integrate the motion of up to 104particles for several centuries. We show that even in the absence of dissipation, the coupling between the horizontal and vertical motions of the particles can lead to a stochastic migration of the particles for one to the other of the 43 × 2 = 86 corotation sites where the arcs are assumed to be trapped. The pressure of solar radiation sweeps out from the arcs the particles with a ratio β_cdof pressure of radiation to solar gravitation larger than ∼0.01, corresponding to ∼50 μm-sized icy particles in the limit of geometrical optics. Poynting–Robertson (PR) drag, on the other hand, has only a small effect on dust particlesalreadyinside the corotation sites. In contrast, PR drag rapidly drives the particles lying outside these sites on unstable orbits. Inelastic collisions between the larger particles remain the most serious problem for the arc stability. We discuss the implications of these results in terms of a population of large particles being the source of dusty arcs. We show in particular that such structures, while not permanent, can be nevertheless statistically common at corotation resonances with a nearby satellite such as Galatea.