SwRITM Researchers Report Breakthough in Understanding the Mystery of Neptune's Ring Arcs
The stability of the narrow ring arcs of Neptune has been a puzzle since their discovery. First detected in 1984 from the Earth in stellar occultations and imaged by the Voyager spacecraft in 1989, the 5 arcs spanning approximately 40 deg in longitude appear to be confined against the rapid azimuthal and radial spreading that results from energy dissipation in inter-particle collisions. Over a decade ago, Voyager data were used to argue in favor of an arc confinement model that relies on both the vertical and mean angular motions of the nearby Neptunian moon, Galatea, to produce a pair of Lindblad and corotation inclination resonances capable of trapping ring particles into a sequence of arcs. However, this mechanism was in some respects problematic, for example, failing to explain late-1990s telescopic results. Namouni and Porco explored the workings of a hitherto neglected ring particle resonance which relies on Galatea's orbital eccentricity and which, together with the Lindblad Resonance, is likely responsible for the angular confinement of the arcs. The action of this resonance, which operates through the precession of Galatea's eccentric orbit forced by the arcs' inertia, will eventually allow a determination of the arcs' mass from future measurements of Galatea's eccentricity.