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The confinement of Neptune's ring arcs by the moon Galatea


Neptune has five narrow ring arcs, spanning about 40 degrees in longitude, which are apparently confined against the rapid azimuthal and radial spreading that normally results from inter-particle collisions. A gravitational resonance based on the vertical motion of the nearby neptunian moon Galatea was proposed1,2 to explain the trapping of the ring particles into a sequence of arcs. But recent observations3,4 have indicated that the arcs are away from the resonance, leaving their stability again unexplained. Here we report that a resonance based on Galatea's eccentricity is responsible for the angular confinement of the arcs. The mass of the arcs affects the precession of Galatea's eccentric orbit, which will enable a mass estimate from future observations of Galatea's eccentricity.

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Figure 1: The motions of arc particles around Neptune.
Figure 2: The effect of ring mass on the resonance location.


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We thank C. Dumas and P. Nicholson for reviews and B. Bottke, R. Canup, M. Evans, P. Goldreich, D. Hamilton, D. Nesvorny, J. Peterson, H. Throop and B. Ward for discussions. We acknowledge support from Southwest Research Institute's Internal Research Grant programme and NASA's Planetary Geology and Geophysics Discipline programme.

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Correspondence to Fathi Namouni.

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Namouni, F., Porco, C. The confinement of Neptune's ring arcs by the moon Galatea. Nature 417, 45–47 (2002).

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