Acknowledgement. We thank Science for their permission to use an excerpt from:

Stone, E. C., and E. D. Miner 1989. The Voyager 2 encounter with the Neptunian system. Science 246 (4936), 1417-1421. (Excerpt from pp. 1419-1420.)

Copyright AAAS, December 15, 1989.

The Voyager 2 Encounter with the Neptunian System

Rings. Earth-based stellar occultation measurements during the early and middle 1980's seemed to imply the presence of partial rings (ring arcs) at several radial distances from Neptune's center. Voyager found a system of prograde, equatorial, circular rings. The outermost of these rings, at a distance of 62,900 km from the planets center, includes three optically thicker "arcs" extending 4 degrees, 4 degrees, and 10 degrees in ring longitude. These arcs are believed to be responsible for all but one of the confirmed ring occultation events seen from Earth. The remaining event was apparently a chance occultation of (1989N2), the second of the Neptune satellites discovered by Voyager.

Narrow rings are thought to be confined by the actions of relatively nearby satellites. 1989N4 and 1989N3 orbit just inside the two narrow rings, 1989N1R and 1989N2R, respectively, and may serve to prevent ring material from spiraling inward toward Neptune. No ring shepherds (satellites) have been found at the outer edges of these rings, although satellites with diameters of <=12 km would have escaped detection. It is not presently known whether additional smaller shepherding satellites exist or if other confinement mechanisms are at work. The brighter arcs within 1989N1R are also an enigma. Such material, if azimuthally unrestrained with the ring, should spread relatively evenly around the ring within only a few years.

Particle sizes within the rings appear to be smaller than was the case with the Uranus rings. The rings were not readily detected by the radio occultation experiment, although additional processing of the data may yield useful ring information. Stellar occultation measurements by PPS and UVS revealed 1989N1R. It should be noted the sigma Sagitarii passed behind the leading 4 degree arc of 1989N1R; Voyager 2 did not pass behind any of the ring arcs in 1989N1R during the radio occultation experiment. The central core of the ring was 10 km in radial width, consistent with imaging measurements. There was a marginal detection of 1989N2R by PPS. Comparison of high-phase and low-phase images of the rings shows that the dust content of 1989N2R and 1989N3R is about twice that of the other rings and much more than that of the main rings of Saturn and Uranus. The three arcs also seem to have more dust than the remainder of 1989N1R.

Particle impacts detected at both ring plane crossings are indicative of a thousand kilometer thick region of micrometer sized particles. Impact rates reached a maximum of about 280 s^(-1) during the inbound crossing at 85,000 km from the planet center; rates at the outbound crossing (103,700 km) peaked at about 110 s^(-1). These rates imply a number density of order 1e-3 m^(-3). Significant but lower count rates were detected more than 20,000 km above and below the ring plane, as well as over the northern polar region.

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