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

Stone, E. C., and E. D. Miner 1986. The Voyager 2 encounter with the Uranian system. Science 233 (4759), 39-43. (Excerpt from pp. 40-41.)

Copyright AAAS, July 4, 1986.

The Voyager 2 Encounter with the Uranian System

Rings. The characteristics of the Uranus' rings offer the prospect of a better understanding of the origin of ring systems and their associated dynamical processes. Some of the physical characteristics are given in Table 2, which includes 1986U1R and 1986U2R, two new rings discovered in Voyager images. 1986U1R is a narrow ring similar to the others; 1986U2R is broad and diffuse. A number of other possible rings or partial rings (arcs) have been identified in stellar occultation data. The spectral reflectance of the individual ring particles is low (<5%), and at least the epsilon ring is gray. Color images show no statistically significant color differences between the epsilon and other rings.

It was expected that there would be up to 18 small satellites (shepherds) confining the narrow rings between them. Two such shepherd satellites, 1986U7 and 1986U8, were found on either side of the epsilon ring. The outer edge of the epsilon ring, which is quite sharp, closely corresponds to the location of a high wavenumber resonance of 1986U8, and the inner edge is close to a similar resonance of 1986U7. Because these resonances do not overlap, the interaction is the same as that between Mimas and the outer edge of Saturn's B ring and differs from the shepherding interaction at Saturn's narrow F ring. Shepherd satellites for the other Uranian rings were not found, probably because they are too small (<14 km in diameter) and are charcoal black like the ring particles.

Surprisingly, particles smaller than tens of centimeters were absent from the epsilon ring, as evidenced by the similarity of the optical thickness of the ring at different radio and optical wavelengths. The sharp edge of the epsilon ring indicates a ring edge thickness less than 150 m. The relative paucity of smaller particles may be the result of atmospheric drag from the extended hydrogen atmosphere.

A long exposure image at high phase angle did reveal a broad but optically thin distribution of micrometer-size particles inward from the orbit of 1986U1R to at least the outer edge of 1986U2R. The dust distribution is highly structured, most closely resembling Saturn's D ring. There is also evidence from impacts on the spacecraft as it passed through the ring plane of a 4000 km thick band of micrometer-size dust particles 116,000 km from the planet. The maximum impact rate detected by the plasma wave instrument was 20 to 30 per second, corresponding to a maximum number density of approximately 0.001 m^(-3).

Several of the main rings exhibit striking variability in opacity with longitude. Both the delta and gamma rings vary by more than a factor of 2 in width, and the narrow component of the eta ring completely disappears in places. This longitudinal variability in the main rings and the possible presence of numerous adjacent ring arcs suggest that Uranus' rings are dynamic and may be young, rather than having formed at the same time as Uranus.

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Last updated Feb-27-1997