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

Stone, E. C., et al. 1981. Voyager 1 Encounter with the Saturnian System Science 212 (4491), 159-163. (Excerpt from pp. 160-161.)

Copyright AAAS, April 10, 1981.

Voyager 1 Encounter with the Saturnian System

Rings. Since first seen by Galileo in 1610, Saturn's primary distinguishing characteristic has been its rings. The distinct, broad A, B, and C rings, which are easily observable from Earth, were found to be of distinctive character. The outermost ring, A, is probably closest to the pre-Voyager concept of Saturn's rings. Its rather uniform appearance is marked by a few narrow features that correspond to orbital resonances with satellites 1980S1 and 1980S3. There is evidence for satellite-driven density waves in the region of the Encke division. Radio transmission through the rings indicated that the effective diameter of A ring particles is 10 m. The small satellite 1980S28 is just beyond the outer edge of the A ring.

The A and B rings are separated by the Cassini division, which contains five broad rings that have additional structure. The effective particle diameter from radio transmission is about 8 m.

The middle, or B, ring is strikingly different, consisting of numerous narrow ringlets with no apparent large-scale order. It is possible that the ringlets are formed by the action of a large number of moonlets embedded within the ring. The B ring region is also characterised by sporadic radial markings or "spokes," perhaps the result of levitation of small particles above the ring plane. The possible importance of electrostatic charging effects in spoke formation and dynamics is suggested by the detection at radio wavelengths of electrostatic discharges from the rings. These discharges are loosely correlated with Saturn's rotation.

The C ring, which is just inside the B ring, is more transparent than the A and B rings and contains a number of dense ringlets whose locations are regular but evidently unrelated to orbital resonances with the larger satellites. Radio transmission through the C ring indicated an effective particle diameter of 2 m. A still fainter D ring, also comprised of numerous narrow features, was found between the inner edge of the C ring and the planet.

An atmosphere of neutral hydrogen extends 60,000 km (36,000 miles) above and below the main rings and somewhat beyond the outer edge of the A ring. Water ice in the ring material is a potential source for the cloud of hydrogen, which has an estimated number density of 600 atoms per cubic centimeter.

Beyond the A ring are three additional rings. The F ring was discovered by Pioneer 11, which also disclosed a complex distribution of material in this region (5). Voyager 1 images showed a very narrow ring that has local concentrations in some regions and a multicomponent braided appearance. Satellites 1980S26 and 1980S27 provide the gravitational shepherding that maintains the narrow ring of very small particles. The other rings are noted in Table 3.

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