With an orbital radius of 141,000 km, the isolated F ring lies 4000 km beyond the edge of Saturn's main ring system. It is composed of millions of small (1 cm) ice particles travelling around Saturn at speeds of 17 km/s. Pioneer 11 discovered the F ring in 1979 and, although the first images suggested something unusual, it was the Voyager 1 pictures in November 1980 that showed a ring composed of three strands, two of which appeared to intersect giving rise to a "braided" appearance. A number of sudden changes in direction, or "kinks" were also seen. However, nine months later, Voyager 2 images showed a more regular ring with at least four non-intersecting strands. Low resolution images obtained by both spacecraft as they approached Saturn showed a number of "clumps" moving in the vicinity of the F ring. The Voyager images also revealed two small, shepherding moons, Pandora and Prometheus, orbiting on either side of the ring.
The only direct observations of the F ring since the Voyager era took place in 1995. It was detected by the Hubble Space Telescope and a number of ground-based instruments when the rings of Saturn appeared almost edge-on as viewed from Earth. The reduced glare from the main rings enabled the normally faint F ring to be detected. There were also detections of Pandora and Prometheus with the surprising discovery that Prometheus appears to be 20 degrees behind its predicted position. The HST observations revealed a number of localised and extended features in the vicinity of the F ring. These were initially mistaken for small satellites but are now thought to be short-lived phenomena which may be similar to some of the features detected by the Voyagers.
The best data on the F ring remain the high resolution images obtained by the Voyager spacecraft in 1980 and 1981. These are the only images capable of showing direct evidence of radial structure within the ring. In an attempt to make sense of the ring and its changing appearance, the group based at Queen Mary and Westfield College, led by Dr Carl Murray, undertook a complete survey of all the Voyager images showing radial structure. Although coverage of the ring is far from complete they managed to fit orbits to the individual strands. They conclude that the strands can be modelled as individual rings having similar but not identical orbits. There are slight differences in the orientation of the elliptical orbits such that the strands can appear parallel at some longitudes and yet are close to intersection at others. They show that the high resolution Voyager 1 observations were close to a region of intersection while the Voyager 2 images covered the near-parallel region. Therefore there may not have been any fundamental change in the F ring between the two encounters. The group has also confirmed previous suggestions that there was dust in the region between the F ring and the main rings.
Why the F ring has radial structure and how it is maintained is more difficult to explain. The shepherd satellites are known to perturb the F ring and their effects can be detected in the images. According to a study published by Murray and Giuliatti Winter in the journal Nature last year, Prometheus actually enters parts of the F ring at intervals of 19 years causing severe disruption. The 20-degree lag in Prometheus's position may be evidence of such encounters. One should have occurred between 1981 and 1995. It is difficult to envisage a mechanism whereby the ring re-organises itself after these events without invoking the additional confining effects of small satellites orbiting within the strands of the F ring. These could also help to explain the radial structure seen by the Voyagers.
The Voyager 1 observations of the bizarre F ring made it a natural target for follow-up by Voyager 2. There were plans to obtain high resolution images of the ring covering most of its orbit, as well as the tracking of individual clumps to monitor their evolution. However, the scan platform on which Voyager 2's cameras were mounted failed near the time of closest approach and these observations were never made. Hopes for a complete understanding of the F ring now rest with the Cassini mission to Saturn which will be launched in October of this year. As a member of the Cassini Imaging Team, Dr Murray has a particular interest in ensuring that the spacecraft's cameras make key observations of narrow rings and small satellites during the four-year tour of the Saturn system starting in 2004. With the ability to take more than 300,000 images of the Saturn system, Cassini's cameras will have the opportunity to monitor the evolving F ring over a period of years rather than days and will resolve the enigma of the solar system's strangest ring.
Images and further information are available via the World Wide Web
1. Voyager images of the F ring are available in GIF format from... http://www.maths.gmw.ac.uk/~kevin/SSD/fring"
Caption to 'fringV1.gif':
False colour image of the F-ring taken by the Voyager 1 spacecraft in November 1980, showing examples of a braid, a clump and a kink. At least three strands are visible. The image covers approximately 20 degrees in longitude. The edge of the A-ring is visible at the bottom right of the image, with the 30-km-wide Keeler Gap clearly visible. The F ring lies some 4,000 km beyond the A ring.
Caption to 'cassfring.gif':
An artist's impression shows the surface of Pandora, one of the shepherds of Saturn's F ring, a thin wispy band of material just on the outside of the main rings of Saturn. The F ring is brightly shown on the left hand side of the picture. By David Seal. (Courtesy NASA/JPL)
2. Artist's impressions of various stages of the Cassini mission and views from Saturn's satellite are available from...
http://www.maths.qmq.ac.uk/~kevin/SSD/art_cass
3. Further information about the Solar System Dynamics group at QMW and their involvement in Cassini can be found at...
http://www.maths.qmw.ac.uk/~kevin/SSD/cass_qmw.html
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