Conference warhed::astronomy

From:	VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:28:04.89
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Subj:	[ASTRO] Interweaving Strands Braid Saturns Strange F-Ring

Interweaving Strands Braid Saturns Strange F-Ring

A group of astronomers based at London's Queen Mary and Westfield College
has completed a new study of Voyager images of Saturn's intriguing F ring.
This narrow ring is one of the strangest of all planetary rings in the solar
system. The images sent back by the Voyager 1 spacecraft in 1980 showed it
had a highly unusual "braided" appearance. However, pictures obtained by
Voyager 2 nine months later showed a much more regular ring composed of
parallel, non-intersecting strands. The London astronomers now conclude that
many of the F ring's unusual features, including its changing appearance,
can be explained by a ring of at least four strands with similar but
slightly different orbits.

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.

Contact

     Dr Carl D. Murray, Astronomy Unit, Queen Mary and Westfield College,
     University of London, London E1 4NS.
     Telephone: (0171) 975-5456. E-mail: [email protected]

----------------------------------------------------------------------------

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.qmw.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.qmw.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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