Cassini reveals more about Saturn's F ring (Forwarded)

Particle Physics and Astronomy Research Council
Swindon, U.K.

Contacts:

Gill Ormrod, PPARC Press Office
Tel: 01793 442012

Professor Carl Murray, member of the Cassini Imaging Science Subsystem Team
Queen Mary, University of London
Tel: 020 7882 5456

Carolina Martinez, Jet Propulsion Laboratory, Pasadena, Calif.
Tel: 00 1 (818) 354-9382

Preston Dyches, CICLOPS/Space Science Institute, Boulder, Colorado
Tel: 001 720 974 5859

25 October 2005

Cassini reveals more about Saturn's F ring

Images of Saturn's narrow and contorted F ring returned by cameras
onboard NASA's Cassini spacecraft have revealed phenomena not previously
detected in any planetary ring. The findings are reported in Nature
(27th October 2005).

The F ring is notorious for exhibiting unusual structures, like "knots,"
"kinks," and "clumps" that continue to puzzle astronomers. However,
Cassini images have shown that the gravitational effect of the inner
shepherding satellite, Prometheus, appears to produce regular patterns
on the ring including a series of channels or gores and 'streamers' of
particles that temporarily link the ring to the satellite. As an example
of a satellite that enters a ring on a regular basis, the phenomena
posed unique challenges to the understanding of ring-satellite interactions.

The findings of the Imaging Science Subsystem team show that channels
and streamers can be understood in terms of a simple gravitational
interaction as Prometheus approaches and recedes from the F ring every
14.7 hours. Using Cassini data the team developed a model that shows the
mechanism by which Prometheus, as it recedes from its closest approach
to the F ring, gravitationally extracts material from the ring. The
affected particles do not escape the F ring -- the changes to their
orbits produced by Prometheus cause them to oscillate back and forth
across the ring. One orbital period after the encounter the effect is
visible as a channel -- in excellent agreement with the Cassini images.
In this way Prometheus leaves its mark on the F ring long after it has
moved on.

Professor Carl Murray from Queen Mary, University of London is the lead
author of the paper and member of the Cassini Imaging Science Subsystem
team said, "As the closer and more massive of the F ring's two
shepherding satellites Prometheus was always the likely culprit for
causing changes to this narrow ring. Our model provides a plausible
mechanism for the origin of intricate structures detected in the F ring
and suggests that streamers, channels and a variety of other phenomena
can all be understood in terms of the simple gravitational effect of a
satellite on ring particles."

Over time Prometheus is expected to drive deeper into the F ring -- with
more extreme perturbations -- culminating in December 2009 when the two
orbits approach their minimum separation.

Professor Joseph Burns, an imaging team member from Cornell University,
Ithaca, New York and also one of the papers co-authors said, "We're
eager to learn what the satellite will do to this narrow, already
contorted ring, and in turn whether the ring particles will strike
Prometheus, changing its surface."

Professor Murray added, "We see the model we have developed very much as
a first step in understanding the processes at work. There are many
features of the F ring that we have yet to explain but at least we have
uncovered one of its secrets. Ultimately this type of research will help
us to understand how planets form and evolve."

The work described in the Nature paper is a collaboration between
Cassini imaging scientists at Queen Mary, University of London, Cornell
University and the Space Science Institute.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission
Directorate, Washington, D.C. The Cassini orbiter and its two onboard
cameras were designed, developed and assembled at JPL. The imaging team
is based at the Space Science Institute, Boulder, Colo.

About PPARC,
http://www.pparc.ac.uk/Ap/intro.asp

Notes to Editors:

Nature paper
How Prometheus creates structure in Saturn's F ring
Carl D Murray (1), Carlos Chavez (1), Kevin Beurle (1), Nick Cooper (1),
Michael W Evans (1), Joseph A Burns (2) and Carolyn C Porco (3).
(1) Astronomy Unit, Queen Mary, University of London.
(2) Department of Astronomy and Department of Theoretical and Applied
Mechanics Cornell University, Ithaca, New York.
(3) Cassini Imaging Central Laboratory for Operations, Space Science
Institute, Boulder. Colorado.

Images

Patterns in the F ring
[http://photojournal.jpl.nasa.gov/catalog/PIA07750]

A mosaic of 15 images of the F ring taken by Cassini's narrow angle
camera on 13 April 2005 showing the regular channels created by the moon
Prometheus. The individual images have been processed to make it appear
that the ring has been 'straightened'; this makes it easier to see the
ring's structure. The resulting mosaic shows a region 147000 km along
(horizontal direction) the ring and 1500 km across it (vertical
direction). The time between the first and last images in the mosaic is
approximately 2.5 hours.

Prometheus creates a channel once every 14.7 hours as it approaches the
F ring. Each channels is clearly visible across more than 1000km of the
ring and is due to the gravitational effect of Prometheus, even though
the moon does not enter the F ring. The channels have different slopes
because the ring particles closer to Prometheus (just visible at the
bottom right of the image) move slower with respect to the moon than
those further away. This causes the channels to shear with time. The
channels at the right are the youngest and have near-vertical slopes
while those at the left are the oldest and have near-horizontal slopes.
Credit: NASA/JPL/Space Science Institute

Lumpy Prometheus
[http://photojournal.jpl.nasa.gov/catalog/PIA07549]
Saturn's shepherd moon Prometheus reveals its elongated, irregular form
to Cassini in this image. The moon's long axis points toward Saturn.
Prometheus is 102 kilometres (63 miles) across. This view shows the
southern part of the moon's anti-Saturn side (the face that always
points away from Saturn). The image was taken in visible red light with
the Cassini spacecraft narrow-angle camera on June 7, 2005, at a
distance of approximately 438,000 kilometres (272,000 miles) from
Prometheus. Resolution in the original image was 3 kilometers (2 miles)
per pixel. The image has been contrast-enhanced and magnified by a
factor of two to aid visibility. Credit: NASA/JPL/Space Science Institute

For images and information about the Cassini-Huygens mission visit the
NASA, JPL and Cassini imaging team websites.

A movie clip showing Prometheus and the structures it creates is also
available at

http://saturn.jpl.nasa.gov/multimedi...cfm?videoID=95

Background

Prometheus: 102 km (63 miles) across.
Pandora (the second shepherd moon): 84 km (52 miles) across.

Saturn's Rings
The rings are (in order out from the planet) D, C, B, Cassini Gap, A, F,
G and E. The A ring has its own gap called the Encke Gap.