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.