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Prometheus and its pull on the passing particles of Saturn's F ring(Forwarded)



 
 
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Old April 4th 06, 03:15 PM posted to sci.space.news
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Default Prometheus and its pull on the passing particles of Saturn's F ring(Forwarded)

ROYAL ASTRONOMICAL SOCIETY PRESS INFORMATION NOTE

Issued by RAS Communications Officers:
Peter Bond
Tel: +44 (0)1483-268672 Fax: +44 (0)1483-274047
AND
Anita Heward
Tel: +44 (0)1483-420904

National Astronomy Meeting Press Room (4 - 7 April only):
Tel: +44 (0)116-229-7474 or 229-7475 or 252-3312 or 252-3531
Fax: +44 (0)116-252-3531

RAS Web site: http://www.ras.org.uk/

RAS National Astronomy Meeting web site:
http://www.nam2006.le.ac.uk/index.shtml

CONTACT DETAILS ARE LISTED AT THE END OF THIS RELEASE.

EMBARGOED FOR 00:01 BST, TUESDAY, 4 APRIL 2006

Ref.: PN 06/11 (NAM 4)

PROMETHEUS AND ITS PULL ON THE PASSING PARTICLES OF SATURN'S F RING

The rings around all the giant planets in our Solar System are
thought to be stabilised by small 'shepherd moons' that orbit in
or near the rings and stabilize them by gravitational influences.

The narrow F ring of Saturn -- which lies just outside the
spectacular main rings -- is tended by two small shepherds.
Prometheus (100 km in diameter) orbits just inside the F ring,
while Pandora (85 km in diameter) moves around Saturn just
outside the F ring.

Images from Saturn's F ring region obtained by the Cassini Imaging
Science Subsystem (ISS) cameras have revealed structure never seen
before in a planetary ring.

Periodic structures such as azimuthal gaps -- 'channels' of low
optical depth -- and 'streamers' have been discovered. These
features can be seen in movie. The origin of these features
has been explored by a team at Queen Mary, University of London
(QMUL) using numerical integrations.

On Tuesday 4 April, Carlos Chavez of QMUL will be explaining to
the RAS National Astronomy Meeting in Leicester the results of
their computer models, which explain the close and complex
relationship between Prometheus and the tangled F ring.

"The models are in excellent agreement with structures observed
in the Cassini images," said Chavez.

"We have found that the gaps are not due to a lack of particles,
but to a forced change in orbital elements by a close encounter
with Prometheus," he explained. "The moon's gravity temporarily
pulls some of the particles away from the main stream as it
passes by.

"It is like a crowd of people walking in a number of lines in the
same direction down a street. Suddenly, someone else comes from
the other side of the street and collides with a few of them. He
then tells them to come with him, and walks away. Only people in
the closest lines follow him, which produces gaps in the crowd.
However, they return back to the main group shortly afterwards."

The most dramatic case will happen in late 2009, when the F ring
and Prometheus are anti-aligned. Once per orbit during this anti-
alignment Prometheus will be at apoapsis (its furthest point from
Saturn) and the nearby ring particles will be at periapsis
(closest point to Saturn). At that time, Prometheus and the ring
particles are at their closest to each other.

The QMUL team explored how these events will affect collisions
between the ring particles and Prometheus. They found a low number
of collisions -- only 0.6% of the particles collided per orbit.
This was unexpected, since it was originally thought that
Prometheus is a 'thieving moon', stealing particles from the F
ring. What actually happens is that the particles are only
temporarily pulled away and then drift back into the ring.

The ring-moon interactions are also likely to have an effect on
the surface of Prometheus. Like our Moon and most other planetary
satellites, Prometheus has a synchronous rotation, always showing
the same face to Saturn.

The team at QMUL investigated the location on Prometheus' surface
where the particles would be expected to collide. They found that,
in the synchronous co-rotating reference frame, the collisions
surprisingly occurred on the trailing face of Prometheus, and
preferably in the equatorial region.

This scenario has important implications for the surface features
of Prometheus, and the team expects to find differences in albedo
(reflectivity) between the trailing and leading faces.

"It would be like a man colliding with other people while facing
continuously in a particular direction and hitting them with only
one side of his body," said Chavez.

Other members of the QMUL team examining the links between
Prometheus and the F ring a Prof. Carl D. Murray, Dr. Kevin
Beurle, Dr. Nicholas J. Cooper, and Dr. Michael W. Evans.

CONTACT:

Carlos Chavez
Astronomy Unit
Queen Mary, University of London
Mile End Road, London E1 4NF
Tel. +44 (0)207-890-8500 ext. 3287

From 4 to 7 April, Mr. Chavez can be contacted via the NAM press office
(see above).

NOTES FOR EDITORS

The 2006 RAS National Astronomy Meeting is hosted by the University
of Leicester. It is sponsored by the Royal Astronomical Society,
the UK Particle Physics and Astronomy Research Council (PPARC), the
University of Leicester and the National Space Centre, Leicester.

IMAGES:

Images of Prometheus, the F ring with its strands, and a streamer
of particles that seems to connect the inner part of the ring with
the satellite:
http://ciclops.org/view.php?id=547

A more detailed view of the azimuthal gaps in the F ring:
http://ciclops.org/view.php?id=247

MOVIES:

Movie showing the azimuthal gaps or channels:
http://ciclops.org/view.php?id=1361

The QMUL simulation of events close to the anti-alignment case
as seen in the guiding centre reference frame:
http://www.maths.qmul.ac.uk/~cec/NewBigMovie.mov

The QMUL simulation for the anti-alignment case:
http://www.maths.qmul.ac.uk/~cec/Ant...ment_Final.mov


 




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