http://www.colorado.edu/news/releases/2004/351.html

University of Colorado at Boulder News Release

Cassini Spacecraft Images Objects, Density Waves In Saturn's Rings
Nov. 9, 2004

Note to Editors: Colwell will participate in a news briefing Nov. 9 at
the DPS meeting in Louisville, Ky. Contents embargoed until 12:10 p.m.
EST Nov. 9.

A University of Colorado at Boulder-built instrument riding on the
Cassini-Huygens spacecraft is being used to distinguish objects in
Saturn's rings smaller than a football field, making them twice as sharp
as any previous ring observations.

Joshua Colwell of CU-Boulder's Laboratory for Atmospheric and Space
Physics said the observations were made with the Ultraviolet Imaging
Spectrograph, or UVIS, when Cassini was about 4.2 million miles, or 6.75
million kilometers, from Saturn in July. Saturn orbits the Sun roughly 1
billion miles from Earth.

Colwell and his colleagues used a technique known as stellar occultation
to image the ring particles, pointing the instrument through the rings
toward a star, Xi Ceti. The fluctuations of starlight passing through
the rings provide information on the structure and dynamics of the
particles within them, said Colwell, a UVIS science team member.

He likened the Saturn system to a mammoth phonograph record, with the
planet in the middle and the rings stretching outward more than 40,000
miles, or 64,000 kilometers. The size of the ring particles varies from
dust specks to mountains, with most ranging between marbles and
boulders, he said.

The Cassini observations show dramatic variations in the number of ring
particles over very short distances, Colwell said. The particles in
individual ringlets are bunched closely together, with the amount of
material dropping abruptly at the ringlet edge.

"What we see with the new observations is that some of the ring edges
are very sharp," said Colwell. The sharp edges of small ringlets are
especially evident in the C ring and in the so-called Cassini Division
on either side of the bright B ring, Saturn's largest ring.

The Cassini observations with UVIS show that the distance between the
presence and absence of orbiting material at some ring edges can be as
little as 160 feet, or 50 meters, about the length of a typical
commercial jetliner, he said.

The sharp edges illustrate the dynamics that constrain the ring
processes against their natural tendency to spread into nearby, empty
space, said Colwell. "Nature abhors a vacuum, so it is likely gravity
from a nearby small moon and ongoing meteoroid collisions confine the
particles in the ring."

Colwell presented his findings at the 36th annual Division of Planetary
Sciences Meeting held in Louisville, Ky., Nov. 8 to Nov 12.

The stellar occultation process using UVIS also shows very
high-resolution views of several density waves visible in the rings,
including a previously unstudied one, he said. Density waves are
ripple-like features in the rings caused by the influence of Saturn's
moons -- in this case, the small moon, Janus.

"Small moons near Saturn's rings stir the ring particles with their
gravitational pull," Colwell said. At certain locations in the rings,
known as resonances, the orbit of a particular moon matches up with the
orbit of certain ring particles in a way that enhances the stirring
process, he said.

The density waves, which resemble a tightly wound spiral much like the
groove in a phonograph record, slowly propagate away from the resonance
toward the perturbing moon, he said. "This can create a wave in the ring
that looks like a ripple in a pond," said Colwell.

"The shapes of these wave peaks and troughs help scientists understand
whether the ring particles are hard and bouncy, like a golf ball, or
soft and less bouncy, like a snowball," Colwell said. He noted that a
density wave analysis by scientists involved in NASA's Voyager 2 mission
that visited Saturn in 1981 were used to determine the mass and
thickness of the planet's rings.

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 in Washington, D.C.

CU-Boulder Professor Larry Esposito of LASP is the principal
investigator for the $12.5 million UVIS instrument, designed and built
for JPL at CU-Boulder.

About The Image

The false color representation of two density waves in Saturn's A ring
was made from the stellar occultation observed by Cassini's UVIS
instrument when the spacecraft was 4.2 million miles from Saturn. Bright
areas indicate the denser regions of the rings. The bright bands in the
left part of the image are the "peaks" of a density wave caused by
gravitational stirring of the rings by the moon, Janus. A smaller
density wave in the right half of the image is produced by the moon
Pandora. UVIS observed the brightness of the star Xi Ceti as the rings
passed in front of it, and the flickering of the starlight was converted
into the ring density depicted by the image. The image represents a
distance of about 450 miles, and the smallest features are about
one-half mile across. Image courtesy NASA/JPL/University of Colorado at
Boulder.