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Posted by baalke on May 22, 2007, 5:22 pm
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http://www.jpl.nasa.gov/news/news.cfm?release=2007-062
Cassini 'Cat Scan' Maps Clumps in Saturn's Rings
Jet Propulsion Laboratory
May 22, 2007
Saturn's largest and most densely packed ring is composed of tightly
packed clumps of particles separated by nearly empty gaps, according
to
new findings from NASA's Cassini spacecraft.
These clumps in Saturn's B ring are neatly organized and constantly
colliding, which surprised scientists.
"The rings are different from the picture we had in our minds. We
originally thought we would see a uniform cloud of particles. Instead
we
find that the particles are clumped together with empty spaces in
between," said Larry Esposito, principal investigator for the Cassini
ultraviolet imaging spectrograph at the University of Colorado,
Boulder.
"If you were flying under Saturn's rings in an airplane, you would see
these flashes of sunlight come through the gaps, followed by dark and
so
forth. This is different from flying under a uniform cloud of
particles."
Because previous interpretations assumed the ring particles were
distributed uniformly, scientists underestimated the total mass of
Saturn's rings. The mass may actually be two or more times previous
estimates.
"These results will help us understand the overall question of the age
and hence the origin of Saturn's rings," said Josh Colwell, assistant
professor of physics at the University of Central Florida, Orlando,
and
a team member of the Cassini ultraviolet imaging spectrograph. A paper
with these results appears in the journal Icarus.
Scientists observed the brightness of a star as the rings passed in
front of the star on multiple occasions. This provided a measurement
of
the amount of ring material between the spacecraft and the star.
"Combining many of these occultations at different viewing geometries
is
like doing a CAT scan of the rings," said Colwell. "By studying the
brightness of stars as the rings pass in front of them, we are able to
map the ring structure in 3-D and learn more about the shape, spacing
and orientation of clusters of particles."
The observations confirm that the gravitational attraction of ring
particles to each other creates clumps, or "self-gravity wakes." If
the
clumps were farther from Saturn, they might continue to grow into a
moon. But because these clumps are so close to Saturn, their different
speeds around the planet counteract this gravitational attraction so
that the clumps get stretched like taffy and pulled apart. The clumps
are constantly forming and coming apart once they reach about 30 to 50
meters (about 100 to 160 feet) across.
"At any given time, most particles are going to be in one of the
clumps,
but the particles keep moving from clump to clump as clumps are
destroyed and new ones are formed," added Colwell.
In the dense B ring, the classical cloud model of the rings predicted
that particles collide about twice per hour on average. "Our results
show that the particles in the B ring spend most of their time in
almost
continuous contact with other particles," said Colwell. These clumps
may
act like super-sized particles, changing the way the rings spread due
to
collisions.
The clumps are seen in all regions of the B ring that are not opaque.
One surprising aspect of the measurements is that the clumps in the B
ring are broad and very flat, like big sheets of particles. They are
roughly 10 to 50 times wider than they are thick. Scientists are also
surprised that the B ring clumps are flatter and have smaller spaces
between them than those found in the neighboring A ring.
A picture of the rings based on these results is available at:
http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov
and http://lasp.colorado.edu/cassini/whats_new/ .
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. The Cassini orbiter was designed, developed
and
assembled at JPL. The ultraviolet imaging spectrograph was built at,
and
the team is based at the University of Colorado, Boulder.
------------------------------------------------------------------------
Media contacts: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
Zenaida Gonzalez Kotala 407-823-6120
University of Central Florida, Orlando
2007-062
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