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Cassini
'Cat Scan' Maps Clumps in Saturn's Rings
May
22, 2007
This
false-color image of Saturn's main rings was made by
combining data from multiple star occultations using the
Cassini ultraviolet imaging spectrograph.
Image
credit: NASA/JPL/University of Colorado
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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:
CHG052207_02_01
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.
Source:
NASA / JPL
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