Saturn Has a 'Giant Sponge
- From: baalke@xxxxxxxxxxxxx
- Date: Thu, 7 Feb 2008 18:01:12 -0800 (PST)
http://www.jpl.nasa.gov/news/features.cfm?feature=1595
Saturn Has a 'Giant Sponge'
Jet Propulsion Laboratory
February 05, 2008
One of Saturn's rings does housecleaning, soaking up material gushing
from the fountains on Saturn's tiny ice moon Enceladus, according to
new
observations from the Cassini spacecraft.
"Saturn's A-ring and Enceladus are separated by 100,000 kilometers
(62,000 miles), yet there's a physical connection between the two,"
says
William Farrell of NASA's Goddard Space Flight Center in Greenbelt,
Md.
"Prior to Cassini, it was believed that the two bodies were separate
and
distinct entities, but Cassini's unique observations indicate that
Enceladus is actually delivering a portion of its mass directly to the
outer edge of the A-ring." Farrell is lead author of a paper on this
discovery that appeared in Geophysical Research Letters January 23.
This is the latest surprising phenomenon associated with the ice
geysers
of Enceladus to be discovered or confirmed by Cassini scientists.
Earlier, the geysers were found to be responsible for the content of
the
E-ring. Next, the whole magnetic environment of Saturn was found to be
weighed down by the material spewing from Enceladus, which becomes
plasma -- a gas of electrically charged particles. Now, Cassini
scientists confirm that the plasma, which creates a donut-shaped cloud
around Saturn, is being snatched by Saturn's A-ring, which acts like a
giant sponge where the plasma is absorbed.
Shot from Enceladus' interior, the gas particles become electrically
charged (ionized) by sunlight and collisions with other atoms and
electrons. Once electrically charged, the particles feel magnetic
force
and are swept into the space around Saturn dominated by the planet's
powerful magnetic field. There, they are trapped by Saturn's magnetic
field lines, bouncing back and forth from pole to pole. The fun ends,
however, if their bouncing path carries them inward toward Saturn to
the
A-ring. There they stick, in essence becoming part of the ring. "Once
they get to the outer A-ring, they are stuck," says Farrell.
"This is an example of how Saturn's rings mitigate the overall
radiation
environment around the planet, sponging up low- and high-energy
particles," says Farrell. By contrast, Jupiter has no dense rings to
soak up high-energy particles, so that planet's extremely high
radiation
environment persists.
The Cassini observations confirm a prediction by John Richardson and
Slobodan Jurac of the Massachusetts Institute of Technology. In the
early 1990âs, Hubble Space Telescope observations revealed the
presence
of a large body of water-related molecules in orbit about 240,000
kilometers (almost 150,000 miles) from Saturn. Richardson and Jurac
modeled this water cloud and demonstrated it could migrate inward to
the
A-ring. "We relied on their predictions to help us interpret our
data,"
said Farrell. "They predicted it, and we were seeing it."
At the time of their prediction, the source of the water cloud was
unknown. The source was not identified until 2005 when Cassini
discovered the stunning geysers emitted from Enceladus.
Data for the discovery that Saturn's A-ring acts like a sponge were
collected in July 2004 when Cassini arrived in orbit around Saturn,
making its closest flyby over the A-ring. "We skimmed over the top of
that ring fairly close," said Farrell.
Hot spots on the inside wall of the plasma donut -- the part colliding
with the A-ring -- were emitting radio signals. These signals behaved
as
a sort of natural radio beacon, indicating the local plasma density at
the inner edge of the donut. The signals were detected by Cassini's
Radio and Plasma Wave instrument. The team used these signals to
monitor
the density of the plasma (the higher the frequency, the greater the
density) and hence witness the change in gas density with time.
"As we approached the A-ring, the frequency dropped, implying that the
plasma density was going down because it was being absorbed by the
ring," said Farrell. "What really drove this home was what happened to
the signal when we passed over a gap in the rings, called the Cassini
division. There, the frequency went higher, implying that the plasma
density was going up because plasma was leaking through the gap."
The research was funded by NASA through the Cassini-Huygens project.
Cassini-Huygens is an international collaboration among NASA, the
European Space Agency, and the Italian Space Agency. The Cassini
orbiter
was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena,
Calif.
For more information about Cassini, visit:
http://saturn.jpl.nasa.gov/home/index.cfm
------------------------------------------------------------------------
Written by: Bill Steigerwald, Goddard Space Flight Center
.
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