Saturn Has a 'Giant Sponge'

Feature February 5, 2008


Saturn Has a 'Giant Sponge'

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 1990's, 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

Media contacts: Carolina Martinez (JPL) 818-354-9382 and Bill Steigerwald (Goddard)
301-286-5017

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