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Cassini finds Saturn moons are active

San Antonio — June 13, 2007 — Saturn's moons Tethys and Dione are flinging great streams of particles into space, according to data from the NASA, European Space Agency and Italian Space Agency Cassini mission to Saturn. The discovery suggests the possibility of some sort of geological activity, perhaps even volcanic, on these icy worlds.

These results appear in this week's issue of the journal Nature.

The particles measured by Cassini were traced back to the two moons because of the dramatic movement outwards from their orbit of electrically charged gas in the magnetic environment of Saturn. Known as plasma, the gas is composed of negatively charged electrons and positively charged ions, which are atoms with one or more electrons missing. Because they're charged, the electrons and ions can become trapped inside a magnetic field.

Saturn rotates in just 10 hours and 46 minutes. This sweeps the magnetic field and the trapped plasma through space. Just like a child on a fast-spinning merry-go-round, the trapped gas feels a force trying to throw it outwards, away from the center of rotation.

Soon after the Cassini spacecraft reached Saturn in June 2004, its instruments revealed that the planet's hurried rotation squashes the plasma into a disc, and that great fingers of gas are being thrown out into space from the discís outer edges. Hotter, more tenuous plasma then rushes in to fill the gaps.

Now, Dr. Jim Burch, vice president of the Space Science and Engineering Division at Southwest Research Institute® (SwRI®), and his colleagues on the Cassini Plasma Spectrometer team have made a careful study of these events using the instrument. They have shown that the direction of the ejected electrons points back towards Tethys and Dione. "It establishes Tethys and Dione as important sources of plasma in Saturn's magnetosphere," said Burch.

Until this discovery, only Saturn's moons Titan and Enceladus were known to be active worlds. "This new result seems to be a strong indication that there is activity on Tethys and Dione as well," said Andrew Coates from the Mullard Space Science Laboratory, University College London, co-author and member of the Cassini Plasma Spectrometer team.

Activity is a draw for planetary scientists, as it means that the planet has yet to become geologically dead or is perhaps being supplied with energy. The activity on Enceladus was detected first by Cassini's Dual Technique Magnetometer. This led the flight team to schedule a particularly close pass of Enceladus, which revealed a wealth of data about Enceladus' alien geysers — and spectacular pictures, too.

"The best results arise when we combine a variety of data sets to understand the observations," said Michele Dougherty, Imperial College in London, and Principal Investigator of the magnetometer.

More flybys of Dione and Tethys are scheduled in the future, which will allow the magnetometer team and the other instrument teams a close-up look at the moons. Before that happens, the teams have to go back and search for further signs of activity in the data already collected during the Tethys and Dione flybys of 2005.

In addition, Burch says that, having detected the electrons, they will try to determine the composition of the Tethys and Dione plasma using ion data.

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 Cassini Plasma Spectrometer team is based at Southwest Research Institute. The magnetometer team is based at Imperial College in London, working with team members from the United States and several European countries.

Editors: An image to accompany this story is available at http://www.swri.org/press/2007/cassini.htm. More information on the Cassini mission is available at http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov, and http://saturn.esa.int .

For more information contact Maria Martinez at (210) 522-3305, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510 or Carolina Martinez, Jet Propulsion Laboratory, (818) 354-9382.

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