Image:Saturn's icy moon Dione
NASA/JPL/Space Science Institute
Tectonic faults and craters on Saturn's icy moon Dione are revealed in this color-enhanced image that mixes infrared, ultraviolet and green light observations. The color variations might be caused by subtle differences in the surface composition or the sizes of grains making up the icy soil, but scientists aren't sure.
updated 6/15/2007 1:46:58 PM ET 2007-06-15T17:46:58

Streams of hot gas swirling around Saturn have been traced to two icy moons previously thought to be geologically dead worlds.

The finding, detailed in the June 14 issue of the journal Nature, suggests Saturn's satellites Tethys and Dione might be volcanically active after all.

Known as plasma, the gas is composed of negatively charged electrons and positively charged ions, which are atoms with one or more electrons missing. After being ejected from the moons, the charged particles become trapped inside the magnetic field surrounding Saturn, called the magnetosphere.

A great escape
The particles remain trapped only temporarily, however, because Saturn spins so fast about its axis-a day there is only 10 hours and 46 minutes long-that it drags its magnetosphere and the trapped plasma inside it rapidly through space.

In 2004, NASA's Cassini spacecraft revealed Saturn's rapid rotation flattens the plasma into a disc, and that giant fingers of gas are being thrown out into space from the disc's outer edge. Scientists think cold, fast-spinning gas particles get hurled outwards, away from the center of rotation, by the same centrifugal force that pushes your body against a car door when the vehicle makes a sharp turn. Hotter, more tenuous plasma then rushes in to fill the gaps. The ejected plasma particles get swept away by the sun's own streaming particles, called the solar wind.

Studying the electron component of the plasma, Jim Burch, an astronomer at the Southwest Research Institute in Texas and a member of the Cassini Plasma Spectrometer team, and his colleagues traced the particles back to the orbits of Tethys and Dione.

Slideshow: Cosmic Sightings: From Earth to stars "No matter where we looked, the source distance always mapped to the moons' orbits," Burch told

"The implication is that there is a source of plasma on the two moons and that it created a 'donut' [of plasma] that goes around the planet," he added.

Active worlds?
Prior to the new discovery, the only Saturn moons known to be active were Titan and Enceladus.

"This new result seems to be a strong indication that there is activity on Tethys and Dione as well," said study team member Andrew Coates of the University of College London.

Some scientists had suspected Diones might be geologically active because NASA's Pioneer 10 probe detected plasma in the Saturn system in 1979. But those findings were cast into doubt when subsequent observations in the 1980s by the Voyager spacecraft didn't find any evidence of plasma in the moon's orbit.

"It was a controversy," Burch said.

The new finding suggests the plasma rings might be transient, Burch said, and that the charged particles don't last long enough for a plasma donut, or "taurus," to completely encircle the planet.

A spacecraft might "go through the part where it's still there," Burch said, "or it may go through the part where the moon's almost come all the way around again and most of [the particles] are gone."

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