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.
"No matter
where we looked, the source distance always mapped to the moons' orbits," Burch
told SPACE.com.
"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 11 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 partiwcles don't last long enough for a plasma donut, or "torus," 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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