Saturn, the sixth planet from the Sun, is a gas giant and the second largest in the solar system. As of 2007, a total of 57 individual moons have been identified around the planet, plus 3 unconfirmed moons that could be small dust clumps in the rings. 35 moons have been named. Many of the moons are very small: out of 57, 31 are less than 10 km in diameter, and another 13 less than 50 km.
Tethys and Dione are two small moons, and recent observations
from the Cassini mission show that they are both more active than previously thought, flinging great streams of particles into space, possible due to geological activity, perhaps even volcanic, on the two icy worlds.
Tethys is an icy body similar in nature to Dione, and its density indicates that it is composed almost entirely of water-ice, with numerous cracks caused by faults in the ice, at the surface. Dione is composed primarily of water ice, but the increase density suggests it must have a considerable fraction (~ 46%) of denser material like silicate rock in its interior.
The movement of electrically charged gas in the space where Saturn's magnetic field is most active indicates that some of the plasma particles originate from the two moons and gets trapped inside the magnetic field.
Due to the rapid rotation movement of Saturn, of just 10 hours and 46 minutes, the magnetic field and the trapped plasma are rapidly swiped through space, which tends to throw the trapped gas outwards, away from the center of rotation.
Jim Burch of the Southwest Research Institute, USA, and his colleagues, have made a careful study of these events using the Cassini Plasma Spectrometer (CAPS), and showed that the direction of the ejected particles makes Tethys and Dione the most likely origins of the gas.
"It establishes Tethys and Dione as important sources of plasma in Saturn's magnetosphere," says Burch, so another one of Saturn's moons, Enceladus, is not the only active world in Saturn's system.
This discovery could lead to determinations of the composing of the Tethys and Dione plasmas, helping the scientists to better understand the processes taking place in and on the two moons.
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