Tethys and Dione are active moons of Saturn, reveals study

Data from NASA/ESA/ASI Cassini mission to Saturn has revealed that two of the giant, ringed-planets moons, Tethys and Dione, are active.

Paris, June 14 : Data from NASA/ESA/ASI Cassini mission to Saturn has revealed that two of the giant, ringed-planet's moons, Tethys and Dione, are active.

Until this result, among Saturn's inner moons, only Enceladus was known to be an active world, with huge geysers spraying gases hundreds of kilometres above the moon's surface.

The spacecraft traced streams of plasma in Saturn's magnetic field to Tethys and Dione.

The plasma is basically a gas composed of negatively charged electrons and positively charged ions, which are atoms with one or more electrons missing. Since they are charged, the electrons and ions can get trapped inside a magnetic field.

As Saturn rotates around itself in just 10 hours and 46 minutes, it sweeps the magnetic field and the trapped plasma through space; the trapped gas feeling a force trying to throw it outwards, away from the centre of rotation.

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

Now, Jim Burch of the Southwest Research Institute, USA, together with his colleagues has made a careful study of these events using the Cassini Plasma Spectrometer (CAPS).

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.

"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, and a collaborator on this latest work, 'Tethys and Dione as sources of outward-flowing plasma in Saturn's magnetosphere'.

Having detected the electrons, the team is now on the lookout for the ions, so that the composition of the Tethys and Dione plasmas can be determined.

The findings appear in the June 14 issue of the scientific journal Nature.

ANI