Two more active moons around Saturn (Forwarded)

ESA News
http://www.esa.int

13 June 2007

Two more active moons around Saturn

Saturn's moons Tethys and Dione are flinging great streams of particles
into space, according to data from the NASA/ESA/ASI Cassini mission to
Saturn. The discovery suggests the possibility of some sort of geological
activity, perhaps even volcanic, on these icy worlds.

The particles were traced to the two moons because of the dramatic
movement of electrically charged gas in the magnetic environs 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 are charged, the electrons and ions can get trapped
inside a magnetic field.

Saturn rotates around itself 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 centre of rotation.

Soon after Cassini reached Saturn, in June 2004, it revealed that the
planet's hurried rotation squashes the plasma into a disc and that great
fingers of gas are indeed being 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, and colleagues
have 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," says
Burch.

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. "This new result seems to be a strong
indication that there is activity on Tethys and Dione as well," says
Andrew Coates from the Mullard Space Science Laboratory, University
College London, and a collaborator on this latest work.

Activity is a draw for planetary scientists as it means that the planet
has yet to reach equilibrium, or is perhaps being supplied with energy.
The activity on Enceladus was detected first by Cassini's Dual Technique
Magnetometer (MAG). 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," says Michele Dougherty, Imperial College,
London, and Principal Investigator of MAG.

In the case of Dione and Tethys, more fly-bys are scheduled in the future,
which will allow the team and the other instruments a close-up look at the
moons. Before that happens, the team has 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 now
be on the lookout for the ions, so that the composition of the Tethys and
Dione plasmas can be determined.

Notes for editors

The findings will appear in the 14 June 2007 issue of the scientific
journal Nature. The article, 'Tethys and Dione as sources of
outward-flowing plasma in Saturn's magnetosphere', is by J. Burch, J.
Goldstein, W. Lewis, D. Young, A. Coates, M Dougherty and N. André.

The Cassini-Huygens mission is a cooperative project of NASA, ESA and the
Italian Space Agency (ASI). The CAPS team, consisting of scientists from
Europe and the US, is based at the Southwest Research Institute in San
Antonio. The MAG team is based at Imperial College in London, and also
consists of team members from the United States and several European
countries.

For more information

Jim Burch
Cassini CAPS Co-Investigator, Southwest Research Inst., Texas, USA
Email: Jburch @ swri.org

Andrew J. Coates
Cassini CAPS Co-Investigator, Mullard Space Science Lab, UK
Email: Ajc @ mssl.ucl.ac.uk

Michel Dougherty, co-author
Cassini MAG Principal Investigator, Imperial College, Blackett Laboratory,
UK
Email: M.Dougherty @ ic.ac.uk

Jean-Pierre Lebreton
ESA Huygens Project Scientist
Email : Jean-Pierre.Lebreton @ esa.int

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