Electrically charged gas in the magnetosphere of Saturn suggests volcanic activity on two of its moons. The direction of the ejected electrons points back towards Tethys and Dione. Previously it had been thought only only Enceladus was an active world.

CLICK ABOVE FOR FULL SIZE. Plumes of icy material extend above the southern polar region of Saturn's moon Enceladus, as imaged by the Cassini spacecraft in January 2005. The monochrome view is presented along with a colour-coded image on the right. The view in this image is perpendicular to the tiger stripe fractures that straddle the south pole. Another plume view, was taken one month later and looks along the tiger stripe fractures. Images like these are being analyzed by scientists as they seek to explain the processes that could be producing such incredible features. Imaging scientists believe that the plumes are geysers erupting from pressurized subsurface reservoirs of liquid water above 0°C. The mosaic is an orthographic projection centred at 46.8 degrees south latitude, 188 degrees west longitude, and has an image scale of 67 metres per pixel. The original images ranged in resolution from 67 metres per pixel to 350 metres per pixel and were taken at distances ranging from 11 100 to 61 300 kilometres from Enceladus. Credits: NASA/JPL/Space Science Institute

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.

The plasma tori of the two satellites are approximated in blue for Tethys (indicating lower energy plasma) and green for Dione (indicating higher energy plasma). The orbit of Cassini is shown in black with red lines showing the positions of observed inward plasma injections. Butterfly electron pitch-angle distributions consistent with outward plasma flow are observed throughout the regions in between the inward injections. Credits: SWRI (W. Lewis)

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.

Magnificent blue and gold Saturn floats obliquely as one of its gravity-bound companions, Dione, hangs in the distance. The darkened rings seem to nearly touch their shadowy reverse images on the planet below. This view looks toward the unlit side of the rings from about 9 degrees above the ring plane. The rings glow feebly in the scattered light that filters through them. Dione is 1,126 kilometers (700 miles) across. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Feb. 4, 2007, at a distance of approximately 1.2 million kilometers (800,000 miles) from Saturn. Image scale is 75 kilometers (47 miles) per pixel. Credits: NASA/JPL/Space Science Institute

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.

Source: ESA