The Cassini scientists’ news release confirms the suspicions of some (expressed here in this post and this post and this post, too). Namely, that Enceladus is not the only geologically active moon of Saturn. Now add Tethys and Dione, imaged here:
Nice little moons, I’d say. Tethys is third out from the planet among the large moons, and Dione is fourth. (Enceladus is number two.)
(These moons) are flinging great streams of particles into space, according to data from the Cassini mission to Saturn. The discovery suggests the possibility of some sort of geological activity, perhaps even volcanic, on these icy worlds.
Jim Burch of the US-based Southwest Research Institute and his colleagues have traced plasma trapped in Saturn’s magnetic field with the space probe’s plasma spectrometer. Here’s the news release explanation:
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.
(Burch and his colleagues) 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.
I wonder how these findings will affect plans for upcoming flybys of these moons. Cassini will pass 10,000 miles from Tethys in thirteen days–the only scheduled close encounter in the next year. The proposed mission extension–which will keep staff and scientists on duty for another two years–supposedly has just one more Dione encounter.