BY DR EMILY BALDWIN
Posted: 22 April, 2009
Tiny charged particles of ice streaming from Enceladus offer tantalising clues to the interior of this enigmatic Saturnian moon.
The Cassini spacecraft has been exploring the Saturnian system for the last five years, and during its flybys of 500-metre wide Enceladus, revealed active jets at the moon's southern pole that spew gas and water vapour out into space from so-called tiger-stripe fractures. During two daringly close flybys of the moon, at altitudes of just 25 and 52 kilometres, the Cassini Plasma Spectrometer scooped up some of the plume gases for analysis.
While the instrument was designed to detect charged gas, or plasma, it also turned up a surprise, tasting tiny grains of ice, too. The nanometre sized dust grains can only be present if they are electrically charged, and analysis reveals that the particles have both positive and negative electrical charges.
"We found we were getting unexpected signatures and large variations in the flux measurements," said Dr Geraint Jones of University College London's Mullard Space Science Laboratory (MSSL) in his talk at the European Week of Astronomy and Space Science Conference this morning. "We saw these localised enhancements of high energy features caused by the presence of heavy negative ions and we saw that positively charged grains curve away from Saturn, while negatively charged grains curve towards the planet."
The grains are bent by the electric and magnetic fields in Saturn's imposing magnetosphere, and are split into positive and negative charge across individual jets, with negative grains on one side and positive ones on the other. Jones and colleague Dr Chris Arridge, also from MSSL, suggest that the grains may be charged through collisions in the vent below Enceladus's surface before they emerge into the plume.
Arridge used the observations to develop computer simulations in order to learn more about the processes in operation at this enigmatic moon. Plasma in Saturn's magnetosphere flows past Enceladus at over 80,000 kilometres per hour, but in order for the dust particles to even be tasted by Cassini, this river of plasma must be significantly slowed down, both in and near the plume, to speeds of less than 3,200 kilometres per hour. It is thought that the reduction of speed occurs because the particles are injected into the plasma stream, just as cars joining a motorway can significantly slow the flow of traffic.
These new results provide further evidence that the material in the Enceladus plume has a huge influence on the moon's surroundings. "Eventually these grains will drift out of the zone where the flow is stagnated and carry on with everything else in the Saturnian system, perhaps being swept up in ring systems," concluded Arridge.