Charged Dust From Inside Saturn's Moon Enceladus

Dr Geraint Jones and Dr Chris Arridge, both from University College London’s Mullard Space Science Laboratory, will present the results on behalf of the Cassini Plasma Spectrometer (CAPS) instrument team on Wednesday 22nd April at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.

Cassini has been exploring Saturn and its moons since 2004. Five hundred kilometre (300 mile)-wide Enceladus, discovered from Slough in 1789 by William Herschel, has been found by Cassini’s suite of instruments to possess active jets near its southern pole that spew gas and water out into space over thousands of kilometres. During two particularly close flybys of the moon in 2008, skimming only 52 and 25 km from the surface at around 15 km per second (54000 km per hour), the CAPS instrument on the spacecraft was pointed to scoop up gas as it zoomed through the plume.

The CAPS instrument is designed to detect charged gas (plasma), but its measurements in the plume revealed a surprise: the instrument also detected tiny ice grains whose signatures could only be present if they were electrically charged. These grains, probably only measuring a few nanometres across (billionths of a metre – 50 000 times thinner than a human hair), fall into a size range between gas atoms and much larger ice grains, both of which were sampled directly during previous Enceladus flybys. The particles have both positive and negative electrical charges, and the mix of the charges varied as the Cassini spacecraft crossed the plume.

Jones and Arridge suggest that the grains may be charged through so-called triboelectric processes, through bumping together in the vent below Enceladus's surface before they emerge into the plume. This provides important hints to the conditions in the vents, and in turn may help with understanding conditions in the interior.

Drs Jones and Arridge are intrigued by what their discovery reveals about Enceladus: “What are particularly fascinating are the bursts of dust that CAPS detects when Cassini passes through the individual jets in the plume” says Jones. “Each jet is split according to charge though”, adds Arridge, “Negative grains are on one side, and positive ones on the other”.

As Arridge will explain in his presentation, as these charged grains travel away from Enceladus, their paths are bent by electric and magnetic fields in Saturn's giant magnetosphere. In this way Saturn's magnetosphere acts as an enormous mass spectrometer for the plume particles, allowing scientists to constrain their masses. Arridge has begun modelling the paths of these newly-discovered particles.

Ionised gas (plasma) in Saturn’s magnetosphere flows past Enceladus at over 80000 km per hour. Arridge’s results show that for this enormous mass spectrometer to work and for these dust particles to reach Cassini, this river of plasma must be significantly slowed down, in and near the plume, to speeds of less than 3200 km per hour. This slowing of the plasma is a result of the plume injecting particles into the plasma stream - making the whole flow slow down in a similar effect to when cars join a busy motorway. These new results provide further evidence that the material in the Enceladus plume has a huge influence on the moon’s surroundings.

Future Cassini flybys will help further understand the processes that occur at Enceladus and in its vicinity. William Herschel could not have suspected that the tiny point of light that he found in 1789 would turn out to be such an exotic place.