Nevertheless, amongst the 317 configurations which were favourable for Cassini, the Enceladus aurora has never appeared on images of the South pole, and has only shown itself seven times in the North pole. ‘But every time it appeared the aurora formed exactly in the place we had predicted,’ Professor Gérard hastens to add. ‘Our predictions have thus been confirmed, at the same time as the random nature of the phenomena has been brought to light. When they can be observed, the Enceladus auroral footprints on Saturn are on average 100 times less intense than those of Io on Jupiter.’

Whilst in the case of Io the aurora on Jupiter were primarily due to Io’s volcanic activity, in the case of Enceladus the phenomena’s origins are found in its cryovolcanism. What is involved here? The subterranean rocks of Enceladus are warmed by the effects of the enormous waves caused by the very close presence of the voluminous Saturn. This mixing and milling of the rocks causes a warming up of water pockets (0°) situated under the satellite’s surface. The liquid water then makes its way to the surface through the numerous faults which punctuate it. The water, in vapour and ice states, is spouted into the atmosphere. These geysers can eject up to 300 kg of water per second to an altitude of 1000 to 1500km. This distance represents several times the diameter of Enceladus, which is 500km.

These ejecta feed an enormous reservoir of particles which surround Enceladus. Subsequently excited by the solar wind they are then captured by Saturn’s magnetosphere and forced to follow the field’s lines of force, along which they acquire considerable energy. When they enter Saturn’s atmosphere via the poles these particles collide with the atmosphere’s molecules, which become de-excited in emitting auroral light. The auroral footprints of Enceladus on Saturn have turned out to be astonishingly large: their angular size by some way exceeds that of Enceladus. The aurora thus represents not only the satellite’s discharge but also and above all that of the cloud of particles which surrounds it.