The irregularity of this cyrovolcanism was confirmed by a second typeof measurement, obtained during a close encounter between Cassini andEnceladus. On August 11, 2008, Cassini approached Enceladus to within55km. ‘This was something quite exceptional,’ adds Professor Gérard,‘as it mustn’t be forgotten that the radius of Enceladus is only 250kmand that this encounter was programmed from a distance of a billionkilometres.’ On the approach to this encounter Cassini passed through abundle of particles ejected by the Enceladus geysers. Variations in theintensity of the particle flow were measured up to a factor of 10, andan abrupt extinction of the signal was even recorded, demonstrating thesporadic nature of the cyrovolcanism on Enceladus, which is in turn atthe root of the irregularity of its auroral footprints on Saturn:auroral activity can only occur when cyrovolcanism is or has just beenactive.

‘Spectroscopic studies of the aurora on Saturn enable the detection of hydrocarbon signatures (primarily methane) in its atmosphere, as on Jupiter,’ explains Jacques Gustin. ‘These molecules absorb Saturn’s auroral emissions depending on their wavelengths. If the charged particles which pour into Saturn’s atmosphere encounter these hydrocarbons the emission relative to their de-excitation will be absorbed in this spectral region. On the other hand if the emission takes place above the hydrocarbon layer the aurora will not produce this methane absorption signature. The more energetic are the particles which bring about the aurora the lower they will descend into Saturn’s atmosphere, and thus the greater will be the auroral light they produce. On the basis of a model of Saturn’s atmosphere it is thus possible to determine the maximum altitude at which the observed aurora occur. Thus the Enceladus auroral footprints observed by Cassini take place above an altitude of 1100km.’