A natural satellite does not gravitate around its planet completely disinterestedly: the couple in reality permanently interacts gravitationally but also electromagnetically. In certain cases this coupling is rendered visible in the form of a magnificent luminous trail, called an aurora, which is unfurled within the planet’s atmosphere, whilst its origin is to be found in its satellite. The journal Nature is publishing the first images of Enceladus’ auroral footprint on Saturn, obtained by the Cassini space probe. Three researchers at the ULg’s Atmospherical and Planetary Physics Laboratory (LPAP) have been taking part in the adventure: Professor Jean-Claude Gérard and the researchers Denis Grodent and Jacques Gustin.

These are not the first images of a satellite’s auroral footprints on its planet: a large scale observation campaign by the Hubble Space Telescope in 2007 enabled Io, a satellite of Jupiter, to snatch this first place (read the article Surprise discovery related to the polar auroras of Jupiter). In 2009 it was Ganymede, another Jupiter satellite, which disclosed its magnetic image to the Jovian poles, once again under Hubble’s watchful eye. Today it is the turn of Enceladus, an ice satellite of Saturn. Its auroral footprint was detected by the UVIS spectrograph on board Cassini, in orbit around Saturn since the Summer of 2004. Even if there exist fundamental differences between the magnetospheres of Saturn and Jupiter, scientists have for some years been predicting the existence of an electromagnetic coupling between Saturn and Enceladus similar to that which ties Io to Jupiter.

Nevertheless it took six years and the Cassini probe working inSaturn’s environment before this prediction could be confirmed. Thisdelay was down to geometric factors, to which was added the sporadicnature of the auroral phenomena connected to Enceladus.

‘It needs to be said that the analyses we are publishing are related to observations carried out in August 2008,’ Denis Grodent begins by pointing out. ‘Furthermore the detection of an auroral footprint requires that the poles are visible to the instrument taking the measurements. However Cassini spends most of its time in Saturn’s ecliptic plane which allows it only a sidelong glance at the gas giant’s polar phenomena. But during the year 2008 a deviation of Cassini’s orbit had been programmed: the orbit thus became more and more inclined, as far as passing above Saturn’s poles and dropping down the equatorial plane. During this turn around Cassini was able to accurately observe Saturn’s poles for several months. It was also particularly close to the giant at that point (at only five radii of Saturn). This event conjunction allowed polar observations of large angular resolution.’