Cassini-Huygens photographs Saturn’s auroras
Spectacular images sent to Earth by the ASI-ESA-NASA probe
25 Nov 2009
The Cassini-Huygens probe, which set off to study for Saturn 12 years ago, has been sending us generous and surprising gifts for some time and the latest images sent to Earth by the joint ASI-ESA-NASA mission are no less so. This unique and remarkable document is the first film in which one of Saturn’s auroras is clearly visible (at the moment this is also our solar system’s "tallest" aurora). Cassini took four days to "capture" the movement in the northern region of the planet from the night zone to the day zone, thus giving us this spectacular and flickering succession of moving lights and shadow that the NASA web site called a "ghostly dance". The photo on the left is only one of the over 500 taken by Cassini over 81 hours, then reprocessed to create the film. It shows a previously unseen vertical profile of the auroras, which gather together like long curtains in the video. These "arcs of light" can extend for over 1200 kilometres on the edge of Saturn’s northern hemisphere.
The auroras are characteristic phenomena on Earth, on Jupiter, on Saturn and on a few other planets. They occur especially at high latitudes, close to the magnetic poles, when charged particles (solar wind) coming from the magnetosphere pass through the atmosphere. The new images from Cassini will help scientists to better understand the mechanisms that cause them. Cassini "has offered us a dazzling spectacle of auroras, in a form that we suspected existed but had never recorded on Saturn," explains Andrew Ingersoll, of the California Institute of Technology, a member of the team that processed the Cassini images and contributed to creating the video. "To be able to observe these phenomena on other planets," continued Ingersoll, "helps us a lot in understanding them a bit better when we study them on Earth." For example, you can immediately notice the different length of the arcs produced by the auroras on Saturn; over twice as long as those on Earth (which do not exceed 500 km). This is due to the different composition of the atmospheres: on Earth there is a large amount of oxygen and nitrogen, whereas Saturn’s atmosphere is predominantly hydrogen. The picture on the right was taken on 21 November, during Cassini’s flyby of Saturn’s moon Enceladus. It shows the area of the South Pole. Several jets of vapour and other particles are clearly visible.
The sensational image on the left has quite rightly toured the world. A bit like Cassini’s "visiting card", the probe "took" it in December 2004 having entered Saturn’s orbit less than six months earlier on 1 July 2004.