Thunder & Lightning! Thunderstorm on Saturn Releases a Septillion Joules of Energy

The largest and most intense storm of the last 20 years was recently discovered on the planet Saturn. Initial measurement data on this thunderstorm is published today as the cover story of the journal nature. This is a further highlight for an Austrian Science Fund FWF project which has been running for two years. The project focuses on analysing data from NASA’s space probe Cassini – however, for the latest storm a global network of amateur astronomers was also called up for duty. It was thus possible to calculate that the storm has so far released a septillion joules of energy. Furthermore, the measurements provide support for a hypothesis which predicts that Saturn’s storms have a seasonal dependence.

Not a fine day on Saturn: a gigantic storm is developing in the northern hemisphere. © Use of this photo for editorial purposes is free of charge, subject to attribution: NASA/JPL/SSI

Other planets have freak weather, too. Saturn’s quirks are being investigated by the Space Research Institute of the Austrian Academy of Sciences. A current project of the Austrian Science Fund FWF is concentrating on measuring Saturn’s atmospheric electricity. When the project began in August 2009, the plan was actually to analyse an older thunderstorm in more detail. But then came December 5, 2010.

Lightning Action On that day, an instrument to measure radio and plasma waves aboard NASA’s probe Cassini detected the first lightning flashes of a storm which was forming in Saturn’s northern hemisphere. What nobody knew then was that this storm would later turn out to be the largest storm which Cassini had ever measured on Saturn, because the first photos by Cassini’s Imaging Science Subsystem initially showed a small, bright cloud. Even these photographs had been taken by chance, as the head of the FWF project and lead author of the current Nature publication, Dr. Georg Fischer, explains: "It is not possible to point this camera as the mood takes you. It was pure chance that it happened to be pointing in the right direction at the right time. Since it would take a while to analyse the photos, I immediately sent out an appeal to the global network of amateur astronomers. I asked them to please keep an eye on the thundercloud. A real lightning action, so to speak."

The rapid reaction was worth the effort, making it possible to follow the dramatic development of the storm which Dr. Fischer explains as follows: "Three weeks after its discovery, the storm already extended over more than 10,000 kilometres. Two months later it encircled the whole planet. And now, seven months after the discovery, it covers an area of four billion square kilometres. That’s eight times the surface area of the Earth."

Fast as Lightning A crucial part of Dr. Fischer’s project is the measurement of lightning activity with the aid of the radio waves emitted. These radio waves, called Saturn Electrostatic Discharges (SEDs), usually occur as short, individual emissions. Not so in this case. The sequence of individual flashes is so rapid that almost continuous radio emission is measured. There are up to ten flashes per second. Taken together with this lightning activity, the storm achieved a total energy of a septillion – 10 to the power of 24 – joules in the first three months of its existence. This corresponds to the total annual solar energy reaching the Earth.

For Dr. Fischer the dimensions of the storm are just as impressive as the timing and the localisation of the storm: "Cassini has been observing Saturn since 2004, and during this time, thunderstorms have only been observed in the southern hemisphere. A Saturn year lasts 29.5 Earth years, and August 2009 saw the start of Saturn’s spring. I had put forward the hypothesis that this would mean that the thunderstorms would change to the northern hemisphere. The fact that it only took such a short time to gain support for this hypothesis has surprised me. This is a successful conclusion to our current FWF project."

Scientific contact
Dr. Georg Fischer
Space Research Institute
Austrian Academy of Sciences
Schmiedlstraße 6
8042 Graz, Austria
T +43 / 316 / 4120 - 664
M +43 / 699 / 12068 - 896
georg.fischer(at)oeaw.ac.at

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Vienna, 7th July, 2011