(Sen) - A huge storm seen to erupt on Saturn nearly two years ago is still intriguing planetary scientists long after its visible effects subsided.

An oval vortex formed high in the stratosphere following the eruption. It has been closely monitored by infrared instruments on NASA's Cassini spacecraft as well as two giant Earth-based observatories, the Very Large Telescope (VLT) operated by the European Southern Observatory in Chile and NASA's Infrared Telescope Facility on Mauna Kea in Hawaii.

Bright cloud material from the storm encircled the whole of the planet in its northern hemisphere where spring was beginning. Waves of energy rippled hundreds of kilometres upwards like two enormous beacons in Saturn's stratosphere.

They were expected to cool down and disappear but instead the two hot spots merged to form a giant vortex by late April 2011, to the amazement of professional planet watchers. They were also surprised that the temperature of the vortex became about 80 C warmer than the surrounding atmosphere. Huge spikes were detected in the amount of gases such as ethylene and acetylene, to the puzzlement of European Space Agency and NASA scientists.

The evolution of Saturn’s storm at infrared wavelengths from January 2011 through to March 2012 using data collected by the Cassini spacecraft, ESO’s Very Large Telescope and NASA’s Infrared Telescope Facility. Credits: ESA–C. Carreau

Leigh Fletcher, of the UK's University of Oxford, is lead author of a study of the storm published in the journal Icarus. He told Sen: "Saturn's atmosphere is usually a fairly bland place without the giant whirlpools and belts and zones you see on Jupiter. So the storm that erupted in December 2010 came as a big surprise.

"We got the first hint that something strange was going on when Cassini's radio and plasma wave instrument ramped up its levels of radio emission indicating lightning strikes. However it couldn't pinpoint their location.

"Then a few days later amateur astronomers reported a small white spot in Saturn's northern hemisphere which grew so rapidly that it became its most prominent feature within days. This was bright white material dredged up from within Saturn's atmosphere being spread by its powerful winds blowing at speeds of hundreds of metres a second."

He added: Technically the storm is over, it's the after-effects that we're now seeing in this huge vortex high in the stratosphere, hundreds of kilometres above where the storm erupted."

The planetary scientists' measurements of the temperatures, winds and composition of the clouds in Saturn's atmosphere have helped them to study the storm and its after-effects in detail.

Dr Fletcher added: "It's the first time we've seen anything like it on any planet in the Solar System. It's extremely unusual, as we can only see the vortex at infrared wavelengths – we can't tell that it is there simply by looking at the cloud cover."

The huge vortex formed briefly became larger than the famous Great Red Spot that has raged for hundreds of years on Saturn's big sibling planet Jupiter. The material within the vortex and its temperature were different from its surrounding atmosphere too, as in the Great Red Spot. But there the similarities end.

Dr Fletcher said: "Jupiter's vortex is embedded deep down in the turbulent 'weather zone', whereas the vast vortex on Saturn is higher up in the atmosphere where, normally, you wouldn't expect anything like it to have formed.

"Although there are parallels to be drawn between the two, the mechanisms by which they were formed and the length of time they are going to exist seem to be very different."

The increase in levels of the odourless gas ethylene when the ovals merged was observed by researchers at NASA's Goddard Space Flight Center in Greenbelt, Maryland. They likened it to a burp and its origin is still a mystery. It is not usually seen on Saturn.

How the storm developed when it could be seen at optical wavelenths from Cassini. Credit: NASA/JPL-Caltech/Space Science Institute

The team that detected the gas using Cassini's composite infrared spectrometer (CIRS) instrument still is exploring the origin of the ethylene, but has ruled out a large reservoir deep in the atmosphere.

Dr Fletcher told Sen: "It is far more likely that it was produced high up and brought to it’s current location. It is expected to be there in the photochemical soup that results from the breakdown of methane in the high atmosphere. It’s just that the levels are far higher than expected."

Goddard's Michael Flasar, the CIRS team lead, said: "We've really never been able to see ethylene on Saturn before, so this was a complete surprise." Their findings are to be reported in the Astrophysical Journal.

The storm was not only the first of its kind to be studied by a spacecraft in orbit around the planet, but also the first to be observed at thermal infrared wavelengths.