PARIS, 28 June 2006 - The European Space Agency's new mission to Venus has discovered that the planet has a bizarre double vortex which whirls in the atmosphere above its south pole, ESA said on Tuesday.

The "double eye", formed by winds of super-hurricane force, was spotted by the unmanned Venus Express spacecraft in its very first swing around Venus, it said.

Previous missions to Venus have spotted a similar structure over its north pole and glimpsed at stormy atmospheric behaviour at the south pole.

The winds on Venus spin westwards at hundreds of kilometres per hour, taking only four days to complete the rotation of a planet that is just under the size of Earth.

This "super-rotation," combined with the natural recycling of hot air in the atmosphere, would logically induce a vortex over each pole, but the mystery is why there should be two vortices.

"We still know very little about the mechanisms by which the super-rotation and the polar vortexes are linked," said Hakan Svedhem, the mission's project scientist.

"Also, we are still not able to explain why the global atmospheric circulation of the planet results in a double vortex and not a single vortex... Atmospheric vortexes are very complex structures that are very different to model, even on Earth."

Venus Express is Europe's first dedicated mission to Earth's closest planet.

It went into orbit around Venus on April 11, equipped with scanners aimed at deciphering the enigmatic Venusian atmosphere.

The planet seems to be a case of runaway global warming, and understanding the mechanisms that drive this could be of use in combatting man-made climate change on Earth.

Venus's mean surface temperature is 457˚Celsius - hot enough to melt lead and even hotter than Mercury, the closest planet to the Sun. Its atmosphere is 96 per cent carbon dioxide, with roiling, yellowish clouds of sulphur and sulphuric acid droplets.