A hurricane-like storm two-thirds as wide as the Earth at Saturn's south pole.

Credit: NASA/JPL/Space Science Institute

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SYDNEY: A hurricane-like storm two-thirds as wide as the Earth has been raging at Saturn's south pole, images from NASA's Cassini spacecraft reveal.

Such hurricane-like weather systems have never before been seen on other planets, and scientists remain uncertain of its cause.

The dark eye of the storm spans about 8,000 kilometres and is surrounded by a bright ring of clouds that tower some 30 km to 75 km above the centre.

"It looks like a hurricane, but it doesn't behave like a hurricane," said Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology in Pasadena. "Whatever it is, we're going to focus on the eye of this storm and find out why it's there."

A movie taken by Cassini's camera over a three-hour period reveals winds around Saturn's south pole blowing clockwise at 560 km per hour. The camera also recorded the shadow cast by the ring of towering clouds, and two spiral arms of clouds extending from the central ring. These ring clouds are two to five times taller than the clouds of thunderstorms and hurricanes on Earth.

Such 'eye-wall' clouds are a distinguishing feature of hurricanes on our planet. They form where moist air flows inward across the ocean's surface, rising vertically and releasing a heavy rain around an interior circle of descending air that forms the eye of the storm.

Though scientists are unsure whether such moist convection is driving Saturn's storm, the dark eye at the pole, the eye-wall clouds and the spiral arms together indicate a hurricane-like system.

Distinctive eye-wall clouds have not been seen on any planet other than Earth. Even Jupiter's Great Red Spot, much larger than Saturn's polar storm, has no eye or eye-wall, and is relatively calm at the centre.

However this giant Saturnian storm differs from hurricanes on Earth because it is locked to the pole and does not drift around like terrestrial hurricanes. Also, since Saturn is a gaseous planet, the storm forms without an ocean at its base.

"The clear skies over the eye appear to extend down to a level about twice as deep as the usual cloud level observed on Saturn," said Kevin Baines of the U.S. space agency NASA's Jet Propulsion Laboratory in Pasadena, California. "This gives us the deepest view yet into Saturn over a wide range of wavelengths, and reveals a mysterious set of dark clouds at the bottom of the eye."

Observations taken over the next few years, as the south pole season changes from summer to fall, will help scientists understand the role seasons play in driving the dramatic meteorology at the south pole of Saturn.