Unlike Earth hurricanes, the storm's position appears fixed

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Measuring 5,000 miles (8,000km) across, the storm is the first hurricane ever detected on a planet other than Earth.

Scientists say the storm has the eye and eye-wall clouds characteristic of a hurricane and its winds are swirling clockwise at 350mph (550km/h).

However, unlike Earth hurricanes it seems stuck at the pole, not drifting.

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

Though Jupiter's Great Red Spot storm moves anti-clockwise, and is far bigger than the storm on Saturn, it does not have the eye and eye-wall that mark out a hurricane.

We've never seen anything like this before. It's a spectacular-looking storm

Michael Flasar, Nasa astrophysicist

But Saturn is a gaseous planet therefore this storm does not have an ocean at its base.

The Saturn storm is bigger not only in diameter than an Earth hurricane, but in height too, with a ring of huge clouds towering 20-45 miles (30-70km) above the well-developed eye - two to five times higher than in storms on Earth.

Unknown phenomenon

One Nasa scientist, Michael Flasar, told Reuters news agency that the storm looked just like water swirling down a bath plug hole, only on a colossal scale. "We've never seen anything like this before," Mr Flasar said. "It's a spectacular-looking storm."

Fourteen frames of the storm were captured by the Cassini spacecraft over the course of three hours on 11 October 2006.

Cassini was passing about 210,000 miles (340,000km) from the ringed planet as it continues its exploration of Saturn and its moons.

Cassini entered into orbit around Saturn on 1 July 2004. Later that year, it released the piggybacked Huygens probe towards the planet's largest moon, Titan.

Huygens touched down on Titan on 14 January 2005, sending back data on the moon's atmosphere, weather and its surface.

The Cassini-Huygens mission is a co-operative project of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).