The storm on the giant ringed planet is around 8000 kilometres wide, measuring around two-thirds the diameter of Earth, Nasa said on Friday.

  

The images were captured over a period of three hours on October 11, by the US space agency's Cassini spacecraft as it passed about 340,000 kilometres from the planet as part of its exploration of Saturn and its moons.

 

Michael Flasar, an astrophysicist involved in the mission at Nasa's Goddard Space Flight Centre, said the storm looked just like water swirling down the drain in a bath tub, only on a gigantic scale.

 

He said: "We've never seen anything like this before … It's a spectacular-looking storm."

 

Eye of the storm

 

The storm has a well-developed eye ringed by towering clouds that soar 30 to 75 kilometres above those in the dark centre, two to five times higher than clouds in earth's thunderstorms and hurricanes, Nasa said.

 

Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology in Pasadena, said: "It looks like a hurricane, but it doesn't behave like a hurricane.

 

"Whatever it is, we're going to focus on the eye of this storm and find out why it's there."

 

Flasar said scientists had more work ahead to understand the Saturn storm.

 

He said: "I'm hoping that as we puzzle over it, it will become even more exciting as we start to connect the dots in our brains.

 

"But right now, the wheels are a little creaky. We're all arguing with each other about what it might or might not be."

 

Different to Earth

 

Saturn, the second-biggest planet in the solar system with an equatorial diameter of 119,000 kilometers and the sixth from the sun, is about 1.2 billion kilometres from Earth.

 

Scientists said it was unclear whether Saturn's storm was a water-driven system.

 

Jupiter's Great Red Spot, which swirls counter-clockwise, is far bigger, but is less like a hurricane because it lacks the typical eye and eye wall, said scientists.

 

It differs from Earth hurricanes in part because it remains stuck at the pole rather than drifting as such storms do on this planet and because it did not form over a liquid water ocean, with Saturn being a gaseous planet, Nasa said.