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WASHINGTON (PTI): Astronomers have discovered a strange vortex swirling in the atmosphere high above the South Pole of the Saturn moon Titan, hinting that winter may be coming to the huge body's southern reaches.

The polar vortex, or the mass of swirling gas, captured by NASA's Cassini probe during a flyby of Titan last month appears to complete one full rotation in nine hours, while it takes Titan about 16 days to spin once around its axis.

"The structure inside the vortex is reminiscent of the open cellular convection that is often seen over Earth's oceans," said Tony Del Genio, a Cassini team member at NASA's Goddard Institute for Space Studies in New York.

"But unlike on Earth, where such layers are just above the surface, this one is at very high altitude, maybe a response of Titan's stratosphere to seasonal cooling as southern winter approaches. But so soon in the game, we're not sure," he was quoted as saying by LiveScience.

When Cassini arrived at the Saturn system in 2004, Titan -- which is 5,150 km wide -- had a vortex and a visible "hood" of relatively dense haze high above its north pole. It was winter there until Saturn's August 2009 equinox, which marked the arrival of spring in the northern hemisphere and fall in the southern reaches of the planet and its moons.

While Titan's northern hood still remains, the circulation in the upper atmosphere has been moving from the warming North Pole to the cooling south, the researchers said.

This shift seems to be causing downwellings over Titan's South Pole, along with the formation of high-altitude haze and a vortex there, they said.

Cassini's cameras first noticed the southern hazes in March, and the spacecraft's visual and infrared mapping spectrometer instrument (VIMS) captured false-colour images on May 22 and June 7.

"VIMS has seen a concentration of aerosols forming about 200 miles above the surface of Titan's south pole," said Christophe Sotin, a VIMS team member at NASA's Jet Propulsion Laboratory (JPL) in California. "We've never seen aerosols here at this level before, so we know this is something new."

Cassini scientists will keep a close eye on Titan's South Pole for further developments, which could shed light on the moon's complex, methane-based weather system.

"Future observations of this feature will provide good tests of dynamical models of the Titan circulation, chemistry, cloud and aerosol processes in the upper atmosphere," said Bob West, Cassini deputy imaging team lead at JPL.