Huge vortex spied on Saturn moon

Nasa's Cassini spacecraft has spotted a swirling green mass of gas at the south pole of Saturn's moon Titan.

Scientists say the polar vortex is turning faster than the moon itself rotates on its axis.

The formation of the vortex may be related to the changing seasons; Titan's southern winter is on its way.

The vortex may eventually form a "polar hood" - an area of dense, high altitude haze - just such a feature was seen over the north pole during winter.

Titan, Saturn's largest satellite, is covered by a thick haze composed of carbon-based (organic) molecules.

Cassini's visible-light cameras caught signs of a yellowish area forming in the haze over this region as early as March 27.

Clouds were first spotted massing around the south pole on 22 May when the spacecraft looked at infrared wavelengths.

"We've been watching this vortex become more developed in the last three to four months, and now, captured in exquisite detail, we're seeing finer-scale features within the detached haze than have been seen to date," said Jason Perry, from the University of Arizona, Tucson.

Since Cassini arrived in the Saturn system in 2004, Titan has had a visible "hood" high above the north pole.

It was northern winter during this period, and much of the high northern latitudes were in darkness.

But the hood - an area of denser, high-altitude haze compared to the rest of the moon's atmosphere - was high enough to still be illuminated by sunlight.

Saturn's August 2009 equinox signalled the beginning of spring in the northern hemisphere and autumn in the southern hemisphere for the planet along with moons such as Titan.

Now the high southern latitudes are moving into darkness. Scientists think the formation of the vortex at Titan's south pole may be related to the coming southern winter and the start of what will be a south polar hood.

On Saturn, Jupiter, Neptune, Venus and Earth, similar vortices are centered on the north or south pole.

However, the polar vortices on these planets are each distinct. Titan's south polar vortex also appears extraordinary in that it contains what appears to be a cloud of condensing organic material containing carbon, hydrogen and nitrogen.

Cassini despatched the piggybacked Huygens probe to Titan in early 2005. Huygens sent back valuable data from its descent through the moon's thick atmosphere and subsequent touchdown on the surface.