Last June, Cassini spacecraft sent to Earth amazing images revealing the existence of lakes on Saturn's Moon, Titan, filled with methane.

The lakes were situated on the Titan's north pole, but scientists did not know precisely what fueled them.

Now, researchers have spotted on Titan a mammoth cloud half the size of the contiguous United States which might be what's filling up the lakes.

"This cloud system may be a key element in the global formation of organics and their interactions with the surface," said study team member Christophe Sotin of the University of Nantes, France.

The cloud was detected by Cassini spacecraft on December 29, 2006 and is about 1,500 miles (2,400 km) in diameter, engulfing the entire north pole of the Titan's Moon.

It turned visible only recently, getting out of shadow as winter passes to spring on Titan.

Like its lakes, Titan's clouds are very bizarre: they are made of ethane, methane and other organics, not water.

Scientists had imagined a cloud system, but this is the first proof, and a detailed one!

This finding just enhances the hypothesis of the "methane-ologic cycle": methane rains down onto the Titan's surface to form lakes and then evaporates to form clouds.

Earth observations depict the Titan cloud system moving with the seasons and a Titan's season means about seven Earth years.

Astronomers imagine that the cloud activity lasts for about 25 Earth years before sharply decreasing for four to five years and after that reemerges for another 25 years cycle.

Thus, the newly discovered cloud is expected to move to the Titan's pole in several years, with the season change.

A Jan. 13, 2007 flyby spotted the cloud in the same place.

"With 16 more flybys to come this year, we should have the opportunity to monitor the evolution of this cloud system over time," said study team member Stephane Le Mouelic, also of the University of Nantes.

Photo credit: NASA.