Titan, the methane moon

"Titan has long intrigued space scientists, as it is the only moon in the Solar System to have a dense atmosphere -- and its atmosphere, like Earth's, mainly comprises nitrogen. Titan's atmosphere is also rich in methane, although the source for this vast store of hydrocarbons is unclear.

"Methane, on the geological scale, has a relatively limited life. A molecule of the compound lasts several tens of millions of years before it is broken up by sunlight. Given that Titan is billions of years old, the question is how this atmospheric methane gets to be renewed. Without replenishment, it should have disappeared long ago.

"When the US-European spacecraft Cassini sent down a European lander, Huygens, to Titan in 2005, the images sent back were of a rugged landscape veiled in an orange haze. There were indeed signs of methane flows and methane precipitation, but nothing at all that pointed to any sea of the stuff.

"But a flyby by Cassini on July 22 last year has revealed, thanks to a radar scan, 75 large, smooth, dark patches between three and 70 kilometers (two and 42 miles) across that appear to be lakes of liquid methane, scientists reported on Thursday.

"They believe the lakes prove that Titan has a "methane cycle" -- a system that is like the water cycle on Earth, in which the liquid evaporates, cools and condenses and then falls as rain, replenishing the surface liquid."