Scientists have found evidence that there may be some form of life on Saturn's biggest moon Titan.

They have discovered clues that might show that microscopic aliens are breathing in Titan's atmosphere and feeding on fuel at the surface.

Data from Nasa's Cassini probe has analysed the complex chemistry on the surface of Titan - the only moon known to have a dense atmosphere.

Its surface is covered with mountains, lakes and rivers which has led astronomers to call it the most Earth-like world in the solar system.

Organic chemicals had already been detected on the 3,200-mile wide moon. But the liquid on Titan methane, rather than water, and scientists expect life there to be methane-based.

The startling discoveries, made using an orbiting spacecraft, are revealed by two separate teams reporting in two science publications, Icarus and the Journal of Geophysical Research.

The paper in Icarus shows that hydrogen gas flowing down through Titan's atmosphere mysteriously disappears at the surface, suggesting it could be being breathed by alien bugs.

The second paper reports that there is a lack of the chemical acetylene on the surface, leading scientists to believe that it may be being consumed once it reaches Titan.

Scientists had expected sunlight interacting with chemicals in the atmosphere to produce acetylene that falls down to coat Titan. But Cassini detected no acetylene on the surface.

Experts caution that there could be other explanations for the results observed.

But taken together, they fulfill two important conditions necessary for methane-based life to exist.

Nasa astrobiologist Chris McKay, of California, who drew up the list of conditions, said: 'We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth.

'If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.'

Look familiar? An artist's conception showing Titan's earth-like surface with Saturn appearing dimly in the background through the thick atmosphere

Roger Clark, a Cassini team scientist, said: 'Titan's atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again.

'All that implies Titan is a dynamic place where organic chemistry is happening now.'

But Mark Allen, principal investigator with the Nasa Astrobiology Institute Titan team said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.

'Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,' Allen said.

'We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results - for example, reactions involving mineral catalysts.'

Scientists say Titan resembles Earth as it was four billion years ago and so could be a new cradle for life. They believe that when the sun swells up, swallowing the Earth, conditions for life could be ideal on a warmed-up Titan.

Professor John Zarnecki, of the Open University, who landed a probe called Huygens on Titan in January 2005, said: 'We believe the chemistry is there for life to form. It just needs heat and warmth to kick-start the process.

'In four billion years time, when the sun swells into a red giant, it could be paradise on Titan.'