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WASHINGTON - Images taken by Nasa's Mars Global Surveyor spacecraft suggest the presence of liquid water on the Martian surface, a tantalising find for scientists wondering if the Red Planet ever has harboured life.

The orbiting US spacecraft allowed scientists to detect changes in the walls of two Martian craters that may have been caused by the recent flow of water, a team of researchers said in a paper appearing on Wednesday in the journal Science.

Scientists previously had established that two forms of water - ice at the poles and water vapour - exist on Mars, but liquid water is crucial to nurture life.

The scientists compared images of the Martian surface taken seven years apart and found the existence of 20 newly formed craters caused by impact from space debris as well as the evidence suggesting liquid water trickling down crater walls.

"These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," Michael Meyer, lead scientist for Nasa's Mars Exploration Programme, said in a statement.

The paper said water seemed to have flowed down two gullies in the past few years, even though liquid water cannot remain long on the planet's frigid, nearly airless surface because it would rapidly freeze or evaporate. This seemed to support the notion that liquid water may exist close enough to the planet's surface in some places that it can seep out from time to time.

The scientists proposed that water could remain in liquid form long enough on the surface to transport debris downslope before freezing. The two bright new deposits are each several hundred meters or yards long.

They cited a possible alternative explanation that these features were caused by movement of dry dust down a slope.

Scientists long have wondered whether life ever existed on Mars. Liquid water is an important part of the equation. On Earth, all forms of life require water to survive.

Among the planets in our solar system, only Earth has a more hospitable climate than Mars, and some scientists suspect Mars once sheltered primitive, bacteria-like organisms.

Previous missions found evidence that the Red Planet at one time boasted ample quantities of water, and the question is whether liquid water is still present.

"As with many discoveries, the possibility that liquid water may be coming to the surface of Mars today poses many questions," the scientists wrote. "Where is the water coming from? How is it being maintained in liquid form given the present and most likely past environments? How widespread is the water?"

They also wondered whether the water could be used as a resource for future missions to explore Mars.

The two sites are inside craters in the Terra Sirenum and the Centauri Montes regions of southern Mars.

"The shapes of these deposits are what you would expect to see if the material were carried by flowing water," added Michael Malin of Malin Space Science Systems in San Diego, lead author of the paper. "They have finger-like branches at the downhill end and easily diverted around small obstacles."

The researchers first reported the discovery of the gullies in 2000, but this is the first time they have revealed the presence of newly deposited material seemingly carried by liquid water.

Last month, Nasa said the Mars Global Surveyor mission appeared to be at an end, saying it had lost contact after a decade-long mission in which it mapped the surface of Mars, tracked its climate and searched for evidence of water.