Nasa space scientists are backtracking over sensational claims that they have found signs of liquid water still flowing on Mars. Detailed new studies from a spaceprobe flying overhead reveal that deposits on crater slopes were probably just landslides of dry debris.

The discovery is a setback to alien hunters who claim that the Red Planet could still harbour simple life forms.

Other findings by the latest probe to orbit Mars undermine arguments that the planet once had a sustained wet, rainy climate, although it confirms that liquid water has been present.

The deposits, hundreds of meters long, were heralded by Nasa in December as powerful evidence that water was still flowing on Mars.

Scientist claimed they were gullies caused by water surging under pressure from beneath the martian surface. They said the equivalent of up to ten swimming pools of water had flowed down two slopes in the Terra Sirenum and Centauri Montes regions of southern Mars.

Nasa claimed the water had vapourised as it flowed, leaving white deposits that had not shown up on photos of the surface taken just a few years earlier. The discovery – hailed as the most important ever on Mars – came after tens of thousands of gullies were snapped from orbit by Nasa’s Mars Global Surveyor probe over nearly ten years.

Now a second probe, carrying the most powerful camera ever flown to another world, plus other advanced instruments, has put the damper on the discovery. Mars Reconnaissance Orbiter, which arrived at the planet last year, looked closely at the new features.

The MRO probe’s chief scientist, Alfred McEwen, said: “The key question raised by these two deposits is whether water is coming to the surface of Mars today. Our evidence suggests the new deposits did not necessarily involve water.”

One of the fresh deposits is a stripe of bright material several hundred yards long that was not present in 1999 but appeared by 2004. The new orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars reveals the deposit is not frost, ice or a mineral left behind by evaporation of salty water.

Scientists also examined the slopes above this and five other locations with bright and apparently young deposits. They found the slopes were steep enough for sand or loose, dry dust to flow down the gullies.

MRO’s high-res camera also dealt a blow to arguments for a long-term wet climate on Mars by examining branched channels and fan like deposits typical of water flows that were found around several impact craters.

The probe found that the flows were probably caused by the asteroid impacts themselves rather than Martian rain. Nasa points out, however, that other gullies still offer overwhelming evidence that liquid water has flowed on Mars within the last few million years and it is thought Mars was once covered by vast oceans. Europe’s Mars Express has detected huge reserves of water ice around the Martian south pole.

Nasa’s latest lander, Phoenix, is currently on its way to Mars to dig into the icy soil.

Professor Colin Pillinger, who led the brave but ill-fated Beagle 2 mission to Mars in 2003, told Skymania News: “There is still overwhelming evidence of big flows of water in the past on Mars. You really need to get on the ground to study these things.

“It has been proposed that a lander be sent to a region where deposits were detected and that is still a good idea. They say a picture is worth a thousand words but an analysis is worth a thousand pictures.”

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