Michael Malin, the lead researcher in the study of pictures taken by the US National Aeronautics and Space Administration's Mars Global Surveyor in 2004 and 2005, said the photos are "what you would expect to see if the material were carried by flowing water".
 
Search for life

The surface of Mars is too cold for liquid water to exist, but scientists believe underground water filtered to the surface carrying debris downslope long enough to form the gullies before it froze.
"This underscores the importance of searching for life on Mars, either present or past."

Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder
During its Mars exploration missions, Nasa has pursued a "follow the water" strategy to determine if the planet once contained life or could support it now.

Scientists believe ancient Mars was awash with pools of water and researchers have spotted evidence of water ice at the planet's North Pole.

"This underscores the importance of searching for life on Mars, either present or past," Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder, who had no role in the study, said. "It's one more reason to think that life could be there."

Some scientists have said the gullies could have been created by liquid carbon dioxide (CO2) coming to the surface, but Nasa experts say this is unlikely.

Others say materials such as sand or dust can flow like a liquid and produce similar results.

Allan Treiman, a geologist at the Lunar and Planetary Institute in Houston, said in an email to the Associated Press news agency: "Nothing in the images, no matter how cool they are, proves that the flows were wet, or that they were anything more exciting than avalanches of sand and dust."

The Mars Global Surveyor orbiter has taken 240,000 pictures of the surface of Mars since 1997 but the agency lost contact with the spacecraft in November.