NASA scientists announced Thursday that they had found the first evidence of flowing water on Mars.
If confirmed, the evidence gathered by the Mars Reconnaissance Orbiter would be the first discovery of active liquid water in the ground on the red planet.
"We have found repeated and predictable evidence suggesting water flowing on Mars," Michael Meyer, lead scientist for the Mars Exploration program, told reporters.
The US space agency said the orbiter circling Mars since 2006 had monitored numerous instances of what appeared to be water flows occurring in several locations during the Martian spring and summer.
Time-sequence imagery of the Newton crater in the southern mid-latitude region showed finger-like markings spreading along several steep slopes and then fading again once colder temperatures move in.
"The best explanation we have for these observations so far is flow of briny water, although this study does not prove that," said Alfred McEwen of the University of Arizona's Lunar and Planetary Laboratory.
"It's a mystery now, but I think it's a solvable mystery with further observations and experiments," said McEwen, lead author of a study explaining the findings in the journal Science.
NASA experts are still not sure if what they have witnessed is actually water flowing on Mars, where no liquid water has been found to date.
"By comparison with Earth, it's hard to imagine they are formed by anything other than fluid seeping down slopes," said Mars Reconnaissance Orbiter project scientist Richard Zurek of NASA's Jet Propulsion Laboratory.
"The question is whether this is happening on Mars and, if so, why just in these particular places."
McEwen, principal investigator for the orbiter's High Resolution Imaging Science Experiment (HiRISE) that captured the images, said the orbiter recorded "thousands" of the flows over the past three years at seven locations.
It had identified 20 other possible sites of similar flows, he said.
McEwen cautioned that the water flows remained "circumstantial," and said scientists "lack that direct confirmation of water" from other instruments studying the planet, but hope it will be confirmed in future missions and experiments.
In any case, it does not appear that scientists are seeing anything akin to a gushing river on Mars, but more likely a subterranean movement.
"The flows are not dark because of being wet," McEwen said. "They are dark for some other reason," possibly because the briny water runs below the surface and is altering the land's appearance in a way that makes it look dark.
Frozen water has been detected in some of Mars's higher latitudes, and other evidence has suggested that water interacted with the Martian surface throughout the planet's history.
NASA has placed a renewed focus on Mars, with the 30-year space shuttle program now over and efforts under way to build a spacecraft capable of carrying humans to the red planet by 2030.
The space agency's unmanned Curiosity rover, also known as the Mars Science Laboratory, is due to explore a mountain inside the Gale Crater on Mars that should reveal whether signs of life ever existed on the red planet.
The largest US rover ever, built at a cost of $2.5 billion dollars, it is set to launch later this year and land in August 2012.
