Intriguing and often-examined gullies on Mars might not be created by water seeping out from underground springs. Rather, they are likely caused by trickling water from melting snowpacks, an active process that could sustain biology on the Red Planet.

A leading Mars scientist has proposed a new theory regarding gully formation on the planet, backed by images taken from NASA's Mars Odyssey spacecraft. The research bolsters the view that liquid water is sheltered by snow, preventing the fluid from rapid evaporation in Mars' thin atmosphere.

The research was presented today at a space science briefing at NASA Headquarters in Washington, D.C. and will be detailed in the Feb. 20 issue of the journal Nature.

But today, formation of the martian gullies by springs or pressurized flows was called into question. Those now-famous gullies are created by trickling water from melting snowpacks, argues Philip Christensen, a researcher at Arizona State University and principal investigator for Odyssey's camera system.

Furthermore, that snow acts as a greenhouse, protecting the water and allowing it to melt and flow, and not instantly evaporate in the low-pressure atmosphere.

"This snow would make an unbelievably attractive abode for life," Christensen told SPACE.com in an exclusive interview. "You've got sunlight for photosynthesis. You've got temperatures above freezing. And you've got liquid water all within a few inches of the surface at mid-latitudes on Mars over huge areas.

"I would think life, if it exists on Mars, would migrate toward exactly these environments," Christensen said. "The snow sits there and acts as this wonderful blanket that allows all of this melting and trickling to go on."

Christensen points to an image taken by Odyssey of a crater in the southern mid-latitude Terra Sirenum region of Mars. It shows eroded gullies on the crater's cold, pole-facing northern wall, he said. But immediately next to those features is a section he calls "pasted-on terrain."

This smooth deposit of material is thought to be "volatile," composed of materials that evaporate in Mars' thin atmosphere. This material characteristically occurs only in the coldest, most sheltered areas.

Christensen reported that the most likely composition of this slowly evaporating material is water in the form of snow. From this observation, the Mars scientist suspects a relationship between the gullies and the snow.

Snow on Mars is likely to accumulate most on the pole-facing slopes -- the coldest areas. It gathers and drapes the landscape in these areas during one climate period, then it melts during a warmer one. Melting begins first in the most exposed area right at the crest of the ridge. This explains why gullies start so high up, Christensen said today.

Once he started to think about snow, Christensen added, he found a large number of other images showing a similar relationship between "pasted on" snow deposits and gullies in the higher resolution images taken by the Mars Global Surveyor.

Christensen told SPACE.com that snowpack melt that forms the gully-carving streams of water is estimated to equal 12 cubic meters per day per gully. At that rate you could fill a swimming pool in less than a week with the amount of water that trickled down each gully. This is water that may occur every year for hundreds of years before the ice goes away, he said.

Given the commonness of snowpacks on Mars, it's likely that somewhere on the planet a snowpack is melting right now, Christensen said. "This isn't a dead process."

"The snow will be back and these gullies will be rejuvenated and reactivated. What we see today is sort of the fossil remnants of that snow. But it's by no means a dead, inactive process. It will occur again," Christensen said at the NASA press briefing.

The new explanation for gully formation fits well with NASA's Mars exploration strategy to "follow the water" and to look for life, Christensen noted. Those melting snowpacks point to places where future robotic craft should go look, he said.

"If we don't find life in these type of environments," Christensen said, "I would question, where would you find life?"

Lynn Rothschild, a scientist at the Ecosystem Science and Technology Branch at NASA's Ames Research Center near San Francisco, California, also took part in today's NASA briefing. The prospect that liquid water might be near the surface bodes well for life on Mars, she said.

"The real kicker is finding that liquid waterand even very tiny amounts is good enough," Rothschild said. There are clear analogs on the Earth for organisms living under the conditions reported by Christensen, she added.

One such community is snow algae, sometimes called watermelon snow, Rothschild said.

These organisms can survive for long periods, hanging out for much of the year in a resting stage. Then, when conditions are right and a thin film of water under the snow is created, the organisms germinate. Now in a mobile stage, they move up to the surface and photosynthesize, complete their life cycle, and go back into spore form.

"That's something one could envision surviving" in the type of conditions that Christensen has proposed, Rothschild said.

John Mustard, associate professor at the Department of Geological Sciences at Brown University in Providence, Rhode Island, called Christensen's new findings a "fundamentally new perspective" on the origins of the Mars gullies. It won't close the books on these features, however, he added.

"As scientists we continue to want to probe and discover things," Mustard said. Future Mars spacecraft, particularly the 2005 launch of NASA's Mars Reconnaissance Orbiter, will offer new tools to further explore the gullies and how they are formed, he said.

Also taking part in the NASA briefing, Bruce Jakosky, a Mars expert at the University of Colorado, Boulder, said the new research "provides a key, new piece of information" about gully formation on the red planet.

"It's very hard to rule out one idea absolutely, or to rule in one as being the correct answer," Jakosky said. "In the hierarchy of ideas that work and ideas that won't, I think this one is at the very top of the list. It is not a proven idea yet, but it's one that we can explore over the next couple of years. I think we'll see a lot of attention focused on this."