Mysterious gullies on Mars appear to have been etched by melting snow, a finding that offers promising new places to search for signs of life on the Red Planet, a scientist reported yesterday.

New pictures collected by NASA's Mars Odyssey space probe show formations that seem to be remnants of thick snow packs that once draped the slopes of craters, cliffs and other areas. The formations are near the gullies, which could have been carved when most of the snow melted, slicing channels into the ground.

"Mars seems to have quite a bit of snow," said Philip R. Christensen of Arizona State University in Tempe, who conducted the research. "I think the young gullies of Mars were actually carved by melting of extensive snow packs."

If confirmed, the findings would solve the puzzle of how the gullies formed and could provide the best place to search for evidence of existing or extinct microbial life when NASA's next round of unmanned Mars probes arrives in 2004.

"If you were to land on one of those and stick a shovel in the ground, you'd be shoveling snow. And if life ever existed on Mars, I can't think of a more exciting place to go and look," Christensen said at a briefing at NASA headquarters here -- an event that was almost canceled by the terrestrial snowstorm that buried the East Coast this week.

The gullies were first spotted by another NASA spacecraft, the Mars Global Surveyor, which sent back images of the formations in 2000. The gullies baffled scientists because they appeared to be relatively young and the planet has been too cold for too long to have had large amounts of water. Scientists proposed several hypotheses to explain how the gullies might have formed, including that they were created by water seeping out of the ground. But none of the scenarios was fully accepted.

The new conclusion comes from images from the Mars Odyssey, which was launched April 7, 2001, and is studying Mars from orbit. While examining an Odyssey image of gullies in the wall of a crater, Christensen noticed a nearby formation that appeared to be "pasted on" the terrain. Further analysis indicated it could be snow. When he looked at gullies elsewhere in the mid latitudes of the planet, he noticed the same kind of nearby formations.

"I saw it and said, 'Aha!' It looks for all the world like these gullies are being exposed as this terrain is being removed through melting and evaporation," said Christensen, whose findings are being published in the journal Nature.

He speculated that Mars goes through 100,000-year climatic cycles. At the beginning of the cycle, snow falls, creating icepacks. As the snow is warmed by the sun, it begins to melt from below, creating water that flows downhill to create gullies over a 5,000-year period. A layer of snow above the liquid water protects the melt from evaporating immediately.

"Snow provides a wonderful abode for life. The snow itself acts as a little miniature greenhouse," Christensen said.

Water is believed to be crucial to the existence of life either in the past or now. Previously, the only places where frozen water was believed to exist on Mars were in inaccessible locations deep below the surface of the poles.

In May and June, NASA plans to launch two robots to Mars to search for signs of life.

"Imagine an environment where I have sunlight, I have temperatures above freezing and I have liquid water. All of those make for a potential for life -- certainly someplace that would have a lot of interest in terms of future exploration," Christensen said.

The new theory is provocative, other scientists said.

"It's not a proven idea yet, but it's one we can explore over the next couple of years. I think we'll see a lot of attention focused on it," said Bruce Jakosky, a professor in the department of geological sciences and the laboratory for atmospheric and space physics at the University of Colorado at Boulder, who attended the NASA briefing.

Lynn Rothschild of NASA's Ames Research Center in Moffett Field, Calif., agreed that the finding offers scientists better places to look for signs of present or past life on Mars.

"What you've done is expand the envelope in terms of places and latitudes that conceivably could be habitable even today," she said.