The Mars Global Surveyor (MGS) first caught the Red Planet's mysterious gullies on camera in 2000. Since then, numerous explanations have been proposed for the erosion--which marks the edges of craters and cliff sides on Mars--including seeping groundwater and pressurized flowing liquid water or carbon dioxide. Now a paper published online this week by the journal Nature suggests that water from melting snow packs could be the culprit.

Using images from both NASA's Mars Odyssey spacecraft and the MGS as a guide, Philip R. Christensen of Arizona State University theorizes that water was transported from the poles toward the mid-latitudes when the planet's axis was tilted closer to the sun. The resulting water-rich snow layer formed there later began to melt as Mars's axis moved farther away from the sun. This melting yielded gully-carving liquid water that flowed beneath an insulating layer of snow. (The image above, taken by the MGS, shows numerous gullies on the wall of a Martian crater. The arrow points to a remnant of a snowpack.) According to the model, the trenches were etched over a period of 5,000 years approximately 100,000 years ago.

The new hypothesis does not yet close the book on Mars's mysterious gullies. But one of its strengths, Christensen notes, is that it can explain the formation of gullies in strange, isolated places where it would be particularly hard for groundwater to seep to the surface. "The suggestion put forward for the processes that gave rise to the gully formation is a plausible one, and I think it's the most consistent explanation with the observations so far," comments Bruce Jakosky of the University of Colorado, Boulder. "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."