Carved by melting snow? (Martian Gullies).

Ever since 2000, when spacecraft observations revealed that Mars has a multitude of gullies that were probably carved by recent flows of water, planetary scientists have been hard-pressed to find a source of water that could do the job. The incentive to find liquid water on Mars is strong because such water could harbor life.

One leading proposal suggests that the gullies formed when water percolating just beneath the surface built up enough pressure to break through an overlying cap of ice and spill along the surface (SN: 7/1/00, p. 5). But the gullies typically are found at midlatitudes, where temperatures are so cold that the presence of liquid water is unlikely. Moreover, many gullies are isolated and lie near the rims of cliffs and craters, where researchers don't expect to find groundwater seepage.

In the Feb. 20 Nature, Philip R. Christensen of Arizona State University in Tempe suggests that instead of the gully-forming water seeping up from below, it falls from above in the form of snow. When he examined pictures of the gully regions taken by the Thermal Emission Imaging System aboard the Mars Odyssey spacecraft, he saw light-colored material that to him resembled blankets of snow. Christensen recalls that seeing the images "was like a lightbulb going off in my head; I've never had a moment like that before."

To form gullies, he theorizes, the surface of a snow deposit mixes with the dust prevalent in the Martian atmosphere, creating a mixture that readily absorbs sunlight. This top layer would act like a thermal blanket, transmitting solar energy to more pristine layers of snow just a few centimeters beneath. In about 5,000 years, the heat would melt enough subsurface snow to erode the underlying rock and carve the gullies, Christensen calculates.

Each gully includes a channel hundreds of meters wide, tens of meters deep, and several kilometers long. According to Christensen's model, a gully is only visible where the snow layer that created it has completely melted. Many others remain hidden, he says.

The deposition of snow could explain why some gullies reside in isolated spots: Snow happened to drape those regions at one time but has since melted. The model can also explain why gullies lie near but not at the very tops of craters and cliffs, Christensen says. Snow deposited any higher than a few hundred meters from the summits would evaporate.

Though unproven, Christensen's model of gully formation "is more compelling than any of the others that have been put forward," comments Bruce M. Jakosky of the University of Colorado in Boulder. The bottom line, he says, is that "as we continue to analyze the images and other data in more detail, we are finding increasing support for the idea that Mars has and has had water at and near the surface."

Mars might have harbored life long ago, but the planet today "appears to be right on the verge of being habitable by microbes," says Jakosky.