Magazine article Science News

Carved by Melting Snow? (Martian Gullies)

Magazine article Science News

Carved by Melting Snow? (Martian Gullies)

Article excerpt

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. …

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