spacetoday.net: Melting snow responsible for Martian gullies

Posted: Thu, Feb 20, 2003, 2:28 PM ET (1928 GMT)
Martian gullies and remnant snow bank (arrow) (NASA/JPL/MSSS/P. Christensen)

Gullies discovered on the sides of several Martian craters nearly three years ago are likely formed by melting snow, scientists reported yesterday. In a paper published in Thursday's issue of the journal Nature, a group led by Philip Christensen said that snow, rather than subterranean ice, is the likely source of the water thought to form the gullies. Scientists first spotted the gullies in Mars Global Surveyor images in 2000. Their relatively unaltered appearance led scientists to conclude that the gullies formed relatively recently in Martian history—within the last 100,000 years—but researchers were unsure of the source of the water. Christensen used images from Mars Odyssey to study the gullies in greater detail, and noticed that in one crater the gullies were located next to a section of "pasted-on" terrain of smooth material made of volatiles that Christensen believes is snow. The snow was likely deposited there when the planet's orbital tilt, which varies over time, was greater than it is today: this allowed the water ice at the poles to sublimate and eventually be deposited as snow in the craters, where it remains today in shadowed regions that face the poles, where the gullies are seen. This discovery could have implications for astrobiology, since a long-lasting source of gradually melting water ice would provide a good habitat for primitive life to form.