for National Geographic News
Melting snow appears to have formed a large number of Martian gullies sometime in the past few million years, according to a new study.
Gullies were first discovered on Mars eight years ago, and experts have been debating since then about what created them.
Despite initial theories that they might be proof of liquid water on modern Mars, the most recently formed gullies appear to have been made by dry landslides.
(Read "Dry Debris, Not Water, Caused Recent Flows on Mars" [March 3, 2008].)
But experts agree that older gullies on the red planet were most likely carved by a fluid.
"There are meanders and streamlined islands in the middle. They divert around obstacles. They do all the things typical of a fluid," said planetary scientist Jay Dickson of Brown University.
One theory says that spring water occasionally bursts from deep underground, creating the features.
Now recent evidence supports a second theory—originally suggested several years ago—that these gullies were created by melting snow, said Dickson, who presented his findings last week at the 39th Lunar and Planetary Science Conference in League City, Texas.
Dickson and colleague James Head III, also at Brown, combined existing data with the latest high-resolution images to catalog the types of terrain on which the gullies formed.
They observed that the gullies tend to appear on slopes facing toward the equator in one range of latitudes and toward the pole at another range.
The gullies also occurred only at elevations that would be conducive to ice and snow accumulating on the ground and then melting, he said.
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