for National Geographic News
Eroded gullies on the flanks of a Martian crater may have been carved by flowing meltwater as recently as a few hundred thousand years ago, a new study says.
Evidence presented at a meeting in May supported the theory that Mars gullies were created by melting snow, but didn't suggest a specific date.
Other scientists examining similar gullies in other craters had hypothesized that they might have been formed by dry avalanches or seepage from springs.
These prior studies were only able to look at the gullies in isolation, said study co-author David Marchant, a planetary scientist at Boston University.
The new study used the latest high-resolution photos and technology from orbiting satellites to observe the gullies' geological setting.
They found that the gullies originate from cirque-like depressions high on the pole-facing slopes of the crater—depressions created by long-gone glaciers.
These glaciers flourished when wobbles in the Martian orbital axis caused the planet to tilt more strongly than it now does, creating ice age conditions. When the tilt again changed, the glaciers' ice turned to water vapor, but the depressions lingered.
(See photos of the red planet.)
Similar conditions can be found in Antarctica's Dry Valleys, Marchant added.
Called "beheaded" glaciers, these features have debris-covered glaciers on the lower parts of mountain slope. But higher up the glaciers have depressions as much as 160 to 250 feet (50 to 75 meters) deep where the ice has dissipated into water vapor.
On Mars, snow could have fallen relatively recently on the crater rim, drifting into the depressions.
"You don't need much snow to create a gully," Marchant said. "The little bit that falls is transported to favored areas. There it can melt. When it does so, it does so quickly and creates enough meltwater to carve gullies."
The slope and shapes of the debris fans at the base of the gullies indicates that they are geologically quite young, he added.
"So these things fit into a relatively young time on Mars history perhaps even almost up to the present."
Thus Mars is by no means a dead planet, he said: "It undergoes climate cycles much like Earth."
(Related: "Mars Water Discovered, 'Tasted' by Lander -- A First" [August 1, 2008].)
James Head III of Brown University led the new study, which will appear tomorrow in the journal Proceedings of the National Academy of Sciences.
He said by email that deposits in and around these gullies would be a great place to someday send a rover.
(Related: "Mars Rover Sets Out on Risky Crater Mission" [June 29, 2007].)
"They may be one of the few places where water is periodically present in the recent past," he said. "This could have implications for biology."
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