Mars' southern polar ice cap is believed to erupt in a violent fit of heated gas every spring in a process that helps explain why the Red Planet has dark spots in that region, scientists said Wednesday.

Jets of carbon dioxide gas burst from the ice cap as it warms every spring, carrying dark sand and dust that fall back to the surface as dark splotches, concluded Phil Christensen of Arizona State University, who analyzed images from NASA's Odyssey orbiter.

"If you were there, you'd be standing on a slab of carbon dioxide ice," he said in a statement.

But gas builds beneath the slab, raising it, and bursts through vents in weak spots, he concluded.

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For years, scientists have puzzled over the origin of the dark spots, which range from 50 feet to 150 feet wide. The spots only appear in the spring, last for several months, and then restart the cycle.

Scientists have thought the spots were part of the ground revealed as the ice disappeared.

But Christensen, using the Odyssey camera, found that the spots were as cold as the carbon dioxide ice, suggesting they were a layer of dark material that covered the ice.