Theory: Carbon dioxide, not water, formed Mars canyons

Satellite image of Mars' largest canyon, Vallis Marinaris

By Richard Stenger
CNN.com Writer

(CNN) -- Colossal canyons across Mars formed eons ago by massive floods of carbon dioxide and solid debris instead of liquid water, according to a controversial report this month.

The theory suggests that the red planet experienced cold and dry conditions for most of its geological history, a scenario that would reduce the likelihood it ever harbored Earth-like life forms.

Planetary scientists supporting the more commonly accepted view that water erosion created many martian features disputed the new hypothesis, published in the August edition of the journal Icarus.

According to geologist and report author Nick Hoffman, when craters with steep walls and other unstable terrain collapsed, large amounts of underground liquid carbon dioxide were released.

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The quick drop in pressure vaporized some of the liquid, just like carbon dioxide released from the valve of a fire extinguisher. The freed gas then formed clouds of gaseous and frozen carbon dioxide, mixed in with water ice, dust and rubble.

The resulting mixture, which included some liquid carbon dioxide as a lubricant, transformed into a chaotic avalanche, capable of traveling down slope for up to thousands of kilometers, and carved out deep and long canyons on the surface, said Hoffman, a professor at La Trobe University in Australia.

Kenneth Tanaka, a planetary scientist with the U.S. Geological Survey, said the hypothesis seemed sound.

"I think it's a valid theory. I tend to side more with (Hoffman) than some of the other Mars scientists that are more enamoured with liquid water," he said.

In fact, Tanaka suggested that a carbon dioxide mixture formed red planet features like gullies and washes in the recent geological past rather than liquid water, as proposed last month by scientists studying close-up images taken by the Mars Global Surveyor orbiter.

Other red planet researchers disputed Hoffman's findings.

"His physics are good. It makes sense chemically. I'm just not sure the geological evidence supports it," said Aaron Zent of NASA's Ames Research Center.

Look downstream of the canyons where the erosion agent stopped flowing and one finds flat expanses and evidence of pooling and coastlines, Zent said.

"I think the explanation (Hoffman) invokes would not make a shoreline and puddle. You need water for that," he said.

Hoffman remains adamant. If Mars once had a significant amount of surface water, then atmospheric carbon dioxide should have dissolved into rocks to form carbonates, but decades of orbiting and surface surveys have turned up scant traces of them, he said.

The theory has serious implications in determining whether or what kind of life the red planet might have harbored.

"If life evolved on Mars then it did not do so in a warm surface ocean, but deep underground under high pressure, hidden in the rocks," Hoffman said in a statement.

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White Mars
Icarus Journal of Solar System Studies
La Trobe University
NASA Ames Research Center

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