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By Ken Croswell
Published on New Scientist (November 21, 2016)
Credit: Spirit. NASA/JPL/Cornell University.
A dash of hydrogen or methane in the atmosphere could have kept Mars warm enough for water to flow.
Ever since the 1970s, we�ve known that chilly Mars must have once been warm enough for rivers. But we�ve struggled to explain how a world much farther from the Sun than Earth is could get so warm � especially at a time when the Sun was dimmer.
Today, the thin Martian atmosphere is mostly carbon dioxide, which is a greenhouse gas, but it traps little heat. Models suggest that even a thick carbon dioxide atmosphere would not have lifted ancient Mars�s temperature above the freezing point.
Now Robin Wordsworth of Harvard University and his colleagues have calculated that if just a few per cent of a mainly carbon dioxide atmosphere is made up of molecules of hydrogen or methane it could make all the difference. When these gases collide with carbon dioxide molecules, they absorb light in a key wavelength range, which allows the planet to retain enough heat that water can flow.
�It�s really exciting,� says James Kasting at Pennsylvania State University in University Park, whose own team has previously calculated that much more hydrogen than this would have been needed.
�We had to wave our arms a lot to justify that much hydrogen in the atmosphere,� Kasting says. �This new paper allows that same hypothesis to work with a lot less arm waving.�
As to whether it was hydrogen or methane that did the actual warming, Kasting says it probably depends on whether Mars ever spawned life. If it did, hydrogen-eating bacteria may have converted much of the atmospheric hydrogen into methane.
Ken Croswell is an astronomer and the author of Magnificent Mars.
"Ken Croswell has done a superb job in outlining what is known, and what is suspected, about the next home of mankind beyond the Moon."--Sir Arthur C. Clarke. See all reviews of Magnificent Mars here.
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