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 CO2 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 CO2 atmosphere is made up of molecules of hydrogen or methane it could make all the difference. When these gases collide with CO2 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 be needed.
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