How Greenhouse Gases Saved Mars

The same heat-trapping processes that are causing so much trouble on Earth once made Mars a very pleasant place

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The idea of water flowing on the surface of Mars went out with the early 20th century astronomer Percival Lowell, who swore up and down — and incorrectly — that he could see canals on the Red Planet. But it came roaring back during the Space Age, when NASA’s Mariner 9 probe spotted what looked uncannily like dry river valleys etched across Mars’ face. Generations of orbiters along with rovers like Opportunity and Curiosity have long since sealed the case. Mars had copious surface water billions of years ago, which means the planet also had a thick, atmosphere — and perhaps even the right conditions to nurture life.

What that atmosphere was made of, however, has long been a matter of dispute. The usual suspects — carbon dioxide and water vapor for starters — seem obvious, since they would have kept the planet warm enough to allow water to survive. But the greenhouse gasses that heat up the modern Earth wouldn’t have had the same effect on a smaller Mars in an earlier solar system. Now, planetary scientists writing in Nature Geoscience think they’ve found the special sauce that helped keep Mars toasty: massive quantities of hydrogen gas belched from Martian volcanoes. “I’ve been working on and off for 20 years to try and figure it out,” says co-author James Kasting, of Penn State. “But I managed to ignore hydrogen.”

Mars clearly needed some sort of heat-trapping help in its atmosphere back then. Not only is the planet half again as far from the sun’s warmth as Earth is, but 3.8 billion years ago, when Martian rivers flowed, the sun was only about three-fourths as bright as it is today. “This is something experts in solar evolution are very confident about,” says Kasting.

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When he started working on the problem back in the 1980’s, Kasting, like most everyone else, assumed CO2 was the key. What he and other scientists only realized later was that the dimmer sun would have caused the gas to condense and fall as dry-ice snow. It wouldn’t have lasted long enough in the atmosphere to warm much of anything.

With carbon dioxide off the table, scientists proposed a different scenario: first, the bombardment of comets and asteroids that cratered the moon and Mars early in the solar system’s lifetime brought in huge amounts of ice. The ice evaporated in the heat of impact, adding a burst of water vapor to the atmosphere — another powerful greenhouse gas. Some of that water would have rained out to create the rivers and lakes whose ancient presence is still etched into the Martian surface. That’s probably true as far as how at least some of the water arrived on the planet is concerned, but the rest?

“We think the amount of water you’d get that way falls short by a factor of 10,000,” says Kasting. He didn’t have a better answer, however, until a few years ago. “Someone asked me at a conference, ‘Have you thought of hydrogen?’ and I said, ‘You can’t do it with hydrogen.’”

(PHOTOS: Mars Like You’ve Never Seen It Before)

That was before he knew about research showing that, actually, you could, including papers by the University of Chicago’s Raymond Pierrehumbert and Caltech’s David Stevenson — the latter of whom has argued that an Earth-size planet with a hydrogen atmosphere could stay warm even if it escaped its own solar system to wander among the stars. Hydrogen is transparent to incoming solar light but blocks much of the resulting heat from escaping the same way other greenhouse gasses do. On Earth we rarely see its effects since free hydrogen in our atmosphere combines too easily with other molecules.

But Mars’s particular geochemistry, Kasting agues, would have caused its volcanoes to produce all of the hydrogen the planet needed to stay warm, and its atmospheric chemistry could have allowed the gas to survive. There would have been plenty of the stuff to have boosted the atmosphere’s heat-trapping capacity and kept water in liquid form long enough to have sculpted the features space probes are finding today.

(MORE: Enough Already. Does Mars Have Life or Not?)

And then the atmosphere went away. The reasons still aren’t fully clear, but that same bombardment that delivered water to the planet would presumably have blasted some of the atmosphere back into space — not so difficult, considering that Mars’ gravity is less than 40% as strong as Earth’s. Another factor: the solar wind, whose steady stream of outward-flowing particles could have stripped away the atmosphere’s outer layers.

That’s the conventional scientific wisdom, anyway, but planetary scientists should soon know for sure: NASA’s MAVEN space probe, launched Nov. 18 for a Sept. 2014 arrival, is designed to study what remains of Mars’ long-lost atmosphere for clues about exactly how and when it largely vanished — with the water that flowed on the Martian surface soon to follow.

(VIDEO: Watch MAVEN’s Prep and Launch)