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Self-imposed climate change may have killed Martian life

Hungry microbes blamed for chilling out the party too much

Microbial life may have flourished on early Mars but those early Martians may have been the cause of potentially life-ending climate change on the Red Planet.

The early Martian atmosphere was most likely rich in carbon dioxide and hydrogen, and if the first microbes on Mars behaved like those on early Earth, gulping CO2 and H2 and expelling methane, the researchers found an effect akin to, but opposite from, Earth's warming climate: Mars froze instead. 

"According to our results, Mars' atmosphere would have been completely changed by biological activity very rapidly, within a few tens or hundreds of thousands of years," said Dr Boris Sauterey, co-author of the research which was published in Nature. "By removing hydrogen from the atmosphere, microbes would have dramatically cooled down the planet's climate."

Researchers at the University of Arizona, The Sorbonne, and several other French institutions say their climate models show that Mars during the Noachian era (between 4.7 and 3.1 billion years ago) could have seen as much biomass productivity as early Earth oceans, where life was plentiful. 

During that era, Sauterey told, Mars' temperature may have spanned between -10 and 20 degrees Celsius (14 - 68 degrees Fahrenheit), "But its atmosphere was quite different from that of Earth. It was as dense, but richer in carbon dioxide and hydrogen, both of which acted like potent warming gasses." 

University of Arizona professor Regis Ferrière, one of the project's leaders, said that the team's goal was to create a model of Mars' crust based on known data about its composition and the Martian atmosphere to see if, as atmospheric gasses diffused into the ground, they would be enough to support life.

"And the answer is, generally speaking, yes, these microbes could have made a living in the planet's crust," Ferrière said. The team further found that Martian microbes would have been happiest "in the upper few hundred meters" of dirt. 

As Martian methanogenic microbes continued to replace hydrogen and carbon dioxide with methane, the researchers said that Mars' atmosphere thinned, likely within just a few tens or hundreds of thousands of years. 

Whether or not life remains on Mars has yet to be decided, but it would have had to adapt to find a new energy source and would have had to go much further underground to avoid a freeze. "For the moment, it is very difficult to say how long Mars would have remained habitable," Sauterey said. 

Earth is nice - why's Mars ice?

If the addition of tons of methane cooling Mars' atmosphere sounds counterintuitive, that's understandable - after all, it is cited as one of the biggest causes of climate change on Earth, with 80 times more warming power than carbon dioxide, at least in the short term. Why would its addition to the atmosphere, if not warm Mars, at least keep the temperature steady?

Ferrière said that all has to do with hydrogen. "On ancient Mars, the atmosphere was rich in both CO2 and H2, and the collision of these molecules turned H2 into a very powerful greenhouse gas - much more so than on Earth," he told The Register

As our Martian neighbors consumed their planet's hydrogen, methane simply couldn't keep up, and neither could life on Mars. 

NASA scientists behind the Mars Atmosphere and Volatile Evolution (MAVEN) probe sent to Mars eight years ago have already explained why the atmosphere of the Red Planet is so thin: Its core froze, causing Mars' magnetic field to dissipate. Without the protection of a magnetosphere, Mars has been hammered by solar winds and seen the bulk of its atmosphere stripped off.

These latest simulations complement MAVEN's results, Ferrière told us, giving a potential answer to the "why" of the loss of Mars' atmosphere: The planet's core cooled because of carbon-hungry microbes that stripped the planet of its greenhouse gas blanket. 

The team behind the microbe modeling said that future Mars missions will be needed to verify their findings, but unfortunately one of their ideal test sites - Hellas Planitia - is also one of the most wind-swept, dusty and inhospitable regions of the planet - even to robot explorers. ®

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