Subterranean microbes may have populated ancient Mars

Ancient Mars may have had an environment capable of harboring an underground world teeming with microscopic organisms, French scientists reported Monday. But if they existed, these simple life forms would have altered the atmosphere so profoundly that they would have triggered a Martian ice age and wiped themselves out, the researchers concluded.

The results offer a murky view of the ways of the cosmos. Life — even simple life like microbes — “could actually often cause its own demise,” said the study’s lead author Boris Sauterey, now a post-doctoral researcher at the Sorbonne University. The results “are a bit bleak, but I think they’re also very encouraging,” he said in an email. “They challenge us to rethink the way a biosphere and its planet interact.”

In a study in the journal Nature Astronomy, Sauterey and his team said they used climate and terrain models to assess the habitability of Martian crust about 4 billion years ago, when the red planet was thought to be aquatic and much more hospitable was than today. They surmised that then-hydrogen-guzzling, methane-producing microbes might have been thriving just below the surface, with several inches (tens of centimeters) of dirt, more than enough to shield them from intense incoming radiation.

Anywhere on Mars that is free of ice could be teeming with these organisms, just like early Earth, Sauterey says. However, the presumably warm and humid climate of early Mars would have been jeopardized by so much hydrogen being sucked out of the thin, carbon dioxide-rich atmosphere, Sauterey said. When temperatures dropped nearly minus 400 degrees Fahrenheit (minus 200 degrees Celsius), any organisms on or near the surface would likely have burrowed deeper to survive.

In contrast, given the nitrogen-dominated atmosphere, microbes on Earth may have helped maintain temperate conditions, the researchers said. Kaveh Pahlevan of the SETI Institute said future models of Martian climate must take French research into account Mars was born wet with warm oceans that lasted millions of years. The atmosphere then would have been dense and composed mostly of hydrogen, serving as a heat-trapping greenhouse gas that was eventually transported to higher altitudes and lost in space, his team concluded.

The French study looked at the climate impact of possible microbes when the Martian atmosphere was dominated by carbon dioxide, so it’s not applicable to earlier times, Pahlevan said. “What their study makes clear, however, is that if (that) life had been present on Mars at that earlier time, “they would have had a major impact on the prevailing climate,” he added in an email.

The best places to look for traces of this past life? The French researchers propose the unexplored crater Hellas Planita, or plain, and Jezero crater on the northwestern rim of Isidis Planita, where NASA’s Perseverance rover is currently collecting rocks for return to Earth in a decade. Next on Sauterey’s to-do list: investigate the possibility that microbial life might still exist deep within Mars. “Could Mars still be inhabited today by microorganisms descended from this primitive biosphere?” he said. “If yes, where?”

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