Many scientists believe tiny Martian microbes could flourish beneath the red planet's ice-cemented polar caps, or even in briny puddles of ultra-salty water—which they believe may be locked under Mars' soil in seasonal flows. But here's one important question about the possibility of life on Mars: What the heck does it eat?

According to Gary King, a biologist at Louisiana State University, the surprising answer could be a scentless, atmospheric gas: carbon monoxide. In a new study in the science journal PNAS, King has concluded that enough of the gas seeps into Mars's soil from the planet's atmosphere to feed hearty lifeforms. Such organisms which could look like Alkalilimnicola ehrlichii, a carbon monoxide-munching microbe found in California in 2007.

"We now know of an energy source for microbial systems that could exist anywhere near the surface."

"This is a very important piece of work for Mars astrobiology," says Chris McKay, an astrobiologist at NASA who was not involved in King's research. "What this research means is that we now know of an energy source for microbial systems that could exist anywhere near the surface of Mars."

The three things you need

As McKay explains it, if you're going to have Earth-like life on Mars today, then you need to account for three things: nutrients, water, and energy. "Nutrients aren't really an issue," he says. "Mars has an abundance of carbon dioxide, nitrates, atmospheric nitrogen, and small amounts of many other nutrients. As for water, the theory that there could be these brines [of saltwater under the soil] has been around for years… What has always needed serious explanation is a potential source of energy. Now we have one."

For the most part, scientists had discounted the idea that hypothetical Martian microbes could get their energy from the planet's atmosphere. The reason is simple: Mars's atmosphere is incredibly thin and dominated by carbon dioxide, which is not a viable energy source. Only a tiny sliver of Mars's total atmosphere is carbon monoxide. It's created as sunlight breaks an oxygen atom off from atmospheric carbon dioxide.

"This is something that's at our fingertips."

In this study, King took many different samples of Earth soils that scientists believe to be Mars-like (such as Hawaiian volcanic ash and salt flat sands of Utah). When he used them to simulate subsurface ultra-salty brines like Mars would have, the team found that an impressive amount of carbon monoxide was able to seep in, more than enough to support plenty of growing microbes.

How to eat carbon monoxide

If you've heard of carbon monoxide, a simple two-atom gas, it's probably from warning about how you should have a detector in your house because it's fatal to humans. So how can a microorganism get its energy from this stuff?

King explains that we already know of organisms on Earth that can get energy from gas via a process called oxidation. In simple terms, it means reattaching an oxygen atom to the gas, which transforms it back into carbon dioxide. In doing so, the reaction releases a burst of energy. King says scientists already know of several compounds in the Martian soil that the plucky hypothetical microbes could use to oxidize their carbon monoxide, such as the molecule perchlorate.

Sadly, we have no current way to test whether King's team is right. Neither the Curiosity nor the Opportunity rover is equipped with the tools to detect life. They're also not near the areas where such life could exist, and can't travel there. (Our current best guess is a Martian feature called the recurring slope lineae, a seasonally ebbing swath of land identified in 2011 by the Mars Reconnaissance Orbiter.)

But NASA is currently evaluating a proposed mission called Icebreaker Life that would be sent in 2021 and would be able to detect microorganisms. McKay and King agree that this new research provides all the more reason to go forward with such a mission.

"Mars is a planet that's really no longer just a part of our imagination. This is something that's at our fingertips," says King.

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