Many scientists believe that the Red Planet might be harbouring microbial life beneath its polar ice caps or even in the puddles of salt water, which might be located underground.
But just as all other live organisms, microbes need three things: nutrients, water and a reliable energy source.
According to the biologist Gary King from Louisiana State University, the latter could be supplied in the form of a scentless atmospheric gas, abundant in the planet’s atmosphere – carbon monoxide (CO).
His recent study, published in the Proceedings of the National Academy of Sciences, has shown that enough of the gas seeps into Mars’s soil from the atmosphere to nourish microbes capable of utilizing it as an energy source. King claims this mechanism both could have been active in the past and is still operational to this day.
“This is a very important piece of work for Mars astrobiology,” said 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.”
As for the remaining two key ingredients of life as we know it, McKay claims they’re easy to account for. “Nutrients aren’t really an issue. 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.”
While CO is toxic to most organisms on Earth, some microbes use it to drive their metabolism, gaining energy by oxidising the substance into carbon dioxide (CO2). In simple terms, this means reattaching an oxygen atom to the gas, thereby transforming it back into CO2 – a reaction that produces a burst of usable energy. One such microbe, called Alkalilimnicola ehrlichii, was discovered in 2007 in California, USA.
The reason why this mechanism hasn’t already been more thoroughly explored is the planet’s thin atmosphere, dominated by CO2 (around 95%), which is not a viable source of energy.
King set out to determine whether Earth’s microbes could indeed utilize CO under conditions similar to those found on Mars – low pressure, high CO2 concentrations, low oxygen levels, and low to moderate temperatures.
In order to accomplish that, he took several soil samples from three different salty systems on Earth – the Big Island of Hawaii, Chile’s Atacama Desert and the Bonneville Salt Flats in Utah – that closely resemble the conditions of Mars.
After simulating subsurface ultra-salty brines like the Red Planet would have, he found that an abundance of CO did in fact seep into the soil, more than enough to support a large number of microbes.
Unfortunately, neither the Curiosity, nor the Opportunity rover is equipped with tools to detect life and located anywhere near the areas where such life could exist (these areas are called Recurring Slope Lineae, or RSL – seasonal dark streaks some scientists think are caused by salt water at or near the surface).
King believes his findings are relevant to the ongoing exploration of the Red Planet and its possible future colonization, as CO-oxidizing microorganisms might some day be used in efforts to terraform Mars into a place more hospitable to human life.
“In order to develop any kind of a soil system that could support anything complex, you would have to have a complex microbial community,” he told Space.com.
“You would need a variety of biosynthetic capabilities. You would need a variety of different elemental transformation capabilities — maybe nitrogen fixers,” he added. “These halophiles (salt-loving organisms) would be part of that.”