When it comes to searching for microbes on Mars, sending a robotic rover to the most arid environment on Earth is a fine place to start. And, should we find these single-celled organisms on the red planet, scientists have an idea as to what we might name them.
As described in a February 2019 study published in the journal Frontiers in Microbiology, a team of researchers explored the extreme environment of Chile's Atacama Desert. They wanted to develop strategies that future robotic explorers could use to seek out the hiding places of Martian microbes.
In 2020, both NASA and the European Space Agency will launch their first life-hunting rovers to the red planet (the Mars 2020 and ExoMars rover missions, respectively), so mission managers will need to know where to look.
Mars ... on Earth
The Atacama Desert is about as extreme as it gets for life to eke out an existence. Not only is the region bone-dry — the core of the desert doesn't get any rainfall for decades – because of its elevation, it also receives high levels of damaging ultraviolet radiation. Plus the soil is extremely salty. These factors should make the Atacama Desert toxic for life, but according to team leader Stephen Pointing, a professor at Yale-NUS College in Singapore, some of the bacteria species just below the surface "survive right at the limit of habitability." And this is very good news for the prospect of finding microbes on Mars.
Pointing's team deployed an autonomous rover-mounted drill and sampling device in the Atacama Desert to see if it could extract soil samples containing microbes down to a depth of 80 centimeters (a little over 2-and-a-half feet). As a comparison, samples were also dug up by hand. Through DNA sequencing, the researchers found that the bacterial life in the samples from both methods were similar, confirming these hardy bacteria exist, and that the autonomous extraction method was successful. This test run shores up hope that if microbes also thrive just below the Martian surface, a robot can find them.
"These results are a cause for optimism that bacterial life could tolerate the conditions of the Martian subsurface," Pointing says. But, finding microbial biosignatures on Mars, he cautions, could be very challenging for a remotely operated Mars rover as they found that the subsurface population of bacteria were extremely patchy, correlating with increased salt levels that restricted the availability of water for the microbes to have access to.
"The patchy nature of the colonization suggest that a rover would be faced with a 'needle in a haystack' scenario in the search for Martian bacteria," he says.
Previous studies have described the ubiquitous population of "relatively unremarkable" photosynthetic bacteria (microorganisms that get their energy from sunlight) that populate the surface of the desert. Things start to get a lot more interesting— and, indeed, more alien— just below the surface, Pointing adds.
"We saw that with increasing depth the bacterial community became dominated by bacteria that can thrive in the extremely salty and alkaline soils," he says. "They in turn were replaced at depths down to 80 centimeters by a single specific group of bacteria that survive by metabolizing methane."
These specialized microbes have been found before in deep mineshafts and other subterranean environments, but they've never been seen beneath the surface of an arid desert. "The communities of bacteria that we discovered were remarkably lacking in complexity, and this likely reflects the extreme stress under which they develop," Pointing says.
Finding highly specialized microbes that can thrive in the extremely dry, salty and alkaline Mars-like soils in the Atacama Desert suggest methane-utilizing bacteria could also thrive on the red planet.
If you recall the kerfuffle about the discovery of elevated levels of methane observed on Mars by various spacecraft over the years (most recently, measurements made by NASA's Curiosity rover), you'll understand why Mars methane is a big deal. On Earth, biological and geological processes generate methane, and, in turn, microbes can metabolize methane for energy.
The discovery of methane in the Martian atmosphere could mean there is some kind of active biology going on underground. To confirm this, we need microbe-seeking missions that will drill below the surface — and now we have a strategy to track them down.
What to Call Martian Microbes
Should microbial life be found on Mars, it would undoubtedly be the most significant scientific discovery in human history. But, in the proud human tradition of naming new things, what would we call our newly discovered Martian neighbors? Would we just copy the system of how we name life on Earth?
"The way we assign Latin names to [terrestrial] bacteria is based on their evolutionary relationship to each other and we measure this using their genetic code," says Pointing. "The naming of Martian bacteria would require a completely new set of Latin names at the highest level if Martian bacteria were a completely separate evolutionary lineage – that is they evolved from a different common ancestor to Earth bacteria in a 'second genesis' event."
Granted, if we find the genetic code of Mars life to be similar to Earth life, it could be that life was transferred from Earth to Mars in the ancient past via a massive impact – a mechanism known as panspermia — but if we truly find a novel genetic code that emerged on Mars, the implications for our understanding of life would be profound.
Pointing concludes: "If we find truly 'native' Martian bacteria I would love to name one, and call it Planeta-desertum superstes, which translates in Latin to 'survivor on the desert planet.'