Mars may have had microbes similar to early life on Earth

In the 2014 science fiction thriller The Signal, three friends from MIT become embroiled in an increasingly bizarre interaction with an alien life form which ends up even stranger than it first appears. While The Signal takes its narrative in some unconventional directions, it’s the sort of story which we keep telling ourselves over and over. What might it be like to finally encounter aliens?

The universe is so large that it defies comprehension. For many, it’s a foregone conclusion that there must be other life out there, in our own galaxy or beyond it, and we can’t help but imagine what it might be like to meet them face to face. So far, we don’t have any reliable evidence of intelligent life elsewhere in the cosmos. In fact, we don’t have evidence of any life, intelligent or otherwise, anywhere besides our own lonely planet.

It’s possible, perhaps even likely, that we just haven’t looked in the right places or in the right ways. As our methods and tools improve, and as we peer more deeply into our own universe, we might find that life has sprung up more than once in our very own solar system.

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A recent study published in the journal Nature Astronomy provides new evidence that the ancient conditions on Mars may have been suitable for microbial life, at least for a little while. Boris Sauterey, a researcher from the University of Arizona and the Université Paris Sciences et Lettres, and colleagues, modeled the red planet’s historical environment and found that it might have been perfectly suited for the sorts of life which got their start on our own planet billions of years ago.

The story of the proposed Martian microbes begins 4.1 billion years ago during Mars’s Noachian period. At the time, Mars would have been a dramatically different place than it is today.

“Even though we know some stuff about how Mars was [during the Noachian], it’s still very uncertain. What we know almost for sure is that there was liquid water flowing on the surface around this time. That means necessarily it had to have a relatively warm climate to allow liquid water to flow. The only way we can reproduce such a warm climate with the models at our disposal is to assume the atmosphere was much denser than it is today,” Sauterey told SYFY WIRE.

Today, the atmosphere in Mars is roughly 95% carbon dioxide and researchers believe it would have been largely carbon dioxide in the past as well. There just would have been a lot more of it, there also might have been a bunch of dihydrogen. Sauterey explained that something interesting happens when you introduce dihydrogen into a carbon dioxide-rich environment.

“If you put dihydrogen into it, the two types of molecules interact with one another and create a greenhouse gas. The combination of carbon dioxide and dihydrogen could have allowed for warm climates,” Sauterey said.

CO2 and H2 would have done more than just heat up the place, however, they are also the chemicals which some microbes consume for energy. According to the study, an atmosphere rich in those two chemicals would have not only made Mars comfortable enough to live on, it also would have provided an ample food source for microbes. The kinds of microbes researchers were interested in are methogenic hydrogenotrophs. In other words, microbes which eat hydrogen and produce methane as a waste product.

If these methane-making microbes did, in fact, exist on Mars in the distant past, researchers imagine they would have lived inside the planet’s crust. Warmer temperatures and free flowing water are a couple of big steps toward habitability, but the surface temperatures on the red planet would have remained pretty harsh.

“Cosmic radiation and UV would have potentially killed anything living on the surface. The crust would have provided a shield from those harsh surface conditions,” Sauterey said. “Another factor is temperature, even if there were relatively warm conditions on Mars, it still would have been cold. Potentially, in order to find the right temperature, they might have gone deeper into the crust.”

Martian microbes would have been trapped between two competing forces. If they traveled too close the surface, they’d be at risk of damage from radiation. If they went too deep into the crust, they’d lose out on the chemical food they'd need to survive. Consequently, they might have found nice little bands a few meters or a few kilometers beneath the surface where conditions were just right. Sadly, the good times were destined to end.

“We know that Mars was likely habitable at some point, but now it’s really not. In between, Mars lost most of its atmosphere, most of its liquid water, and its climate got super cold. At some point, habitability on the surface was lost,” Sauterey said.

If we assume that this primitive microbial biosphere existed on Mars, they may have signed their own death warrant without even knowing it, and it all goes back to the atmosphere. The paper demonstrates that a microbial population would have fed on dihydrogen and carbon dioxide and produced methane as a biproduct. On Earth, methane is a powerful greenhouse gas but on Mars during the Noachian it would have been small potatoes.

“Dihydrogen was a super potent warming gas on Mars. They would have removed the gas that was warming the climate, replaced it with another gas that also warmed the planet but not as much, and the climate would have cooled down quite significantly,” Sauterey said.

Over time, the amount of ice on the surface of the planet might have increased until Mars was nearly a solid snowball. That ice shell would have reduced the amount of dihydrogen and CO2 getting underground, separating the microbes from their food supply. In short, they would have caused their own demise and that might be a more common result in the universe than we think.

“It could be that life just screws things up. As soon as it shows up on a planet it actively deteriorates its planetary environment. Life itself might be the limiting factor for the [prevalence] of life… the question is what is most representative of what happens in the universe. Is it what happened on the Earth, where life successfully maintained itself for billions of years or is it something more like what we describe for Mars?” Sauterey said.

The answer to that question remains to be seen, but we might get a chance to settle the debate over Martian life in the near future. The team also modeled pockets of the planet where populations of microbes might have persisted — if indeed they ever existed — and one of them just so happens to be at Jezero crater where the Perseverance rover is collecting rock samples. If there was ever life there, just beneath the surface, we might find signs of it when we recover Percy’s rock samples in 2030.