About 3.5 billion years ago, two of the planets that orbited the sun may have had biospheres of similar bulk. One, Earth, evolved in a way that allowed life to flourish and splinter into endless forms most beautiful. Mars, the other world, followed a different path.
Today the Martian surface is hostile to life as we know it, but as this scientific story goes, Mars may have once hosted a rich abundance of microbes. Residing in the planet’s briny underworld and shielded from the lethal radiation that bathes the surface, these organisms could have grown in nooks and fissures, multiplying until their collective heft rivaled Earth’s cache of life. Called methanogens, Mars’s microbes would have inhaled atmospheric hydrogen and carbon dioxide and exhaled methane gas—and in a twist, they may have turned out to be their own worst enemy.
Over time, their growing, insatiable appetite would have robbed the Martian atmosphere of hydrogen—a powerful greenhouse gas during the planet’s early days—ultimately casting a deadly freeze over the planet and driving microbial populations into deeper, warmer crannies. How long those burrowing microbes could have survived in the deep is unknown. It’s possible they were only a short-lived flash of life on an otherwise sterile world.
“Maybe extinction is the cosmic default of life in the universe,” says Boris Sauterey of the Institut de Biologie de l’Ecole Normale Supérieure in Paris. “It’s not the process of life appearing that is limiting; it’s life maintaining itself that is limiting.”
But perhaps, more than 30 feet beneath the surface and encased in ice, these ancient single-celled organisms achieved a state of dormancy—a sort of cryopreserved slumber, ready to perk up when more life-friendly conditions arise.
The Martian interior may not be as lifeless as its face. It could host a world of alien organisms that are capable of waiting thousands of years between each turn of their metabolic engines.
This scenario may sound farfetched, but recent results from scientists modeling the habitability of ancient Mars and studying the hardiness of microbes in labs and beneath our own planet’s surface all point in the same direction: It’s a long shot, but it’s possible life evolved on Mars and still exists . And scientists may just find signs of that life when meteors barrel into Mars and excavate buried layers of ice, or when new spacecraft arrive to plumb this underground realm.
“I wouldn’t put it past the Martian microbe to beat the odds and survive for an extended period,” says Amy Williams of the University of Florida. “Whether it’s still there today, I can’t hazard a guess. But as an astrobiologist, my hope is that it is, and that maybe that knowledge can help us have a deeper appreciation for our place in the universe.”
A temperate Mars of the past
Dry and irradiated, the Martian surface would challenge even the hardiest of Earth’s microbes to survive for more than a moment.
Billions of years ago, though, the planet was warmer and more watery. It’s not clear how long those temperate conditions persisted or exactly how much water there was, but it is clear that ancient Mars contained all the ingredients for life as we know it, including water, carbon-containing organic compounds, and active chemical reactions that provide energy.
Which is why Sauterey, a computational ecologist, decided to see just how habitable early Mars might have been. Previously, his team developed models to characterize how Earth’s early life influenced the planet’s surface conditions some 3.5 billion years ago, when Mars may have been habitable as well.
As described in a paper published in Nature Astronomy, Sauterey and his colleagues considered multiple models of Mars with different atmospheres, surface temperatures, and types of brine, which have different freezing points. They assumed that any organisms populating the planet would have been the sort of hydrogen-gobbling, methane-producing microbes that also populated early Earth—and they assumed that such microbes would be limited to environments at least 10 feet beneath the Martian surface, where life-sustaining brines are plentiful and radiation is not.
The team found that both surface temperature and the type of brine play a crucial role in determining the likelihood of habitability. In the team’s simulations, habitable subsurface environments were less likely to exist on a colder, more ice-covered planet because glaciers limit the amount of hydrogen gas that can reach the subsurface to fuel alien metabolisms. But on a warmer and less icy world—in its most life-friendly form—Sauterey found there was at least a 50 percent chance that swaths of the shallow subsurface were habitable billions of years ago.
“Our result is that Mars, if it was not fully ice-covered, was likely habitable,” he says. “That does not mean it was probably inhabited, because we don’t know how you switch from habitability to inhabitation.”
The team also mapped the most likely habitable subsurface sites on the planet and found that Hellas Planitia—a sweeping impact basin in the planet’s southern hemisphere—could support life in all but the worst of circumstances. Isidis Planitia and neighboring Jezero Crater, where NASA’s Perseverance rover is currently collecting samples for return to Earth, were also among the more habitable locations.
Sauterey and the team then simulated how the burgeoning Martian methanogens might have impacted their environment. They were surprised to find that life on Mars could have been a casualty of its own existence, draining the atmosphere of planet-warming hydrogen; Earth escaped that fate because of the different mix of gases in its atmosphere.
“To an extent, we expected to find that Mars was habitable to these types of organisms,” Sauterey says. “We did not expect to find the opposite influence of life on planetary habitability—that, if that type of life existed on Mars, it would have actually deteriorated the habitability of the planet.”
Sauterey and his colleagues suggest that as they altered the planet’s climate, these doomed Martian microbes may have moved even farther underground, where it’s warmer and more hospitable, but less energy-rich.
Jackie Goordial, a microbiologist at the University of Guelph in Ontario who studies microbes in the permafrost at Earth’s poles, says that life may have been even more likely to exist than the models suggest because Sauterey and his colleagues used somewhat conservative definitions of habitability.
For example, the team used minus 20 degrees Celsius (minus 4 Fahrenheit) as the lowest temperature at which life could survive; on Earth, Goordial says, scientists have observed microbes surviving at colder temperatures. Sauterey’s team also assumed that ice cover would limit the extent of habitable environments by limiting access to atmospheric gases, but on Earth, microbes can feed on hydrogen produced underground, suggesting Martian microbes could exist farther underground than the simulations assume.
“There’s a whole community of scientists who do nothing but look at life being cut off from the atmospheres—and it exists,” Goordial says. “It’s weird life. It’s really cool and it’s certainly applicable to Mars.”
Another team of researchers approached the question of Martian life in a different way: by seeing how long microbes could survive in conditions mimicking those roughly 30 feet beneath the surface. At that depth, the level of incoming solar and cosmic radiation is about the same as the dose sustained on Earth’s surface—but the soils are frozen and dry.
The team chose to study a bacterium called Deinococcus radiodurans—one of the most famous extremophiles, known for its ability to withstand immense doses of radiation. Found in nuclear reactors as well as Antarctic soils, D. radiodurans survives by quickly repairing radiation damage to its DNA.
“The fact that we have these things on Earth—the fact that radiodurans is found in nuclear reactors—is crazy. We didn’t have [reactors] until not even a hundred years ago,” says the University of Florida’s Williams, who was not involved in the new research.
In liquid culture, D. radiodurans can survive a dose of approximately 25,000 Gray (Gy); in contrast, just 5 Gy will kill humans and most other vertebrates.
The team studying D. radiodurans found a way to make the critter even more extreme, as described in a study published in the journal Astrobiology. First they dried out a culture of D. radiodurans. Then they froze it, mimicking the cold, desiccated state beneath Mars—which caused the culture to enter a dormant state. When they challenged the sleeping bacteria with increasing doses of radiation, they found that cells in suspended animation could withstand a dose of approximately 140,000 Gy.
“That’s really an enormously big number; it’s astronomical,” says lead researcher Michael Daly of the Uniformed Services University in Maryland. “One would expect that microorganisms that evolved on Mars are as resistant—if not more resistant—to radiation than D. radiodurans, which evolved on a relatively mild planet called Earth.”
The team repeated the same experiment with five less robust microbes, including E. coli and Saccharomyces cerevisiae (brewer’s yeast), and found that desiccation and freezing similarly increased the cells’ tolerance to radiation—although they still couldn’t tolerate anywhere near the level of exposure as D. radiodurans.
When Daly and colleagues calculated how long a single cell of D. radiodurans could survive roughly 30 feet beneath Mars’s surface, they came up with a startling number: It would take nearly 280 million years to destroy the cell. That number applies to cells in a dormant state, but over time, multiple warming events—like meteor strikes—could temporarily transform the subsurface environment and revive the cells, providing an opportunity for them to reanimate and replicate.
Researchers have observed similarly extreme lifecycles in microbes buried deep beneath Earth’s surface, and scientists have been able to retrieve viable microbes from ancient permafrost cores. Models of life in deep sea sediments also suggest organisms may be able to survive with very little energy input, Goordial says.
“We think these microbes exist in a really slowed down state of metabolism. Perhaps their cells are replicating once every 10,000 years,” she says. “We see this on Earth—although it’s very difficult to study directly … Could something similar be happening in the subsurface of Mars?”
If microbes are there, they’re buried too deeply for current technologies to find. The Perseverance rover’s drill bores less than four inches deep; Daly and his colleagues calculated survival times for microbes 100 times deeper.
In the future, scientists hope to deliver spacecraft to Mars with deeper drilling capabilities. One of those missions, the Mars Life Explorer, was recently ranked among the highest priorities in U.S. planetary science for the next decade—although it wouldn’t launch until the 2030s at the earliest.
Or maybe scientists will get lucky sooner. A recent meteorite impact, detected by NASA’s InSight lander and Mars Reconnaissance Orbiter, gouged a hole in the planet’s crust and launched boulders of previously buried water ice onto the planet’s surface—which Daly says is exactly the type of material he’d want to investigate for the presence of dormant microbes.
“While I don’t expect life to ever walk up to one of our rovers on Mars,” Williams says, “I don’t want to underestimate the ability of life to find a way.”