Even the greatest Mars enthusiasts are reluctant to say that the Red Planet was ever able to cook up complex, multicellular life forms. Mars was a relatively warm, wet world for only about a billion years before it lost most of its atmosphere and water to space and became the frigid desert planet it is today. But on Earth, that first billion years was plenty of time for single-celled organisms to have emerged. Now, a new study in Nature Astronomy argues that something similar may well have happened on Mars.
Researchers suggest that microbes once lived both in bodies of water on the surface and in the Martian crust itself. What’s more, descendants of those original microbes may still be at large, driven deeper beneath the less-hospitable Martian surface, but alive and thriving all the same.
The new study, led by theoretical ecologist Boris Sauterey of Sorbonne University in Paris, does not rely on hands-on examination of Martian meteorites or new findings from NASA’s active Curiosity or Perseverance Mars rovers, but rather on a computer model Sauterey developed with colleagues at the Institut de Biologie de l’Ecole Normale Supérieure, in Paris, and other institutions. The model starts with the assumption that, as dry riverbeds and ocean basins on Mars reveal, the planet was once awash in water. What’s more, as the Spirit and Opportunity rovers—which landed on Mars in 2004 and went out of service in 2010 and 2019 respectively—originally discovered, Martian water was very high in salts. This high salinity helped keep the water liquid even at temperatures as low as -70.15ºC, (-94.27ºF).
“If you add salts to water, it lowers the freezing point,” says Sauterey. “The brinier the water, the colder it can get without freezing.”
All across Mars, even in some of its coldest regions, the presence of liquid water thus could have allowed the appearance of microbes similar to the earliest ones that emerged on Earth—ones which rely on a simple metabolism in which they consume two-atom hydrogen molecules (H2) and carbon dioxide from the atmosphere, and produce methane as waste. The microbes could have taken hold not just in the briny Martian water, but in the soil and deep beneath the surface, which may have been even more hospitable to life thanks to geothermal heating produced by subsurface magma and the decay of radioactive materials. The deeper the microbes went, the warmer the environment would get, with temperatures reaching 47ºC (116ºF). On modern-day Earth, hearty microbes known as extremophiles do perfectly well at temperatures even higher than that and ought to have done so on Mars as well.
But it turns out that when it comes to hydrogen-consuming microbes, there can be too much of a good thing. High H2 levels in the atmosphere have a planetary warming effect; the more of the gas the microbes extracted for nourishment, the lower Martian temperatures would have fallen. “In the context of the early atmosphere of Mars, hydrogen would be a potent warming gas,” says Sauterey. “The microbes would have pulled out a substantial fraction of Mars’ H2.”
Under the computer models, this could have led to a Martian deep freeze in which much of the planet was covered in ice. With the Red Planet turning into a white planet, Mars’ albedo—or its ability to reflect heat and light—would have risen, reducing the amount of sunlight that was absorbed by the surface, and creating a feedback loop that lowered temperatures even further. The H2-consuming microbes would, effectively, have committed a form of suicide—at least on the surface of the planet—sealed within the ice and cutting themselves off from the hydrogen and carbon dioxide they consumed from the atmosphere. Only isolated patches at or near equatorial latitudes would have remained free of ice, allowing gas exchanges with the atmosphere to continue.
Mars, however, was not destined to remain an ice world forever. More than three billion years ago, the planet lost its magnetic field, which allowed the solar wind to strip away much of the atmosphere, driving surface temperatures even lower. As Sauterey explains, when atmospheric temperature and pressure drop low enough, the freezing and evaporation point of water become essentially the same, turning the planetary ice shell to gas and allowing much of it to sublime into space along with the Martian air. But life could have clung-on tenaciously all the same. The tenuous atmosphere that remains —about 1% as dense as Earth’s—would still have sufficient carbon dioxide and H2 to sustain the metabolism of the surviving subsurface microbes, with the gasses able to penetrate as far as 20 km (12.4 mi.) underground.
It’s at those depths that, the models suggest, the microbes might still be hanging on, especially in regions in the mid-latitudes, which were spared the Martian ice covering. Significantly, Jezero crater, which the Perseverance rover is currently exploring, might have been one of the ice-free regions, and under Sauterey’s and his colleague’s computer model, the soil and subsurface of the crater are estimated to have had a 15% chance of once being habitable even during the ice age—not terrible odds as extraterrestrial life goes—raising the possibility that life could remain extant there.
Tantalizingly, Mars rovers and orbiters periodically detect trace outgassing of methane on the Martian surface, a phenomenon scientists have never been able to explain fully. It’s possible—even likely—that some geological process is at play. But there’s also a chance—at least in theory—that what’s being observed is the waste methane of surviving microbes living far below ground.
“That possibility,” says Sauterey, “deserves to be tested.”
For now, it would take more than computer modeling to conduct such tests, with analysis of actual samples of Martian soil the best way to look for evidence of biology. Significantly, Perseverance is busy caching just such samples of soil and rock in titanium tubes and leaving them on the surface of Jezero crater, awaiting a sample-return mission that will fly to Mars, collect the tubes, and bring them back to Earth early in the next decade. If there are fossilized traces or other chemical signatures of the methane-producing microbes that may have lived in the soil of Jezero crater, the samples could reveal them. The earliest Martians may not have been much, biologically speaking, but they may have been there—and still be there—all the same.