Although we know early Mars was wetter, warmer and more habitable than today’s freeze-dried desert world, researchers have yet to find direct proof that life ever graced its surface. If Mars did once host life, key questions remain: How did such life impact the planet, and where could we find evidence for its past existence? A new study considering these mysteries finds that a plausible Martian biosphere could have been instrumental for tipping the planet into its current inhospitable state. The findings further identify certain regions of Mars—including Jezero Crater, where NASA’s Perseverance rover now roams—as most likely to host signs of this past life. And they ominously hint that life may be its own worst enemy on worlds throughout the cosmos.
Re-creating Mars as it was four billion years ago using climate and terrain models, researchers concluded methane-producing microbes could once have thrived mere centimeters below much of the Red Planet’s surface, gobbling atmospheric hydrogen and carbon dioxide while protected by overlying sediment. But that buried biosphere would have ultimately retreated deeper into the planet, driven by freezing temperatures of its own making—perhaps to its doom.
Their study, published in Nature Astronomy, proposes that the interchange among hydrogen, carbon dioxide and methane (all heat-trapping greenhouse gases) would have triggered global cooling that covered most of Mars’s surface with inhospitable ice.
“Basically what we say is that life, when it appears on the planet and in the right condition, might be self-destructive,” says study lead author Boris Sauterey, a postdoctoral fellow at Sorbonne University. “It’s that self-destructive tendency which might be limiting the ability of life to emerge widely in the universe.”
Gaia’s Blessing—Or Medea’s Curse?
In 1965 the late chemist and ecologist James Lovelock—then a researcher at NASA’s Jet Propulsion Laboratory—argued that certain chemical compounds in an atmosphere act as biosignatures indicating life’s presence on another world. On Earth, for instance, the coexistence of methane (from methane-producing bacteria, called methanogens) with oxygen (from photosynthetic organisms) constitutes a potent biosignature: each gas eradicates the other in ambient conditions, so the persistence of both indicates a steady replenishment most easily explained by biological sources.
Lovelock’s work forms the basis of today’s scientific search for alien life. It also informs the Gaia hypothesis, which he codified with biologist Lynn Margulis during the 1970s. This hypothesis, named after a “Mother Earth” deity from Greek mythology, suggests that life is self-regulating: Earth’s organisms collectively interact with their surroundings in a way that maintains environmental habitability. For instance, higher global temperatures from excess atmospheric carbon dioxide also boost plant growth, which in turn siphons more of the greenhouse gas from the air, eventually returning the planet to a cooler state.
In 2009 University of Washington paleontologist Peter Ward put forward a less optimistic view. At planetary scales, Ward argued, life is more self-destructive than self-regulating and eventually wipes itself out. In contrast to the Gaia hypothesis, he named his idea after another figure from Greek mythology: Medea, a mother who kills her own children. To support his “Medea hypothesis,” Ward cited several past mass extinction events on Earth that suggest life has an inherently self-destructive nature. During the Great Oxidation Event more than two billion years ago, for instance, photosynthetic cyanobacteria pumped huge amounts of the gas into Earth’s oxygen-starved atmosphere. This eradicated the earlier dominant life-forms: methanogens and other anaerobic organisms for which oxygen was toxic. “You just look back at Earth’s history, and you see periods where life was its own worst enemy,” says Ward, who was not involved in the new study. “And I think this certainly could’ve been the case on Mars.”
On Earth, though, the flood of oxygen also proved crucial for biological diversification and the eventual emergence of our biosphere’s multicellular ancestors—showing that defining a situation as Gaian or Medean might be a matter of perspective. Until life is found on other worlds, however, we are left to examine the question through theoretical studies such as Sauterey’s.
A Deeper Look for Martian Life
Kaveh Pahlevan, a research scientist at the SETI Institute, who was not involved in the study, says that the work “does broaden the way we think about the effects that biospheres can have on habitability.” But he notes that it considers only the planet-altering effects of one metabolism type. The study would not capture the intricacy of something akin to the Great Oxidation Event, which hinged on the conflicting influences of methanogens and cyanobacteria. Sauterey acknowledges this limitation: “You can imagine that a more complex, more diversified [Martian] biosphere would not have had the negative effect on planet habitability that just methanogens would have had,” he says.
The study highlights how a complex ecosystem, like that of early Earth, may be essential to recovery from otherwise catastrophic environmental change. And in Ward’s view, an ascent toward ever greater complexity might help a biosphere avoid an otherwise-dismal Medean fate. “I truly believe the only way out—the only way any planet escapes once it gets life—is to evolve intelligence,” he says. Only then, Ward says, could inhabitants develop solutions to mitigate Medean tendencies for life to foul its planetary nest.
The study did not consider the possibility of present-day methanogens lurking within the Martian subsurface. Such a situation could help explain enigmatic plumes of methane that scientists have repeatedly detected in the planet’s atmosphere, although lifeless geophysical activity could also account for the plumes as well.
For ancient Mars, however, the study pinpoints places untouched by ice for large swaths of the planet’s history—despite a near-global glaciation from a worldwide cooling event—where such microbes could have once thrived closer to the surface. One spot is Jezero Crater, the current target of the Perseverance rover’s search for biosignature-bearing materials. But it is possible that fossil evidence of early methanogens would be under too much sediment for the rover to reach.
The study also identified two even more promising sites: Mars’s Hellas Planitia and Isidis Planitia regions. These targets fit with a broader rising interest in examining the Martian subsurface for signs of life, says California Institute of Technology geobiologist Victoria Orphan, who was not involved in the study. Sauterey’s research, Orphan says, is “a reference point to help stimulate debates and deeper thinking about future missions.”
Sauterey is careful to point out that the new work is hypothetical—and that just because parts of Mars’s crust were once habitable does not mean the planet was ever inhabited. Whether or not ancient methanogens ever lived on Mars, however, the results of the study illustrate how life itself can set the conditions for its own flourishing—or fizzling—on any world in the cosmos. Even single-celled organisms have the power to transform an otherwise habitable planet into a hostile place. And, Sauterey darkly adds, “with the technological means that we have, humans can do that even faster.”
Editor’s Note (1/13/22): A version of this article with the title “Mars’s Downfall” was adapted for inclusion in the February 2023 issue of Scientific American. This text reflects that version, with the addition of some material that was abridged for print.