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Mars was “very likely” habitable to life more than 3.7 billion years ago and may still be home to resilient organisms today, according to a new study that presents a tantalizing glimpse of how microbial aliens could have emerged and survived on the red planet—and where we might be able to find signs of them.
Modern Mars is cold and desiccated, making it inhospitable to most imaginable forms of life. But there is abundant evidence that this world was warmer, wetter, and more welcoming to life in the past. For instance, NASA’s Perseverance rover is currently exploring the parched remains of what was once a vast ancient lakebed on Mars, known as Jezero Crater, in part to search for traces of extinct aliens that might have lived there billions of years ago.
While there’s no concrete evidence that life has ever existed on Mars, scientists have speculated that microbial organisms called methanogens, which are among the most ancient life forms on Earth, could have been well-suited to conditions on early Mars. Methanogens derive energy from hydrogen and carbon dioxide, which were available on ancient Mars, and they release methane as a waste product.
Now, scientists led by Boris Sauterey, a biologist at the Institut de Biologie de l’École Normale Supérieure in France, have presented a first-of-its-kind study that assesses the viability of hydrogen-eating methanogens on early Mars, and speculates about the survival of any descendants of these hypothetical organisms to the present day.
The team discovered that “subsurface habitability was very likely” on early Mars and that “biomass productivity could have been as high as in the early Earth’s ocean,” according to a study published on Monday in Nature Astronomy.
The results support the exciting prospect that life may commonly arise on other worlds, but also reveal an unexpected dark side: Life may frequently be the agent of its own destruction.
“We evaluated the habitability of Mars and we evaluated the influence that this biosphere, that could have been similar to the primitive terrestrial biosphere, would have had on the Martian climate—and we were surprised,” Sauterey told Motherboard in a call.
“What did not surprise us was the fact that we found Mars to be likely habitable; we had an expectation of that based on previous works,” he continued. “What was surprising was the fact that when we assumed that a biosphere inhabited Mars, the climatic effects of that biosphere were opposite [of Earth]. Instead of warming the planet, and consolidating the planetary habitability, this biosphere would instead cool it down dramatically, degrading the habitability of the planet.”
In other words, the researchers discovered that the activities of early Martian microbes would have triggered a global cooling effect that may have led to their own extinction. Here on Earth, life also has also affected the climate for billions of years, but these feedback loops have generally maintained and even enhanced our planet’s habitability. The new study suggests that transient life forms may emerge on many worlds, but the type of long-term habitability we’ve enjoyed on Earth may be an outlier.
“As soon as you put methanogens on Mars, the climate cools down by 20 to 40 degrees [Kelvin], the planet gets covered in ice, and potentially becomes completely uninhabitable,” Sauterey said. This could be the general rule in the universe and Earth might be an exception.”
“The ingredients of life may be everywhere, but potentially life is just making a mess of it every time,” he continued. “Right now we are a good example of the fact that we can make a mess of a habitable planet,” referring to human-driven climate change and other anthropogenic pressures on Earth, “but even a really primitive ecosystem could do the same.”
This may seem like a bittersweet scenario, but it is tempered by other promising revelations in the study. For one thing, Sauterey and his colleagues conclude that early Mars was almost certainly habitable under certain parameters and in key areas, such as Hellas Planitia or Jezero Crater. This finding bolsters the evidence that habitable environments may have emerged in multiple places not only within our solar system, but also the wider galaxy and universe. Not every habitable location is necessarily inhabited by aliens—and Sauterey cautioned that “we still have no idea how inert matter becomes alive”—but the abundance of these hospitable environments is still an encouraging sign in the search for extraterrestrial life.
The team also examined the possibility that methanogens that thrived on Mars billions of years ago could have taken refuge from the cold deep under the Martian surface. Cut off from access to their atmospheric energy sources, these hypothetical survivors would have had to find a new source of power, but Sauterey noted that methanogens on Earth have managed to make these kinds of shifts in the past.
If descendents of ancient Martians are hidden in subterranean lairs, it might be possible to detect them by scanning Mars to look for underground biomass or sniffing out chemical traces of their activities at the surface. Indeed, NASA’s Curiosity rover has detected unexplained emissions of methane gas at its location on Mars, which could have either a biotic or abiotic origin.
Sauterey hopes that future missions might be able to definitively distinguish whether the elusive methane detections are geological in origin, or are the exhalations of ancient subterranean organisms.
“We can try to detect, for instance, emissions of methane that are not expected even with what we know of the geological process going on at the surface of Mars,” he said.
“Assuming that actually some of them survived, if there are still habitable pockets for those types of organisms on Mars, it’s possible to see if we can characterize chemical signals,” Sauterey concluded. “Maybe we can use life to explain stuff that we have no other explanation for so far.”