Has NASA Already Found Life on Mars?

On July 20, 1976, NASA’s Viking 1 lander became the first spacecraft to land safely on Mars. Shortly after, he sent back our first close-up image of the surface— a dull sight of stones scattered at the feet of the lander.

“I don’t think we would have been surprised if there were blades of grass,” recalls Tom YoungViking’s mission director. Scientists have speculated for years about what scenes would await Viking 1 and its twin, Viking 2, which would land elsewhere on Mars a few weeks later. Most thought Martians would be, at most, just small life forms – although Carl Sagan had mischievously suggested that creatures the size of polar bears could populate the landscape.

Humans have been imagining other inhabited worlds for millennia. And we even havediscovered“life on Mars more than once through the the last century Or two. Yet every time we build sharper tools and take a closer look, all these claims have evaporated, much like the ancient seas of the Red Planet. At the time the Viking landers launched, we knew there were no signs of life visible from orbit.

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But there was still hope that something was moving out there – enough hope to launch the Viking landers on a daring search mission. Their results, however, were not conclusive. Most experts agree that the sister spacecraft failed to find definitive signs of organisms in patches of Martian soil, but some scientists continue to believe that is the case.

Even today, half a century later, the results of these investigations fuel the debate, and the Viking landers remain the most efficient. only missions ever sent to the surface of another world to search for existing extraterrestrial life. The ambiguity that emanated from their soil samples has become emblematic of the uncertainty that hangs over the search for extraterrestrial life, not only on Mars and other worlds from our sun, but throughout the observable universe. Our knowledge of the physical limits of life and the supernatural forms it might take remains so incomplete that we might too easily declare a discovery where none actually exists – or fail to recognize true extraterrestrial biology hiding in plain sight.

As a result, astrobiologists have, for decades, opted for a conservative burden of proof that relies on two aphorisms invented by Sagan when he wasn’t thinking about Martian polar bears: life, he and his co-authors wroteshould be considered a “hypothesis of last resort”, largely because “extraordinary claims require extraordinary evidence.

But what if Sagan was wrong? If we live in a universe where life is common and not extraordinary at all, setting the bar so extraordinarily high for its discovery beyond Earth can backfire. What if the fingerprints of life have been present on Mars forever and we’re too cautious to admit we’ve found them?

Already, growing evidence suggests that ancient Mars may well have been an inhabited planet – and may still be. Our search for life there is a bit like blindfolded searching for apples, except we don’t know what an “apple” might look like, and apples may exist only in scattered pieces. What’s needed, many scientists say, is a clearer way to separate signs of “life” from “non-life,” a quantitative method that leverages statistics and probability to guide our interpretations of biology’s potential fingerprints.

“There are all these different lines of evidence that keep coming together that make me say, ‘My God, it’s getting harder and harder to explain everything that’s happening on Mars abiotically,'” says Amy Williamsastrobiologist at the University of Florida. “I’m not yet ready to say we’ve found evidence of life, but I think the story is building to help us understand what that potential is – to assign a probability to it instead of just saying ‘yes’ or ‘no.’

Today’s Mars is barely habitable at best, but 3.5 billion years ago the planet was almost certainly a more suitable world for life. Ancient Mars was warmer, with a thicker atmosphere and a global magnetic field that shielded its surface from cosmic radiation; seas and lakes filled its basins and rivers flowed through its valleys. Over the eons, Mars lost its magnetic field and, with it, its bodies of water and its thick, insulating atmosphere.

Yet over the course of Mars’ life, organic compounds – the building blocks of life – have fallen onto the planet, brought by meteorites and cosmic dust.

“If we live in a universe where life takes advantage of aqueous environments when there is juicy chemistry at work – which is how I imagine the universe, but it’s an unvalidated intuition – then something should have started happening in some of these places on Mars,” says the astrobiologist. David Grinspoon from the Institute of Planetary Sciences. “So there’s this predisposition to think, ‘Either there should have been an origin of life on Mars, or we’re really wrong about something on Earth.'”

Since 2012, NASA Curiosity rover looked for signs of ancient habitable environments in Gale Crater. And in 2021, the agency Perseverance The rover landed in Jezero Crater to specifically search for ancient biosignatures, not current life. Today, based on the results of these robotic explorations, many experts suspect that ancient Mars was indeed a biological world, although they cannot yet prove it.

“The evidence for habitable environments and life in early Mars gets stronger every time we look at it,” says Chris McKayastrobiologist at NASA Ames Research Center.

The most convincing clue comes from a reddish mudstone found by Perseverance which is spotted with “leopard spots” or mineral assemblages containing chemically modified iron and organic matter. Such stains on Earth’s rocks are usually the work of mineral-eating microbes. In fact, scientists have yet to find a convincing way to explain observation on Mars without biology; for such chemistry to occur without Over the course of its life, it appears that the rock would have undergone a set of events or conditions that seem rather improbable, given our current understanding of its history and environment.

From the rocks scattered around Gale Crater, Curiosity contributed another intriguing observation: a particular abundance long-chain alkanes, carbon molecules which, on Earth, sometimes come from fatty acids like those in cell membranes. Scientists think Martian chains are fragments larger fatty acids that were degraded long ago by cosmic radiation and then, much later, by the Curiosity instrument that studied them. If that were the case, Williams says, the original molecules would have been difficult to deliver to abiotic sources, such as meteorites; the longest fatty acid would be longer than anything still found in meteorites.

Although not definitive, these observations from Mars are certainly exciting. “This is what you might call permissive evidence of life: it could be that microbes left this material behind,” says McKay. “But it’s not convincing.”

To definitively answer the question of whether these molecules carry biological fingerprints, scientists must first rule out any possible abiotic explanations, an effort that requires a complete understanding of the environmental context of a potential biosignature. That’s hard enough to do on this planet, and it’s even harder to do remotely.

Or we could bring samples from Mars to study them in laboratories here on Earth. NASA had planned to use its Sample return to Mars mission to recover pieces of leopard-spotted rock and other prime specimens collected by Perseverance, but the plan became so complex and expensive that Congress overturned it. Astrobiologists still hope that the samples will eventually find a cheaper and more convenient way back to Earth.

The search for life beyond Earth should be the agency’s top priority, Young says. And the mystery of our apparent cosmic loneliness could be solved by a cache on rocks just waiting to be retrieved on Mars.

“I think it’s really important for us to do it,” he said. “Every leader who misses the opportunity to bring back these samples has left a dark mark on their leadership. »

Until – or unless – a sample returns to Mars, astrobiologists are forced to ask what, exactly, would make a potential biosignature “compelling.” Of course, we can all imagine a few scenarios in which the evidence of extraterrestrial life would be unequivocal: prime numbers encoded in a radio transmission, a fossilized trilobite on Mars, aliens res embedding to squids in the oceanic moons of the outer solar system.

However, when it comes to ancient Mars, most experts don’t expect to find “a single piece of data, and now we know the answer,” says Rebecca McCauley RenchNASA’s senior scientist for planetary science astrobiology.

But if ringing the bell for “extraterrestrial life” probably rests on a preponderance of evidence rather than any singular, blunt observation, wouldn’t astrobiologists be better off abandoning Sagan’s aphorisms?

“Extraordinary claims require extraordinary evidence” East a concise slogan, but it’s a bit limiting,” says Maison Christopheastrobiologist at Pennsylvania State University. Out of respect for Sagan’s credo, he says, “people will go out of their way to say that everything they found is abiotic.” House suggests that scientists should instead view life-suggestive explanations with the same rigor and sincerity with which they evaluate abiotic conclusions.

Furthermore, chemist Steve Benner says: “What is “extraordinary” as a statement depends on who perceives it. » If it’s not so crazy to imagine that ancient Mars was a biological world, then perhaps “life” isn’t such an extraordinary claim after all.

“What if we lived in a universe where life is very ordinary? request Michael Wongastrobiologist at Carnegie Science. “If we only let the rarity of something guide us toward life, we may miss many lives that are just ordinary life, and we may be attracted to extraordinary nonbiological processes and think that these are life instead.”

To confidently conclude anything from a set of “ordinary” observations, scientists need to construct a quantitative method to determine whether a set of observations is a signature of biology. “This quantitative analysis of the biosignature should be a goal within the community,” says the astrobiologist. David Catling from the University of Washington, “because otherwise it’s just hand waving, and we don’t provide any definitive answers. It’s always interesting, and the findings always move things forward, but we need to be more quantitative about it.”

Let’s say you have 100 Martian robots digging around in various craters and each of them finds something like leopard spots or these long-chain alkanes. You do the work to exclude geology and realize that even if you could explain observations without invoking biology, these explanations each require improbable conditions or processes. Or maybe, said Morgan Cablea planetary scientist at Victoria University of Wellington in New Zealand, you do all this work and end up finding mutually exclusive abiotic explanations.

“When will we cross the threshold to say, ‘Yes, the likelihood that we find this many potential biosignatures means we can remove the word potential’?” Cable said. “It’s almost as if we need to take Carl Sagan’s ‘last resort hypothesis’ and apply it to the abiotic case as well.”

However, for any quantitative approach to work, “you need a certain amount of data,” says Catling. “And at the moment, it’s pretty spotty.”

As tedious as it may seem, for now scientists must be content to continue “as usual” in their search for life on Mars. New tools such as AI-augmented robotic explorers and machine learning algorithms that analyze observations may emerge, but the fundamental goals remain unchanged: continue to search deeper for clues hidden in the Martian rock record and gradually build a more complete understanding of the planet.

In other words, the frustrating but necessary task is to somehow transform quantities of “ordinary” data into a narrative that might tell us something “extraordinary” about Mars and its history. This doesn’t quite encapsulate Sagan’s strict criteria – it’s more of a variation on a theme – but it doesn’t deviate profoundly from them either.

“I think we owe it to the community and to ourselves – to humanity, in fact: if you’re going to make a statement like that,” Williams says, “you have to be right.”

The search for life on Mars is filled with cautionary tales of scientists who made big claims and then had to walk back their words. But according to Benner, a somewhat iconoclastic scientist who works on his own through his nonprofit research organization, the Foundation for Applied Molecular Evolution– this troubled heritage has made today’s astrobiologists overly cautious and unable to see what, to him, is increasingly obvious.

“More likely than not”, there East life on Mars today, he says. And the Viking landers probably detected it half a century ago. The scientific community’s collective failure to recognize this, he says, “is a prime example of how science does not self-correct, even when the facts are clear.” (Benner’s peers find his views on Viking controversial, to say the least.)

Each Viking lander carried three biological experiments, all designed to detect the metabolic murmurs of microbes likely to live in the planet’s soil. An experience added water and nutritious soup on the ground and looked for gases breathed. Another nutritious broth added with a radioactive tracer in the soil, then looked to see if any of these “radiolabeled” traces emerged from exhaled carbon dioxide. The third asked if a process like photosynthesis could incorporate radiolabeled carbon dioxide into organic molecules.

Intriguingly, at both landing sites, Martian soil exposed to the radiolabeled broth appeared to produce radiolabeled gases. “The curve was incredible,” Young recalled. “If you looked at that experience, it couldn’t have been more positive.”

Yet the mission team ultimately declared Mars to be a lifeless world. Another instrument on board, a mass spectrometer designed to detect organic molecules, apparently came back empty. And it was a riddle because no one could explain how life could exist where organic molecules didn’t exist.

“It made everyone, including me, think that ‘life’ is out of the question,” McKay says. “There is no life without organic products.” No organics, no life: For a time, this disappointing conclusion made the mission a failure and hampered NASA’s Mars exploration program.

“What we really learned from Viking is that environmental context is extremely important,” says McCauley Rench. “You can’t just ‘try to detect life.’ This is part of what makes research so difficult.

It would take 20 years and another tantalizing sign of life on Mars to revive the space agency’s interest in the planet. In 1996, scientists discovered what looked like tiny microbial fossils embedded in a Martian meteorite collected in the Allan Hills region of Antarctica. Years later the “discovery” was ultimately ruled out as a misinterpretation, but not before inspiring President Bill Clinton. to describe it in a televised speech as a “justification of the American space program”.

NASA’s renewed Mars program would pursue a more modest, incremental goal: understanding whether the planet was once habitable rather than whether anything still lived there or ever did. But a partial solution to Viking’s riddle of life on Mars appeared unexpectedly in 2008. In the soil of its perch in the Martian Arctic, NASA’s Phoenix lander discovered perchlorate salts, which tend to destroy organic molecules when heated.

If perchlorate was common in Martian soils, McKay and others arguedthis would explain why the Viking mass spectrometer had not detected organic matter: the the perchlorate had chewed them up during the experiment. And it is indeed very common on the planet: in 2013, Curiosity also found perchlorate in Gale Crater, 3,500 miles from the Phoenix lander.

“That’s when the story changed dramatically,” McKay says. “No organics are coming out the window. There was organics there, and there was perchlorate.”

Curiosity and Perseverance each followed the definitive detection of organic compounds at their respective landing sites in a series of discoveries that continually revealed more complex molecules. Mars was proving chemically richer and more dynamic than most scientists had dared to imagine, although some, including the late Gil Levinprincipal investigator of the Viking Labeled Liberation experiment, suspected as much. “Until the day Gil died,” Young said, “he was convinced that life was there.”

In truth, scientists don’t need “life” to explain the results of Levin’s labeled release. This experiment sampled a process called oxidation, in which larger molecules are broken down and stripped of electrons to produce energy. The Martian surface itself is naturally oxidizing: radiation breaks down perchlorate into highly reactive compounds that i do not need heat to break down molecules. This could explain how the added nutrients were broken down to release radiolabeled carbon dioxide. Additionally, some nutrients in the broth could break down into carbon dioxide on their own.

More recent work by Benner and colleagues revisits the third experiment, according to McKay, “the orphan of the Viking trio.” This often-overlooked attempt to see if something like photosynthesis was happening on Martian soils “absolutely yielded what, before launch, was considered a positive result,” House says. But this Viking result made no sense on a planet that was then considered free of organic matter, and it was disavowed as a false positive by the very scientists who were behind it. Called a pyrolytic release experiment, the investigation tested whether labeled carbon atoms, in carbon dioxide or carbon monoxide gases, were incorporated into larger organic molecules. On Earth, photosynthetic organisms achieve this through a process called carbon fixation.

In a published article And a new bookBenner argues that this Viking experiment showed that something in Martian soil appeared to transform inorganic gas into organic matter. It’s the opposite of destroying organic matter, Benner says, and it can’t be explained by perchlorate or any other oxidant. “There is nothing at the moment that explains this result,” he says.

In other words, there is nothing except perhaps life.

Benner makes an important argument that isn’t easy to dismiss, McKay says, as long as his assumptions are correct. “It would be difficult to understand the evidence for reduction in such an oxidizing world without appealing to life,” says McKay. But McKay and Benner both recognize that additional work is needed to truly rewrite our understanding of the often-neglected Viking experience.

For now, Viking’s results may remain ambiguous, at least until scientists return to Mars with a newer suite of life-detecting instruments and much more information about the vermilion surface of our vermilion neighbor. No more being fooled by perchlorate or premature declarations of sterility: we just need to recognize that when we dare to ask big questions, we may not immediately understand nature’s answer.

“I sometimes think about the Viking experiments and how interesting it would be if, in 100 years, we recognized that this was the first time life was discovered on Mars,” Grinspoon says, “and we just didn’t accept it for another 75 years.”