Fifty years ago, on July 20, 1976, the most ambitious unmanned project that NASA ever conceived landed on the planet Mars. The first of two Viking landers set down in Chryse Planitia (Greek for the "Golden Plain"), a smooth, circular, low-lying plain located in Mars’ northern equatorial hemisphere. NASA chose this particular landing area because it was believed at the time that ancient outflow channels might be a good place to look for living creatures.
A few weeks later, Viking II set down in Utopia Planitia ("Nowhere Plain"), a vast plain in Mars' northern hemisphere. (Viking did not find any Federation shipyards.) The site was chosen because orbiter data suggested it had a higher concentration of water ice beneath the surface and in the atmosphere than Chryse Planitia, making it a prime candidate for detecting signs of microbial life.
Both landers were equipped with the most ingenious experiments that NASA ever devised. The Biology Experiment Package contained a suite of three primary metabolic experiments and one control experiment designed to search for organic chemistry. One test looked for microbial "respiration"; another tested for photosynthetic organisms (able to use sunlight); another tested for metabolism by measuring changes in atmospheric gas composition.
The "control" test, the Gas Chromatograph-Mass Spectrometer (GC-MS), was designed to detect organic compounds (the carbon-based building blocks of life) in the soil. The results were negative. Because of that test and the inconclusive results from the other tests, NASA concluded that the positive signals seen in the Labeled Release and Pyrolytic Release experiments were false positives caused by exotic, inorganic soil chemistry rather than living microbes.
Fifty years later, scientists have gone back and forth about exactly what the Viking landers found. More than one scientist has changed their minds about the ultimate question of life on Mars as additional information from other landers has become available.
Carl Sagan, the patron saint of the search for life in the universe, coined the phrase, "extraordinary claims require extraordinary proof." Life should be the "hypothesis of last resort" when trying to discern whether an experiment yields evidence of life. All abiotic alternative explanations must be eliminated before making the "Eureka!" announcement that life has been found.
"There's all these different lines of evidence that keep coming together that make me go, 'Gosh, it's becoming more and more difficult to explain everything on Mars abiotically,'" says Amy Williams, an astrobiologist at the University of Florida. "I'm not yet ready to say that we've found evidence for life, but I think the story is building to help us understand what that potential is—to put a probability on that instead of just saying 'yes' or 'no.'"
There are two major problems with Sagan's (and NASA's) approach to identifying life. The first is that NASA is looking for "life as we know it." The scientists argue that this is the only rational, scientific way to look for life. Until we find life that's based on some other chemistry or biology, we can only guess at what is living and what isn't.
The second problem with the "life as we know it" standard is that it makes scientists extremely cautious.
But what if Sagan had it all wrong? If we live in a universe where life is common and not extraordinary at all, setting such an extraordinarily high bar for its discovery beyond Earth can backfire. What if life's fingerprints have been on Mars all along, and we were too cautious to admit that we'd found them?
Already, a growing pile of evidence suggests ancient Mars could very well have been an inhabited planet—and may still be. Our search for life there is a bit like bobbing for apples blindfolded—except we don't know what an "apple" might feel like, and the apples may exist only in scattered pieces. What's needed, many scientists argue, is a clearer way of separating signs of "life" from "nonlife," a quantitative method that leverages statistics and probabilities to guide our interpretations of biology's potential fingerprints.
Mars today is (probably) a dead planet. With no magnetic field to protect the surface from the sun's relentless bombardment of deadly charged particles, and no atmosphere (about 1 percent of Earth's), the chances of finding life as we know it and can identify it are remote.
And yet...
"If we live in a universe where life takes advantage of watery environments when there's juicy chemistry going on—which is the way I picture the universe, but that's unvalidated intuition—then something should have started happening in some of those places on Mars," says astrobiologist David Grinspoon of the Planetary Science Institute. "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.'"
"The evidence for habitable environments and life on early Mars is getting stronger every time we look at it," says Chris McKay, an astrobiologist at NASA's Ames Research Center.
The most compelling clue comes via a reddish mudstone found by Perseverance that's dappled with "leopard spots," or mineral assemblages that contain chemically altered iron and organics. Such spots on Earth rocks are usually the work of mineral-munching microbes. In fact, scientists haven't yet found a convincing way to explain the observation on Mars without biology; for such chemistry to occur without life, it seems, the rock would've had to experience a set of events or conditions that appear rather improbable, given our current understanding of its history and environment.
Sitting on Mars right now is a pile of rocks and soil samples from different regions. They are waiting to be picked up by NASA's Sample Return Mission, a joint NASA-European Space Agency (ESA) mission. The Perseverance rover landed in Jezero Crater in 2021. It has drilled and sealed over 30 ultraclean titanium tubes containing rock cores, regolith, and atmospheric samples, storing most onboard while caching a backup depot at another location.
The plan is to send a retrieval lander to Mars to land near Perseverance and transfer the tubes to a small Mars Ascent Vehicle that would take off and rendezvous with the ESA-built Earth Return Orbiter, which would capture the capsule in orbit and carry it back to Earth for analysis in sterile, high-containment laboratories.
The program is on "pause" and probably dead. With a price tag of $8 billion to $11 billion, it will probably be a lot cheaper to wait for humans to fly to Mars and retrieve it. It's a shame because some of those samples excited scientists when they were collected.
"The most compelling clue comes via a reddish mudstone found by Perseverance that's dappled with "leopard spots," or mineral assemblages that contain chemically altered iron and organics," reports Scientific American. "Such spots on Earth rocks are usually the work of mineral-munching microbes.
"'Extraordinary claims require extraordinary evidence' is a pithy slogan, but it's kind of limiting," says Christopher House, an astrobiologist at Pennsylvania State University. In deference to Sagan's credo, he says, "people will bend over backward to say that whatever they've found is abiotic." Instead, House believes scientists should consider explanations that suggest life with the same rigor and sincerity with which they evaluate abiotic conclusions.
As we probe further and further into the void, our definition of life will almost certainly change. These first steps, tentative, cautious, and full of doubts, are how science progresses. We will find life eventually, and we'll be sure about it.
