For most of the past half-century, scientists have been trying—and failing—to find life on Mars. Beginning with NASA’s twin Viking landers, which touched down on the Red Planet in 1976, this hunt has focused on discovering possible biosignatures—organic molecules that may indicate life’s past existence.
There are, of course, good reasons to suspect the search will ultimately prove successful. Although now inhospitably dry and cold, a wealth of evidence shows that Mars was once a warmer, wetter and more habitable world. Life may have flourished there billions of years ago. Presumably, an epochal discovery could come from using a robot-borne chemistry lab to tease some telltale organic molecules out of an appropriate sample scoured from the right rock. But as highlighted in a recent study published in Nature Communications, this task may be too much for even our best present-day Mars explorers, NASA’s Curiosity and Perseverance rovers. Perhaps evidence for life on Mars is hidden in plain view, merely unrecognized because of the limits of rocketry and current rover technology, and a breakthrough will only come if—or when—we manage to either bring bits of Mars back to Earth or send astronauts to the Red Planet.
The study was conducted by a team of international scientists led by Armando Azua-Bustos of the Center for Astrobiology in Spain. The researchers tested how well standard life-hunting technology could detect biosignatures not on Mars but rather right here on Earth. Using a series of instruments analogous to those on Curiosity, Perseverance and the European Space Agency’s upcoming Rosalind Franklin ExoMars rover, the team sought biosignatures in one of our planet’s most Mars-like environments: the extremely arid Atacama Desert of the Chilean Andes. Specifically, they searched in the rust-colored, iron-rich sedimentary rocks that give the Atacama’s Red Stone region its name. Some rover analogues could not detect any organics in the Red Stone rocks while others found potential biosignatures such as amino acids—but only after treating the samples with chemical reagents that are in short supply on current Mars rovers.
“Maybe we still need to do some work in order to detect evidence of life on Mars,” Azua-Bustos says.
In some sense, the study is a validation of the current rovers on Mars. The team studied Red Stone samples with a roverlike kit, parts of which closely resemble the Sample Analysis at Mars (SAM) instrument onboard Curiosity, as well as a portion of ExoMars’s Mars Organic Molecule Analyzer (MOMA) instrument. Both tools heat rock and soil samples to vaporize organic molecules to gas, which is then sorted, fragmented and weighed. These measurements produce a sort of “fingerprint” for various substances, allowing researchers to identify the types and abundances of molecules within a sample. Using their roverlike proxies, Azua-Bustos and his colleagues did find biosignatures in the Red Stone samples. But they only did so with the right treatments, detecting amino acids and carboxylic acids when the samples were treated with derivatizing reagents—chemicals that make certain organics vaporize more easily. These reagents are the only way to vaporize and thus detect some types of organic molecules with the heat treatments used by SAM and MOMA.
The trouble is that the threshold for using such derivatization reagents on Mars is exceedingly high. Like any consumable on an interplanetary mission, the reagents are only available in a very limited supply, just enough for a handful of attempts. Curiosity’s SAM only has nine single-use sample cups for derivatization. The ExoMars rover is similarly limited: it can only derivatize 12 samples, says Fred Goesmann, MOMA’s principal investigator. Additionally, the reagents can linger in and around a rover’s instrumentation, potentially contaminating unrelated samples. “That’s why it took awhile to use [derivatization] on SAM,” Goesmann adds. “Afterward your instrument is not as it was before,” and cleaning it is difficult.
That’s enough to make any mission planner think twice, especially because derivatization is perhaps best used on inherently marginal samples—those containing very low amounts of organic carbon that fail to reveal much when scrutinized with easier, more preliminary approaches. Although the study team did manage to use the technique to reveal otherwise-hidden organic molecules in some Red Stone samples, Goesmann says that because those samples were so marginal, “it’s hard to say whether we would have decided to use derivatization right away” if the choice was being made for a rover on Mars.
In the Atacama, the team also tested techniques that are more analogous to Perseverance’s SuperCam and Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instruments, which use different methods to detect organics. Those instruments measure how rocks fluoresce, absorb and reflect light when they are hit with different lasers. Certain minerals can also react to the laser light, however, and those abiotic mineral-sourced signals obscured fainter ones from organics in the Red Stone samples, Azua-Bustos says.
Besides finding potential flaws with current rover-borne instruments, the Red Stone study also field-tested technologies that presently do not have a rover analogue, with promising results. For example, the team successfully found faint traces of ancient cyanobacteria in Red Stone samples using the Signs of Life Detector, a device that relies on antibodies to detect specific organic compounds. This device is also less impacted by salts (abundant in Red Stone and Martian soils alike) that can degrade organics during the heat treatments used by SAM and MOMA.
“These kinds of instruments have been designed for planetary exploration, but they have not yet been applied to that,” says Keyron Hickman-Lewis, a paleontologist at the Natural History Museum in London and a participating scientist in the Perseverance rover mission, who was not involved in the Red Stone study. The value of this study, he suggests, may be less in finding flaws in current techniques and more in vetting those that may someday be used in future missions.
In the end, each approach has its own strengths and weaknesses and is “seeing different things,” Azua-Bustos says. No single instrument is likely to yield a conclusive detection of life. Instead only a collection of results from different analyses can provide a suitably comprehensive picture of any site’s potential biosignatures. But including “all the things you can imagine here on Earth with teams across the world,” he says, is a nonstarter for rovers because of their limited size, power supply and operational lifetime.
Upscaled, far more sensitive versions of Curiosity’s SAM, for instance, can be the “the size of a room,” which means they “are never going to be able to fly,” says Mary Beth Wilhelm, an organic biogeochemist and planetary scientist at NASA’s Ames Research Center, who was not involved in the study.
“Maybe rover technology is pushed to its limits by particularly subtle biosignatures,” Hickman-Lewis says. “This is a good argument for Mars sample return.” Fortunately, this is exactly the plan. NASA is leading an international effort to bring some Mars rocks to Earth by the early 2030s. It is already well underway: the Perseverance rover has filled around half of its sample tubes for eventual delivery back home.
Between Martian rocks brought to Earth and better organic-molecule-detecting instruments sent to Mars, the Red Planet’s long-sought biosignatures are progressively running out of places to hide. That is presuming that they exist at all. Life may have never emerged on Mars. Of course, the only way to test that null hypothesis is to continue the search—ideally without overlooking any evidence that may lurk right under our robotic or real noses.