Percival Lowell wasn’t the first to think he’d discovered life on Mars, but he was among the last. In the late 19th and early 20th centuries, the American astronomer published a series of books promoting his theory that observable features on the surface of the Red Planet were the handiwork of an intelligent species on the verge of extinction. The objects of Lowell’s fascination—and the wider astronomy community’s scorn—were the so-called “Martian canals,” which he believed were used to route water from the planet’s ice caps.
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NASA has been robotically exploring Mars since the mid-’60s, and because of these missions we’re now fairly certain that the planet isn’t home to any extraterrestrial engineers. (Sorry, Percy.) But these spacecraft did find an abundance of geological evidence that Mars may once have had liquid water on its surface, a magnetic field, and a thick atmosphere, which top the list in terms of prerequisites for life as we know it. In other words, there’s still a chance that basic life forms once existed on the surface of the Red Planet. And later this month, NASA will take its biggest step yet toward finding out.
On July 30, NASA is expected to launch its new rover, Perseverance, on a one-way journey to Mars. The car-sized robotic geologist will spend its first year on the planet drilling core samples in search of signs of ancient life. (Another robotic mission later this decade will return the samples to Earth.) The rover will collect at least 20 tubes of dirt around its landing site, the Jezero crater, which scientists believe was a river delta nearly 4 billion years ago. If Mars ever hosted life, the stagnant water of the ancient Jezero delta would be the type of place you’d expect to find it.
But don’t expect Perseverance to dredge up any bones or seashells—it’s on the hunt for fossilized microbes, not mollusks. And even finding an intact bacterium would be an astonishing stroke of luck. “That would be a total dream,” says Tanja Bosak, an experimental geobiologist at MIT and a member of the 10-person team that will guide the rover’s sample selection. Instead, the rover is looking for potential biosignatures, the faint molecular traces left behind by microbes billions of years ago. If Perseverance discovers life on Mars, it will be less like encountering a stranger in the woods and more like discovering their footprints.
When she’s not hunting for ancient life on other planets, Bosak studies the earliest life on our own, a process she says is analogous to what Perseverance will be doing on Mars. To track down ancient microbes on Earth, geobiologists look for patterns in rock formations that could only have been formed by biological processes. Stromatolites, for example, are rocks infused with layers of what Bosak calls “organic gunk.” These thin sheets of fossilized algae and other primitive organisms shape sediments in a distinct wavy pattern that is visible to the naked eye.
“With microbes, you never really see only a single cell. It’s always a macroscopic community,” says Bosak. “The fundamental interactions between organic matter and minerals should be the same on Earth and Mars, so we’ll use cameras to look for these different kinds of microbial shapes.”
It would be a big deal if Perseverance finds stromatolites on Mars, but not enough to prove the existence of extraterrestrial microbes. The rover would also have to find an abundance of molecules that are typically associated with life in the same spot. “All cells metabolize,” says Bosak. “They take in molecules from the environment and spew out something else.” This could include basic elements like phosphorus and nitrogen, or more complex organic molecules like calcium carbonate. In a best case scenario, the rover would find fossilized traces of lipids or other biomolecules that are essential for living things. The challenge for Perseverance will be finding these fossilized molecules smeared across a mote of Martian dust.
The first step in this process involves the SuperCam instrument, an array of lasers attached to the rover’s mast that can study rocks at a distance. One laser vaporizes the rock by heating it to 18,000 degrees Fahrenheit. This creates a plasma that the rover can photograph to understand its elemental composition. Another laser interacts with the molecules in the Martian soil without destroying their chemical bonds and, by the way the laser light changes, reveals which compounds are laced in the dirt.
If the SuperCam detects organic molecules or elevated concentrations of elements like nitrogen or phosphorus, Perseverance will roll over for a closer look. Two instruments attached to the end of its arm, PIXL and Sherloc, use more lasers to get a detailed picture of the rock. PIXL uses an x-ray beam to create a fluorescent map of the rock’s elemental chemistry and Sherloc uses an ultraviolet laser the width of a human hair to detect any organic material that may be hiding among the grains of dirt.
“These are the types of techniques we use when we’re studying the earliest record of life on Earth,” says Ken Williford, NASA’s deputy project scientist for the Mars 2020 mission and the director of the Astrobiogeochemistry Laboratory at the Jet Propulsion Laboratory. “The way we find ancient biosignatures on Earth is not just by measuring the bulk chemistry of a rock. We map where that organic matter is in the rock, and that allows us to look for lifelike textures and compositions together.”
Once Perseverance finds a promising patch of red dirt, Bosak and her colleagues will have to make a call about whether to take a core sample at that location to be returned to Earth later. It’s a high-stakes decision—the rover can only stash about a few dozen samples, and once a decision is made there’s no turning back. The rover has a lot of ground to cover in its first year on Mars, so it won’t have time to revisit previous sample sites. And astrobiologists aren’t the only scientists itching to get their hands on some Mars rock. Some samples will be used to answer other fundamental questions, like how long habitable conditions lasted on the Martian surface and what those conditions were like.
The oldest, noncontroversial evidence of life on Earth is around 3.5 billion years old; beyond that point, the microbial fossil record becomes warped beyond recognition by eons of intense geological processes. Williford expects that the rocks examined by Perseverance will be around 300 million years older than the oldest evidence of life on Earth. And if we can barely recognize the oldest life on our own planet, it will likely be even harder to recognize it on Mars. “Any signs of life are much more likely to be highly ambiguous than they are to be anything obvious,” says Williford. Even if Perseverance finds biosignatures that would pass as strong evidence of ancient life on Earth, Williford says the scientific community would likely withhold its judgement until the samples were returned and studied with more sensitive instruments. “The implications are just too enormous,” Williford says.
Of course there’s the possibility that Perseverance turns up empty-handed in its search for biosignatures on Mars. But that doesn’t necessarily mean the planet is devoid of life, says Sarah Johnson, a planetary scientist at Georgetown University. It might just mean that life on other planets looks different from life on our own. But how can you find something if you don’t know what you’re looking for?
In 2018, NASA’s astrobiology program awarded Johnson and an international team of researchers a $7 million grant to figure out an answer. Today, Johnson leads the new Laboratory for Agnostic Biosignatures, which she describes as an effort to understand “life as we don’t know it.” The techniques that Perseverance will use to detect possible biosignatures all presume that life on Mars evolved in a similar way to life on Earth, and so it’s looking for evidence of similar biochemistries. Johnson’s lab is in the business of finding ways to detect life that might not play by Earth’s genetic rulebook, which is a bit like learning to speak a language you’ve never heard of.
“The main idea with agnostic biosignatures is that they include life as we know it, as well as other types of life,” says Johnson. For example, she and her colleagues think that the complexity of a molecule may be an important biosignature that doesn’t depend on a terrestrial biochemistry. There’s a certain threshold of complexity for chemical compounds beyond which it is almost impossible for them to form without the aid of a biological process. The task for Johnson and her colleagues is to figure out how to define that complexity in a meaningful way. “You can’t just look at big molecules, because there are lots of molecules, like polymers, that are really, really big, but they’re just repeating the same subunits,” Johnson says.
Instead, Johnson and her colleagues are looking at complexity as a process. In other words, how many different ‘steps’ does it take to create a given molecule, where each step is something like adding a new type of molecular bond? Their research suggests that there is a threshold of complexity at around 14 or 15 steps; above that, any molecule is almost certain to have been formed by a biological process.
Johnson’s lab is investigating other potential agnostic biosignatures, such as certain types of reduction-oxidation reactions, which transfer electrons between atoms. This is the main source of energy transfer at the microbial level, and searching for different types of redox reactions could potentially be used to identify extraterrestrial life that doesn’t share our specific biochemistry.
Johnson and her colleagues are exploring a variety of agnostic biosignatures, but she says they’re related in that they take a more probabilistic approach to detecting life. “We’re trying to move away from this binary of ‘yes life’ or ‘no life’ to a spectrum of certainty,” says Johnson. “If we think about what we would expect to happen from a biological or random process in probabilistic terms, I think that can move us forward quite a bit. We’re kind of in this world of biohints as opposed to definitive biosignatures.”
It’s still early days for research on agnostic biosignatures, but Johnson is optimistic that the techniques she and her colleagues develop may be able to help analyze the Perseverance samples when they’re returned to Earth later this decade. They may also have a role to play on upcoming NASA missions to Titan and Europa, two moons in the outer solar system that many planetary scientists consider to be leading candidates for hosting life in our solar system.
If there’s life on these alien worlds, there’s a good chance that it will be significantly different from our own. Jupiter’s moon Europa is covered in a thick layer of ice that is believed to conceal a planet-wide ocean, which means that any lifeforms there would have cropped up around hydrothermal vents deep beneath the surface. Saturn’s largest moon Titan has a thick atmosphere rich in carbon compounds and may also have large bodies of liquid water below its surface. Scientists aren’t sure what they’ll find when they arrive, but if Johnson and her team are successful, they’ll have a brand new set of tools to help them recognize an extraterrestrial when they see one.
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