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.
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.