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Will we recognize life on Mars when we see it?

Percival Lowell was not The first to think he discovered life on Mars, but he was among the last. In the late 19th and early 20th centuries, the American astronomer published a number of books in which he advocated his theory that observable features on the surface of the Red Planet were the work of an intelligent species that was threatened with extinction. The objects of Lowell’s fascination – and contempt for the broader astronomical community – were the so-called “Mars channels,” which he believed were used to channel water from the planet’s ice caps.

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NASA has been researching Mars on a robotic basis since the mid-1960s, and with these missions, we are now fairly certain that there are no alien engineers on the planet. (Sorry, Percy.) But these spaceships found a wealth of geological evidence that Mars once had liquid water on its surface, a magnetic field, and a thick atmosphere that, as we know, top the list of life requirements. In other words, there is still a possibility that basic life forms once existed on the surface of the Red Planet. And later this month, NASA will take its biggest step to date to find out.

On July 30, NASA is expected to launch its new rover perseverance on a one-way trip to Mars. The auto-sized robot geologist will spend his first year on the planet drilling core samples in search of signs of old life. (Another robot mission later this decade will bring the samples back 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 almost 4 billion years ago. If Mars ever hosted life, the stagnant water of the old Jezero Delta would be the kind of place you’d expect it to be.

However, don’t expect persistence to dig bones or shells – it’s looking for petrified microbes, not mollusks. And even finding an intact bacterium would be an amazing 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 lead the rover’s sample selection. Instead, the rover looks for possible biosignatures, the weak molecular traces that microbes left behind billions of years ago. When persistence discovers life on Mars, it is less like meeting a stranger in the forest, and more like discovering their footprints.

If she isn’t looking for old life on other planets, Bosak studies earliest life on her own. A process she says is analogous to what persistence will do on Mars. To track down old microbes on Earth, geobiologists are looking for patterns in rock formations that could only have been created through biological processes. For example, stromatolites are rocks infused with layers of what Bosak calls “organic matter”. These thin layers of petrified algae and other primitive organisms form sediments in a clear wave pattern that is visible to the naked eye.

“With microbes, you never really see a single cell. It’s always a macroscopic community, ”says Bosak. “The basic interactions between organic matter and minerals should be the same on Earth and on Mars, so we’re going to use cameras to look for these different types of microbial forms.”

It would be a big deal if Perseverance found stromatolites on Mars, but not enough to prove the existence of extraterrestrial microbes. The rover would also have to find an abundance of molecules typically associated with life in the same place. “Metabolize all cells,” says Bosak. “They absorb molecules from the environment and spit out something else.” This could include basic elements like phosphorus and nitrogen or more complex organic molecules like cholesterol. At best, the rover would find fossilized traces of lipids or other biomolecules that are essential for living things. The challenge for persistence will be to find these petrified molecules smeared over a mote of Mars dust.

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