The hunt for extraterrestrial life has always been bedeviled by false positives—those occasions where scientists think they’ve found life but turn out to lack a wholly convincing case.
The archetypal example comes from NASA’s twin Viking landers, which delivered controversial evidence of life on Mars in the mid-1970s. That evidence was a whiff of radioactive carbon wafting from Martian soil, hinting at microbial metabolism taking place within—but three other life-detection experiments each lander carried only found null results. More muddled data about life on Mars arrived in 1996, when scientists discovered what could have been microbial microfossils inside a Martian meteorite found in Antarctica. But subsequent studies showed the putative microfossils could have easily been produced by several other entirely abiotic routes. Most recently, researchers studying the atmosphere of Venus claimed to see significant amounts of phosphine there—a gas that, on Earth, is chiefly made by microorganisms. Yet soon other scientists had cast doubt on the validity of those measurements, and had postulated the gas—if it was there at all—was from some strange-but-lifeless form of Venusian volcanism.
In each case, the pattern was the same: initial excitement, followed by subsequent skepticism, and eventual dismissal. Time and time again, it seems, astrobiologists are only finding alien signs of life—so-called biosignatures—that are frustratingly inconclusive. This is in large part because astrobiologists by necessity seek the simplest, most robust forms of life that appear possible in harsh otherworldly environments, and the chemicals and structures we often associate with such organisms on Earth can often be produced abiotically. And, of course, the chemistry of alien life might be entirely different from what we observe on our own planet. Is there a better way to look?
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