The real question is not “is there life on Mars?”, but “was there?” It’s not beyond the realm of possibility that primitive bacteria-like life ekes out an existence on the frigid surface of the planet today, but very few scientists expect that. It’s considerably more possible that such life might have appeared briefly on Mars in the distant past when it had a warmer climate and liquid water, perhaps even shallow oceans, on its surface.
That’s why scientists are searching for signs of life in Martian rocks. Fossil life-forms are unlikely, but there may be surviving indications of life in the chemical composition of the rocks. That is precisely what an international team working on data provided by Nasa’s robotic Mars rover Perseverance thinks it might have identified.
They reported their results in a paper in Nature on 10 September, when Nasa also hosted a press conference on the finding. “This very well could be the clearest sign of life that we’ve ever found on Mars,” enthused the acting Nasa administrator Sean Duffy, a supremely unqualified Trump appointee, while the scientists strove to stress how tentative their conclusions are.
The rocks were found within a long dried-up river channel that once fed into the lake filling a crater named Jezero, which shows strong evidence of once having been filled by a lake. The reddish and tan-coloured rocks are a kind of mudstone, apparently sedimented from fine silt. Perseverance has instruments for carrying out automated analysis of the chemical properties and structure of such samples, and the information it has broadcast back to Earth reveals that the rocks contain “organic carbon”.
That designation doesn’t mean the carbon compounds necessarily came from living organisms. But the particular mixture of this material with minerals like iron phosphate and iron sulphide is hard to explain if it was made by purely geological processes. On Earth, compositions like this are found in aquatic environments where iron minerals have been chemically transformed by microbes. What’s more, reactions involving iron sulfide minerals have long been proposed as a kind of primitive metabolism for the earliest forms of life on our planet.
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So the Nasa researchers propose that all this constitutes a “potential biosignature”: evidence for biological processes happening on Mars. Still, it’s very hard to say for sure that a particular molecule, mineral or mixture on some other world couldn’t possibly have been produced “abiotically” – that is, without assistance from life.
We just don’t know enough about these extraterrestrial chemical environments to pronounce confidently on what they can or can’t make. But the claim is reasonable, and deeply enticing. Discovering evidence of life (even if it’s now long extinct) on just one other world would transform our view of how common it is likely to be in the universe.
Bear in mind the thought that, during the review process at Nature (which the journal now makes publicly available), one reviewer initially warned that “the story of this manuscript is misguided, probably wrong, and will lead to the creation of a new controversy about… past life on Mars.” The reviewer reminded the authors of an earlier debacle in 1996, when a team of Nasa scientists claimed to have found “evidence for primitive life on early Mars” in a Martian meteorite found on Earth.
On that occasion, the paper “made headlines around the world with President Bill Clinton even giving a press conference about it.” (Little danger that the current US president would care enough to do that, and if he did, most sensible people would assume he was lying again.) The warning was well posed: that earlier claim was quickly shown to be overblown.
This time around, the scientists are being more circumspect, and perhaps not just to avoid overselling the science. Their paper rightly ends by arguing that the best way to find out more about this intriguing rock would be to have a robotic mission bring back a sample to Earth. But Trump’s proposed cut of almost 50% for Nasa’s science budget will, if approved, surely spell doom for precisely such a mission, which Nasa has been planning for years.
Trump, of course, wants the glory of sending people to Mars instead – something unlikely in his lifetime, let alone his presidency (if those two are now distinct), and which might end up merely contaminating any delicate signs of early Martian life. “Nasa’s commitment to conducting gold standard science will continue as we pursue our goal of putting American boots on Mars’ rocky soil,” waffled Duffy, while the souls of the scientists silently shrivelled.
