![]() ![]() Recognise this, and the argument is no longer about ‘habitability’ in the normal sense. DNA does its job as an entire supermolecule, and there is no biology somehow inherent in its fragments.Įither way, it remains the case that any life extant in the Venusian clouds would have to be chemically quite unlike that on Earth. Arguably this renders the stability of the bases themselves at such low pH no more significant than that of many other organic molecules. What’s more, the bases themselves would be protonated and therefore probably unable to form the Watson-Crick base pairs that are so central to the DNA double helix and to the mechanisms of information storage and transfer in genetics. To use these bases to encode sequence information would require an entirely alien biochemistry. 7 DNA and RNA themselves cannot persist in these conditions, however, because the phosphate ester backbone and the ribose sugar would be degraded by the acid. Sara Seager at the Massachusetts Institute of Technology and colleagues have recently shown that both the purine and pyrimidine bases of the nucleic acids DNA and RNA remain stable in concentrated sulfuric acid. It’s often implied that the extreme acidity of the cloud droplets would be inherently inimical to life, but that’s not a given. It’s not absurd to wonder if sulfuric acid might itself act as a solvent for some kind of life – it can, after all, engage in hydrogen bonding. It’s nonetheless intriguing to speculate about what might be possible in those clouds. 6 (Full disclosure: I was an author on the paper that calculated this activity.) But while the detection itself has been debated, 4,5 it is also hard to see how anything like terrestrial life could exist in the Venusian clouds, because their water activity is far smalle r than the minimal value at which microbes are known to live on Earth. There is, you see, no known geological source of phosphine. Meanwhile, the claimed detection in 2021 of phosphine (PH 3) in the clouds of Venus, 3 which is outside our solar system’s conventional habitable zone, excited speculation about microbial life subsisting in those atmospheric droplets, even though they are made from concentrated sulfuric acid. 2 They identify five exoplanets seen by the Kepler space telescope that fit this criterion, making them prime targets for spectroscopic examination of their atmospheres. Astrophysicist Cassandra Hall and colleagues at the University of Georgia, US, have argued that exoplanet searches for signs of life might be more narrowly focused on the ‘photosynthetic habitable zone’ where both liquid water and oxygen-producing photosynthesis can occur. 1īut perhaps habitable zones are both bigger and smaller than we once thought. Enceladus’s ocean was recently shown to contain phosphates, completing the inventory of elements needed for life as we know it. According to the definition on Nasa’s Exoplanet Exploration site, this zone of a solar system is ‘The distance from a star at which liquid water could exist on orbiting planets’ surfaces.’ That implies the zone is an annulus with a maximum and minimum radius, which is undermined even in our own solar system by the fact that several worlds, notably Jupiter’s moon Europa and Saturn’s moon Enceladus, lie outside the conventional habitable zone but have water below their icy crusts kept liquid by tidal heating. The notion of a habitable zone in astrobiology is looking ever more shaky. ![]()
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