How can we search for life on other planets when we don’t know what it might look like? One chemist thinks he has found an easy answer: just look for sophisticated molecular structures, no matter what they’re made of. The strategy could provide a simple way for upcoming space missions to broaden the hunt.
Until now, the search for traces of life, or biosignatures, on other planets has tended to focus mostly on molecules like those used by earthly life. Thus, Mars missions look for organic molecules, and future missions to Europa may look for amino acids, unequal proportions of mirror-image molecules, and unusual ratios of carbon isotopes, all of which are signatures of life here on Earth.
But if alien life is very different, it may not show any of these. “I think there’s a real possibility we could miss life if [resembling Earth life is] the only criterion,” says Mary Voytek, who heads NASA’s astrobiology programme.
Now Lee Cronin, a chemist at the University of Glasgow, UK, argues that complexity could be a biosignature that doesn’t depend on any assumptions about the life forms that produce it. “Biology has one signature: the ability to produce complex things that could not arise in the natural environment,” Cronin says.
Obviously, an aircraft or a mobile phone could not assemble spontaneously, so their existence points to a living – and even intelligent – being that built them. But simpler things like proteins, DNA molecules or steroid hormones are also highly unlikely to occur without being assembled by a living organism, Cronin says.
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