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.
Step by step
Cronin has developed a way to measure the complexity of a molecule by counting the number of unique steps – adding chemical side groups or ring structures, for example – needed for its formation, without double-counting repeated steps. To draw an analogy, his metric would score the words “bana” and “banana” as equally complex, since once you can make one “na” it is trivial to add a second one.
Any structure requiring more than about 15 steps is so complex it must be biological in origin, he said this week at the Astrobiology Science Conference in Mesa, Arizona.
Cronin thinks he may be able to make that criterion simpler still, by specifying a maximum molecular weight for compounds that can assemble spontaneously.
Astrobiologists welcome Cronin’s suggestion. “I appreciate Lee for developing a biosignature that has minimal assumptions about the biology,” says Voytek.
In practice, though, Voytek notes that a detector compact enough to travel on an interplanetary mission would probably need to be designed to look for carbon-based life.
And even if Cronin’s method works, no scientist would risk claiming to have found extraterrestrial life on the basis of just one line of evidence, says Kevin Hand of NASA’s Jet Propulsion Laboratory and project scientist for the Europa Lander mission now being developed by NASA. That means that future missions will still need to look for multiple biosignatures.