Atmospheres conducive to life

Thirty years ago, when I was an editor at Nature, the journal received a manuscript that left us in a quandary. It was submitted by Carl Sagan, whose international fame as the presenter of the TV show Cosmos (I, like other nerdy kids of my generation, had watched it avidly) already made it hard to consider the paper dispassionately. Sagan was of course a respected planetary scientist, albeit having suffered from the elitist derision that then tended to be directed by many scientists at peers who deigned to popularise their work. But our problem was that the paper wasn’t really reporting anything new or surprising. Its conclusion was that there was probably intelligent life on Earth.

What made this suggestion striking, however, was that it was derived solely from data collected by Nasa’s Galileo spacecraft as it passed 960km above our planet on its way to Jupiter after a gravitational assist via Venus. Sagan had the prescient idea of testing whether the spacecraft’s detectors, including infrared and ultraviolet spectrometers, could gather enough remote-sensing data to make a strong case for our planet being inhabited. Those data also included direct imaging of the surface and plasma-wave detection of radio signals, but much of the argument hinged on the spectroscopic analysis of the atmosphere’s chemical composition.