Free Astronomy Magazine September-October 2019

9 ASTROBIOLOGY Not being able to realistically imagine life other than as we know it on our planet, as- tronomers will look for biosignatures iden- tical to terrestrial ones, having no idea what biosignatures might be associated with life forms based on elements other than carbon or metabolisms different from those of life on Earth. The canonical biosignatures of modern Earth are molecular oxygen (O 2 ) and meth- ane (CH 4 ). Their simultaneous presence in an evolved planetary atmosphere is con- sidered a particularly strong biosignature. However, their mutual interaction implies that they have a relatively short lifetime in Earth-like atmospheres. In fact, stellar UV rays easily break down oxygen molecules, creating radicals that rapidly destroy meth- ane molecules (to put it simply). Therefore, although these two molecules can be pro- duced by abiotic processes, in the absence of biological processes capable of con- stantly replenishing the atmosphere, oxy- gen and methane could not simultane- ously be present in quantities detectable in a planetary spectrum. On Earth, methanogenesis probably began in the Hadean, over 3.5 billion years ago, thanks to the anaerobic metabolism of the first bacterial species. Already in that re- mote aeon, most of the methane present in the atmosphere was probably biogenic, despite the fact that the geological pro- duction of that gas was more prevalant A n artist’s concept of a planet orbiting in the habitable zone of a K-type star. [NASA Ames/JPL-Caltech/ Tim Pyle]