Free Astronomy Magazine September-October 2019

10 SEPTEMBER-OCTOBER 2019 ASTROBIOLOGY than today. Instead, there was still no trace of biogenic oxygen, whose production would have started about a half-billion years later (3 billion years ago or perhaps even earlier), with the advent of photosyn- thesis, the currently dominant metabolism on our planet. Until the middle of the Pro- terozoic (2.0-0.7 billion years ago), the abundance of atmospheric oxygen re- mained very scarce, reaching probably only 0.1% of the present level, and its presence would have been inferred from a possible faraway civilization only through the spec- tral recognition of ozone (O 3 ), a photo- chemical by-product of oxygen that pro- duces a strong signal in the UV even when oxygen itself is too scarce to leave a trace. In the last half-billion years, however, the disequilibrium in the atmospheric compo- sition generated by life has become in- creasingly evident. Being able to analyze the terrestrial atmosphere through a spec- troscope from a distant planet, you would