Back in the 1970s when NASA launched the Viking probes to Mars in search of life, James Lovelock suggested that the Martian atmosphere (and that of Venus, Earth’s evil twin) told a story when compared with Earth. Our nitrogen-oxygen atmosphere is unusual and owes its existence to the biological activity of microbes and plants. Our neighbours have atmospheres of predominantly carbon dioxide and their chemical nature suggests that little if anything biological is going on there.
It’s commonly believed that before about 2.4 billion years ago, Earth’s atmosphere was free of oxygen. There’s a lot of evidence for this, including the absence of oxidised compounds in ancient rocks. There’s considerable debate about the exact composition of the early atmosphere – in particular, how “reducing” it was – that is, what were the levels of hydrogen-rich molecules such as methane and ammonia. This is important for understanding prebiotic chemistry and the emergence of the first things that we would call living organisms. Most biologists are familiar to some degree with the famous, if misleading, Urey-Miller experiment.
An interesting paper in Nature last week examines sulphur isotopes in 2.8 billion year-old sediments from Western Australia. They expected to see evidence for UV-induced photochemical reactions, due to the assumed absence of an oxygen-derived ozone layer at that time – but they didn’t. So was the atmosphere actually oxic when the rocks formed, or is something else going on? It’s a complex problem that illustrates how we often have to rely on assumptions and models and struggle to reconcile them when the data doesn’t fit.