About 2.7 billion years ago, towards the end of the Archaean period, a rain shower left its mark on ash deposits from a volcanic eruption in what is now the South African veldt. As the ash hardened to form tuff rock, the crater-like imprints of the individual raindrops were fossilized. An analysis of these imprints, aided by comparison with similar prints formed during the 2010 Eyjafjallajökull eruption in Iceland, suggests that air density during the Archaean was no more than twice modern levels. At that time, the Sun was dimmer than it is today, but the climate was warm. Most theories to explain this 'Faint Young Sun' paradox have assumed that the atmosphere was denser in the Archaean than it is now, and that the greenhouse effect was stronger, but this latest work seems to rule out higher carbon dioxide levels; nitrogen-pressure broadening remains unlikely, but possible.
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