Fossil raindrops reveal Earth's early atmosphere

SOME things never change. An analysis of a fossilised rain shower suggests air density on early Earth was broadly similar to today's - making it difficult to explain why Earth was warmer than it is now when the sun shone less brightly.

Around 2.7 billion years ago, a volcanic eruption left what is now South Africa blanketed in soft ash. Before it hardened into rock, light rain left imprints in its surface. Using a pipette full of water, fresh volcanic ash and mathematical modelling, Sanjoy Som at the University of Washington in Seattle worked out how the size of imprints related to the speed at which the rain fell through the ancient atmosphere. They say air density was probably no greater than 1.3 kilograms per cubic metre - comparable to today's 1.2 kg/m³ (Nature, DOI: 10.1038/nature10890).

This creates a stumbling block for those who thought the solution to the faint sun paradox lay in a dense early atmosphere. The faint sun could have kept early Earth warm with the help of a thick blanket of greenhouse gases. That explanation now seems moot. "The 'faint young sun' paradox remains," says Som.

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