Fossil raindrops
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Fossil raindrops tell air story

The imprints of raindrops preserved in 2.7bn-year-old rock are being used to figure out what the atmosphere was like on the early Earth.

Scientists have calculated from the size of the depressions what the probable speed of the drops was as they impacted the ground.

This has allowed them to determine the density of air in ancient times.

Dr Sanjoy Som described the fossil raindrops to the BBC's Jonathan Amos.