A technique that uses fossilized raindrops to work out what the air pressure on Earth was billions of years ago has been used for the first time by scientists in the US. By analysing the shapes and sizes of raindrop imprints in volcanic ash, the team has shown that the atmospheric pressure in the Archaean eon was roughly the same as it is today. This is at odds with a popular theory of how the Earth stayed warm enough for life to exist at the time.

Billions of years ago, the Sun was about 20% dimmer than today because a star burns hydrogen more slowly earlier in its fusion cycle. There would therefore have been less radiation reaching the Earth and the surface should have been frozen. However, there is ample evidence of liquid water at the time as well as very primitive forms of life – a mystery known as the "Faint Young Sun" paradox.

Most scientists agree that the Earth must have been able to retain more heat in the past – but the reason why remains controversial. One explanation, proposed in 2009, is that atmospheric pressure was many times today's figure, causing pressure-broadening, whereby carbon dioxide becomes a more efficient greenhouse gas at higher pressures.

Terminal velocity

To test this, astrobiologist Sanjoy Som and colleagues at the University of Washington in Seattle reached back into the history books. In 1851 the British geologist Charles Lyell, proposed that atmospheric pressures of the past could be estimated by analysing the marks made by raindrops that have fallen onto volcanic ash. Some of these marks can still been seen today and Lyell suggested that they would reveal the speed at which the raindrops struck the ground. Raindrops hit the ground at terminal velocity, which is reached when gravity equals air resistance. Because air resistance depends on atmospheric pressure, so does the terminal velocity of a raindrop of a given size.

In the subsequent 150 years, however, nobody has successfully implemented the idea – until now. "The reasons, I think, are that, first of all, raindrop imprints are extremely rare," explains Som. "I guess it was a combination of having excellent field scientists in my colleagues Roger Buick and Jelte Harnmeijer and the strong foundation of fluid mechanics of myself and David Catling. Bringing those two worlds together is not common."

A new use for hairspray

The researchers produced latex impressions of 2.7 billion year old raindrop impressions found in South Africa and produced detailed laser measurements of these. They compared these to water droplets dropped down a stairwell onto recent volcanic ash – some of it from the 2010 eruption of the Icelandic volcano Eyjafjallajökull – that they had hardened with hairspray. By comparing the size of the impressions produced, and assuming that the raindrops that caused the prehistoric impressions were of roughly typical size, they estimated that atmospheric pressure 2.7 billion years ago was 50–105% of the pressure today, which would rule out pressure-broadening as a solution to the Faint Young Sun paradox. Even if the raindrops happened to have been the size of the largest raindrops ever recorded (and such raindrops are extremely rare), it is still questionable whether pressure broadening would be possible.

"I think it's a pretty sound study," says earth and planetary scientist William Cassata of the University of California, Berkeley. "I think it'll be interesting to see if, once other researchers go and look at similar deposits elsewhere in the geologic record, they can establish a coherent trend through time. That would help us to have more confidence, but as a singular constraint it looks very robust."

Applications to astrobiology

If pressure-broadening is not the explanation for the paradox, most scientists believe the explanation is that Earth's atmosphere in the eon contained large quantities of gases such as methane, which are potent greenhouse gases at any pressure. Som, who now works in the exobiology branch of NASA's Ames Research Center in California, is interested in the potential of research in this area to aid astrobiology.

Astronomers have already found hundreds of planets orbiting other stars and he believes that the discovery of an Earth-like planet could happen soon. "The way we're going to probe this extrasolar planet is by measuring the composition of the atmosphere, because life is a big controller of what the atmosphere of a planet can be". These results could then be compared to what we know about Earth's atmosphere today – and in the past when Earth was very a different planet than today, but very much alive with microbial life.

The research is published in Nature 10.1038/nature10890.