I came across this interesting story on the Institute of Physics website today and thought it worth sharing. More puzzles than answers, but it might jog a few ideas.
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 nitrogen and carbon dioxide become more efficient greenhouse gases at higher pressures. ;)
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.