Analysis of imprints made by raindrops in volcanic ash 2.7 billion years ago is shedding light on Earth's early atmosphere and might help in the search for extraterrestrial life.
Astrobiologist Dr Sanjoy Som, of NASA's Ames Research Center, and colleagues, report their findings today in the journal Nature.
His study is the first to use the fossils to estimate Earth's atmospheric pressure at the time the raindrops fell.
"Fossil raindrops have been known about for a very long time but have not been used beyond proof that it rained," says Som.
He says the research could be useful in the search for extraterrestrial life which involves comparing candidate host planets to Earth.
Scientists eventually hope to compare the host planets' atmosphere, to Earth's - not just Earth as we know it today, but as it was in the early days, which was quite a different planet.
"We could tentatively detect evidence of life on other planets based on the composition of the atmosphere," says Som.
A key step to working out atmospheric composition is understanding atmospheric pressure, since pressure equates to the amount of gas crammed into the atmosphere.
Som and colleagues used imprints from ancient raindrops to find out the atmospheric pressure of the Earth 2.7 billion years ago.
The team measured the size of 955 fossil raindrop imprints in South Africa from 3D images generated from latex casts of the craters.
They then did a "low-tech" experiment to calculate the relationship between raindrop crater size and raindrop momentum, which is linked to the mass of the raindrop, and the pressure of the air it falls through.
In the experiment one researcher released droplets of known size down seven floors of a stair well, while Som caught the drops in a tray full of ash from the Icelandic Eyjafjallajokull volcano, mimicking the surface the ancient raindrops would have hit.
The higher the air pressure, the greater the resistance on the falling raindrop, the slower it falls and the smaller the crater it makes.
Using this relationship, Som and colleagues were able to work out the air pressure at the time the ancient raindrops fell.
They estimate that the pressure of Earth's atmosphere at that time was no more than twice what it is today.
"This estimate will allow scientist to make more precise calculations of what type of gases were present in the early Earth's atmosphere," says Som.
Som says the findings will also help scientists in their search for a solution to a long-standing puzzle called the 'Faint Young Sun' paradox.
"The Sun during this early period of Earth was about 20 to 30 percent dimmer than it is now and yet there is evidence of oceans, rivers and rain in the geological record, so the question is what was keeping the Earth warm?" says Som.
"Our findings are consistent with greenhouse gases being the source of the warming and will help atmospheric scientists narrow down which were those greenhouse gases, and how much was present."
Australian microbiologist Professor Malcolm Walter of the University of New South Wales says Som and colleague's work will be applicable to planets other than Earth.
And he describes their methods as "very innovative".
"I've never heard of such an approach before," he says. "I'm sure there are lots of complications and uncertainties there but it is a novel approach and I think it's very interesting."