London: Researchers are using the imprints of raindrops preserved in a 2.7 billion-year-old rock to figure out what the atmosphere was like on the early Earth.
Scientists have used the depressions that the drops left to calculate how fast they were going as they impacted the ground. It allowed them to determine the density of air in ancient times, the BBC News reported.
This "palaeobarometry" approach, revealed at the American Geophysical Union Meeting, will help constrain the models that try to simulate conditions in Archaean times.
Earth 2.7 billion years ago spun much faster, the Moon was closer and the Sun was much weaker. There were no animals or plants in existence back then as the air was simply not breathable.
"There was probably quite a bit of nitrogen in the atmosphere, like today, but there was no oxygen," explained Sanjoy Som from Nasa`s Ames Research Center.
"The oxygen was likely replaced by greenhouse gases such as carbon dioxide and methane," he said.
"My palaeobarometry work cannot tell you precisely what the gases were, but it will assist modellers of atmospheric composition by giving them a constraint," he told BBC News.
Som said the "fossil raindrops" were discovered in Ventersdorp in the North West Province of South Africa in the 1980s.
They consist of lots of pits in the surface of a rock that started out as volcanic ash-fall.
Rain tumbling on to the ash would have dug out small depressions, which were then subsequently covered over by further ash deposits and lithified, or turned to stone.=
Som and colleagues believe the pits should reveal something about ancient air pressure.
Their starting point is that the diameters of the imprints are controlled ultimately by the top speed of the raindrops as they hit the ground.
This number - the terminal velocity - is dependent on air density. In the modern atmosphere it is about 9m/s.
"The rationale here is that if the air back then was thicker, the raindrops would fall slower, and the craters in the ash would be smaller; and conversely, if the air was thinner, the drops would fall faster and the craters would be larger," said Som.
Som`s team conducted experiments which allowed the group to relate the momentum of a raindrop to the size of the imprints made; and then, using theory, to calculate the momentum of a drop of a known size at any air density.
The team concluded that if the biggest imprints in the Ventersdorp rock were formed by the largest raindrops, air pressure in the Archaean could have been no more than twice what it is today.
The study supports the idea that the ancient atmosphere must have had a strong concentration of greenhouse gases.