Scientists have concluded that the Earth's early atmosphere was loaded with greenhouse gases - by examining fossilized raindrop impressions.
Between two and four billion years ago, the sun burned as much as 30 percent less brightly than it does now.
"Because the sun was so much fainter back then, if the atmosphere was the same as it is today the Earth should have been frozen," says Sanjoy Som, a postdoctoral researcher at NASA's Ames Research Center.
But while, theoretically, the planet should have been encased in ice, there's geologic evidence for rivers and ocean sediments at the time.
Now, University of Washington researchers have used evidence from fossilized raindrop impressions from 2.7 billion years ago to deduce the atmospheric pressure at the time, and say that an abundance of greenhouse gases most likely caused the warm temperatures.
The sizes of raindrop impressions depend on their velocity, atmospheric pressure and the composition of the material into which they fall.
At Earth's surface, raindrops won't get any bigger than a quarter-inch in diameter - about the same size as created the largest fossilized impressions - regardless of the atmospheric pressure.
Today, raindrops that size fall at about 30 feet per second. But if the ancient atmosphere was thicker, they'd have fallen more slowly, and left smaller imprints.
If the biggest imprints were formed by the largest raindrops, atmospheric pressure 2.7 billion years ago could have been no more than twice what it is today. But the largest possible raindrops are extremely rare, he said, so it is very likely that the pressure was the same, or even lower, than it is today. That would favor a buildup of greenhouse gases in the atmosphere to explain a warmer Earth.
The finding could prove important in the search for life on planets orbiting other stars, called exoplanets. That's because the Earth of 2.7 billion years ago was very different from what we know today, and yet it too supported abundant life in the form of microbes.
"Setting limits on atmospheric pressure is the first step towards understanding what the atmospheric composition was then," says Som. "Knowing this will double the known data points that we have for comparison to exoplanets that might support life."