March 29, 2012

Raindrop Fossils As Evidence Of A Warm Early Earth

Fossilized raindrop impressions have shown scientists evidence of greenhouse gases in Earth´s early atmosphere.

This means between 2 and 4 billion years ago, the sun may have burned 30% 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.

While the Earth should have been theoretically encased in ice, geologic evidence suggests otherwise. Scientists have studied sediments from rivers and oceans believed to be from the same period as these raindrops, suggesting not everything was completely cold and frozen. So how, then, was the sun able to shine less brightly and rivers and oceans still able to flow?

Researchers from the University of Washington are looking to fossilized raindrop impressions for their answers. These 2.7 billion year old impressions are giving researchers information about the atmospheric pressure at the time. These impressions have also shown greenhouse gases may be a likely cause for such warm temperatures.

The size of each raindrop impression depends on the velocity at which they fell, the atmospheric pressure and the composition of material into which they fell. At Earth´s surface, raindrops normally don´t get any bigger than a quarter-inch in diameter. The fossilized impressions were about the same size, regardless of atmospheric pressure.

In today´s atmosphere, raindrops of this size will normally fall at a rate of 30 feet per second. However, if the ancient atmosphere was indeed thicker as the scientists are predicting, then the raindrops would have fallen at a slower rate of speed and therefore, would have left smaller imprints.

The size of these imprints are helping Som and the University of Washington students determine the ancient atmospheric pressure.

According to their studies, if the largest raindrop impressions were caused by the largest raindrops, then the atmospheric pressure 2.7 million years ago could have been twice what it is today. However, not many of these large raindrops were discovered. This means it is highly likely the pressure of the Earth was the same, if not lower than it is today. Since atmospheric pressure was close to the same level as it is today, SOM and his University of Washington team believe a buildup of greenhouse gases would explain a warmer Earth.

This kind of finding could prove important for other areas of study. By understanding the levels of greenhouse gases on an ancient Earth, scientists can better understand the chances of life existing on other planets orbiting the starts, called exoplanets. Just as the Earth, very different 2.7 billion years ago than it is today, was able to support abundant life in the form of microbes, so too could planets with a similar atmosphere support life of its own.

“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.”


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