Secular Café: 3 billion year old raindrops

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3 billion year old raindrops Mar 28th 2012, 22:47 Quote: Raindrops In Rock: Clues To A Perplexing Paradox by Richard Harris Researchers have analyzed the fossil imprints of of raindrops, like the ones shown here, to study the atmosphere of the Earth, as it was 2.7 billion years ago. The rule at the top is 5 centimeters, or about 2 inches, long. Enlarge W. Alterman/University of Pretoria Researchers have analyzed the fossil imprints of of raindrops, like the ones shown here, to study the atmosphere of the Earth, as it was 2.7 billion years ago. The rule at the top is 5 centimeters, or about 2 inches, long. text size A A A March 28, 2012 The late astronomer Carl Sagan presented this paradox to his colleagues: We know the sun was a lot fainter two billion years ago. So why wasn't the Earth frozen solid? We know it wasn't because there's plenty of evidence for warm seas and flowing water way back then. The question is still puzzling scientists. But new clues to that paradox come from an unlikely source: fossilized raindrops, from 2.7 billion years ago. Back then, the Earth had no trees or flowers or animals birds or fish. But it did have volcanoes. And it did rain. You can see evidence of that in a remarkable fossil, found in South Africa. It records an ancient downpour. "So it rained 2.7 billion years ago on volcanic ash," says scientist Sanjoy Som. "The ash was covered by a very thin but very resistant layer of ash, and all that was again covered by more volcanic ash." That ash turned to rock. And even today, you can still see the little rims around the small impact craters the raindrops created. "So they're flawlessly preserved, which is surprising for rocks that are that old." Som was getting his Ph.D. at the University of Washington, and his advisor suggested that Som examine those fossilized raindrops to see what he might learn about the ancient atmosphere. "Because I have background in aeronautical engineering I found that challenge super exciting and jumped on it," says Som, who is now at NASA's Ames Research Center. The scientists knew that the size of raindrops would tell them how thick the atmosphere was back then. Raindrops fall more slowly through thick air than through thin air. So the thicker the air, the more gently they will fall to earth, and the smaller the craters will be. And the raindrop craters they found were similar to those you get on dusty lava today. So Som and his colleagues conclude that the atmosphere back then was a lot like it is today, maybe a bit thinner, but surely no more than twice as dense. .... http://www.npr.org/2012/03/28/149527...lexing-paradox

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