Raindrops Tell The Story of Earth’s Ancient Atmosphere

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Fossil raindrops housed at the Blue Beach Fossil Museum in Nova Scotia. Image by Rygel, M.C.

Does the word ‘fossil’ bring to mind great swathes of dusty desert, covered in cacti, and the occasional dinosaur bone?

It shouldn’t. Not always, anyway.

Instead, think of rain.  Think of a dim earth covered in a thick atmosphere. Think of raindrops falling to the ground through this dense atmosphere, their imprints fossilized and found billions of years later. That’s right: billions of years.

These ancient raindrops tell the story of the Earth’s climate 2.7 billion years ago.

Recently, researchers at the University of Washington have used fossilized raindrop impressions to discover an intriguing tale about the earth’s atmosphere. Lead author Sanjoy Som conducted the research as part of his doctoral work in Earth and space sciences. He is currently a postdoctoral researcher at NASA’s Ames Research Center in Mountain View, California.

Raindrop Impressions Enable Study of Earth’s Ancient Atmosphere

The story of these ancient raindrops begins on a wet day 2.7 billion years ago, somewhere in the vicinity of modern day South Africa. A raindrop fell and created an impression on the ground.  Then more rain fell, making similar impressions, some of which were fossilized. While we might think of thousands of raindrops as a wet day, Som and his colleagues have discovered a way to make these raindrops tell a story about the composition of the ancient atmosphere.

The story begins with the physics of water. As a drop of water falls, it moves through the atmosphere. The atmosphere impacts the speed at which the water falls. If the atmosphere is thick, the raindrop falls more slowly, like a spoon moving through thick pea soup. If the atmosphere is thinner, the raindrop falls more quickly.

Click to Read Page Two: When The Raindrops Hit The Ground

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