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Oceans Gained

June 5, 2015

When you turn the tap on your kitchen sink, you have a pretty good idea where the water came from — a municipal water plant, or maybe your own well. But scientists are still trying to figure out where that water came from originally — what turned on the tap that made Earth the blue planet.

The newborn Earth was too hot to hold onto much water, which means the water had to be deposited later. For decades, the leading idea said it came from comets — balls of ice and rock left over from the birth of the solar system. According to that idea, when comets slammed into our planet they deposited vast amounts of water, which slowly coated more than two-thirds of Earth’s surface.

But observations of almost a dozen comets suggests otherwise. Spacecraft have measured the ratio of different forms of water in these comets. If comets were the source of Earth’s water, they’d match the ratio found here. But with one exception, they don’t. In fact, there’s a vast difference between Earth’s water and that found in most comets.

The closest match to Earth’s water is actually found not in comets, but in asteroids, which inhabit a broad belt between Mars and Jupiter. Although they’re made primarily of rock and metal, many asteroids have fair amounts of ice. Plenty of asteroids have hit our planet over the eons, so they may have supplied most of the water that fills our lakes and oceans — and trickles into our homes when we turn the tap.


Script by Damond Benningfield, Copyright 2015

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