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Where Did Earth's Water Come From?

AUDIE CORNISH, HOST:

Water is everywhere on Earth - the clouds, the rain, the oceans and rivers, even our own bodies. Where all that water originally came from is a bit of a mystery. NPR's Nell Greenfieldboyce reports that scientists may have found the answer inside some rare meteorites.

NELL GREENFIELDBOYCE, BYLINE: The Earth formed 4 1/2 billion years ago. Compared to other planets, it emerged pretty close to the sun. There, hot temperatures would mean no water ice, no ice to join with the swirling bits of rock and dust that were running into each other and building up our young planet.

LAURETTE PIANI: That's why we do not know exactly where the water on Earth come from, why we need to find a source of water on Earth.

GREENFIELDBOYCE: Laurette Piani works at a French research lab called CRPG. She says that source of water could have been farther out in the solar system, like maybe icy comets or water-rich asteroids that hit the newly formed Earth and watered it. This has long been the prevailing view.

PIANI: To explain the presence of the Earth's ocean and of the Earth's water in general.

GREENFIELDBOYCE: She wondered, though, if water could have been there at the start. So she and some colleagues recently took a close look at a rare kind of meteorite. It doesn't look like anything special.

PIANI: It's a bit like a gray rock.

GREENFIELDBOYCE: But it's also thought to have formed near the sun and is the same kind of primordial stuff that glommed together to create our planet. And it turns out it contains plenty of hydrogen. That's an indicator of its ability to contribute water to a planetary mix. In fact, if you built a planet out of this material, you'd have at least several oceans' worth of water. These findings, described in the journal Science, made Anne Peslier feel really happy.

ANNE PESLIER: I was happy because it makes it nice and simple.

GREENFIELDBOYCE: She's a planetary scientist at NASA's Johnson Space Center in Houston, Texas, who wasn't part of the research team. She says this old idea that Earth's water came from the outer solar system would have required something unusual, like Jupiter having a little trip through the inner solar system to send water-rich asteroids headed our way.

PESLIER: So here, we just don't need Jupiter. We don't need it to do anything weird. We just grab the material that was there, where Earth formed, and that's where the water comes from.

GREENFIELDBOYCE: Still, she says, even if most of the water was there at the beginning, comets and such probably did deliver some of Earth's water later on. Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF HOORAY FOR EARTH SONG, "KEYS") Transcript provided by NPR, Copyright NPR.