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New analysis challenges existing ideas about our planet’s formation

Dry space rocks could have been Earth’s water source

by Sam Lemonick
August 27, 2020 | A version of this story appeared in Volume 98, Issue 33

Photograph of a meteorite analyzed in this study.
Credit: Laurette Piani/Museum of Natural History in Paris
Contrary to current theories, scientists now say meteorites like this one, thought to be made from the same material as Earth, could have supplied our planet's water.

When Earth and other inner planets formed from dust and gas orbiting the sun, they left behind material that scientists use as a record to study planet formation and other aspects of the early solar system. Called enstatite chondrite (EC) meteorites when they fall to Earth, these chunks of rock have chemical compositions similar to that of our planet. But the intense radiation close to the sun means that, while floating in space, these rocks have no water ice. That has left scientists wondering where our planet’s abundant water—both on the surface and locked in minerals underground—came from.

One theory is that comets or carbonaceous chondritic material from the outer solar system delivered water to the otherwise dry Earth after it formed. After analyzing 13 EC meteorites, Laurette Piani of the University of Lorraine and colleagues found that the samples contained more than enough hydrogen to account for all the water on Earth (Science 2020, DOI: 10.1126/science.aba1948). When Earth formed from this material, the hydrogen it contained would have combined with oxygen to make water, the researchers suggest. They also note that the hydrogen-to-deuterium ratio in these meteorites matches that found in Earth’s interior.

That doesn’t rule out water arriving from other sources, according to NASA scientist Anne Peslier, who says some EC water could have been lost to space when the young Earth was still very hot. And Piani’s group notes that isotopic evidence suggests that some surface water did not come from EC material. Questions they hope to address next are where and how hydrogen is stored in EC minerals.



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