New evidence upends thinking on how the Earth got water

An aerial photograph showing the most recent eruption at the Holuhraun lava field in Iceland. This eruption was fed directly from the Iceland mantle plume, and a proportion of the gas emitted is thought to be water vapour sourced from this plume.

The origins of water on the Earth have been a long-standing question puzzling scientists, but a new study provides the strongest evidence yet that that water-saturated dust embedded itself in the rocky clumps that eventually took shape as Earth formed.

That ancient dust saturated with water appears to have collected in the deep mantle.

Scientists at the University of Hawaii tested the hypothesis by studying rocks from Baffin Island in Canada that had come up as lava from the mantle.

The found small droplets of water trapped by glass crystals.

The water had the same composition of current water, that is the ratio of isotopes of hydrogen atoms in the water molecules. That ratio between one hydrogen isotope — deuterium — with one neutron, and another — hydrogen — with no neutrons is unique to every planet, asteroid and comet.

It was the discovery of a different ratio from Earth on Comet 67P/Churyumov–Gerasimenko by Rosetta that disproved a hypothesis that Earth's water had originated on comets. But measurements from Rosetta’s Rosina instrument found that water on comet 67P /Churyumov-Gerasimenko contains about three times more deuterium than water on Earth.

But the problem is finding samples on Earth that haven't been contaminated.

"We needed an undisturbed source of mantle from the Earth’s formation," Lydia Hallis, lead author of the study and a planetary scientist with the University of Glasgow in the United Kingdom, told Live Science.

"Hydrogen is everywhere on Earth. It's difficult to tell if what you're measuring isn't hydrogen from contamination. You're measuring parts per million in a piece of rock so small you can't see it. It took years to get to the point where we knew we were measuring something real, not water from the surface," she said.

Steve Desch, an astrophysicist and professor in the School of Earth and Space Exploration at Arizona State University, told Live Science the new research changed the way scientists would look at the problem.

"The debate about the origins of Earth's water has centered for decades on whether Earth got its water from comets or chondrites (rocky meteorites)," he said. This study suggests that dust and gas around the sun was an important contributor as well.

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