• Thu. Nov 26th, 2020

Where’d Earth’s water come from? Ancient rocks hold a clue

Earth’s oceans may have formed directly from the “star stuff” that coalesced into the early solar system, according to new research – challenging the idea that most of Earth’s water was delivered here by meteorites and comets.

Scientists studying rare meteorites thought to have formed in the hot inner solar system found that the ancient rocks contain more hydrogen than previously thought – and hydrogen, along with oxygen, is one of the two essential ingredients of water.

If the Earth formed from similar materials, then more than enough hydrogen would have been available to combine with oxygen to create all of the world’s water – many times more, in fact.

A piece of the meteorite Sahara 97096, an enstatite chondrite that was found to contain the equivalent of 0.5 percent of its weight in water.L. Piani / Museum of Natural History in Paris

“The Earth might have been wet from the beginning, when it started to form,” said Laurette Piani, a cosmochemist at the Petrographic and Geochemical Research Center in France and lead author of a paper on the new findings published Thursday in the journal Science.

Early models of the formation of the solar system suggested the Earth should be bone dry because it formed so close to the hot young sun. Instead, it’s flowing with life-sustaining water.

Scientists have tried to explain the mystery for years, and developed a theory that the young Earth was bombarded by water-rich meteorites and icy comets from the frozen outer solar system, which became our oceans, lakes, rivers and clouds.

But this exotic proposal doesn’t match tell-tale chemical traces in the Earth’s composition, and it doesn’t directly explain the many oceans worth of water locked up underground in the Earth’s rocks.

Piani and her colleagues examined enstatite chondrite meteorites thought to have formed in the hot inner parts of the solar system more than 4.5 billion years ago, alongside the Earth and the other rocky planets.

Only about 200 enstatite chondrite meteorites have ever been found, but they match specific aspects of the chemical composition of our planet.

That suggests they are similar to the “building blocks” that formed the Earth from an interstellar nebula – “star stuff” – in the early solar system, Piani said.

Feb. 27, 202000:26

The researchers used special procedures to analyze the meteorites, including gently heating their samples for several days to remove any Earth water that contaminated them after they fell from space.

They found more hydrogen in their minerals than expected – enough that the hydrogen in similar materials could have combined with oxygen to create at least three times all the water known to exist on Earth, both on its surface and deep underground.

“Our discovery shows that the Earth’s building blocks might have significantly contributed to the Earth’s water,” Piani said. “Hydrogen-bearing material was present in the inner solar system at the time of the rocky planet formation, even though the temperatures were too high for water to condense.”

The study also suggests that the later bombardment from the outer solar system may have still happened, but it would have made only a small contribution to our surface water, and no significant contribution to the water in the Earth’s rocks.

“The interior water can be explained by inner solar system material,” Piani said. “We calculate that only 5 percent of the surface water was inherited from outer solar system material.”

The study “brings a crucial and elegant element to this puzzle,” wrote planetary scientist Anne Peslier from NASA’s Johnson Space Center in Houston, in a scientific article published in Science. “Earth’s water may simply have come from the nebular material from which the planet accreted.”

Rick Carlson, a geochemist and the director of the Earth and Planets Laboratory at the Carnegie Institution for Science in Washington, said the research strengthened the idea that the Earth formed from a mixture of materials from the early solar system.

It also showed the Earth was relatively “dry” compared to other bodies in the solar system, despite its visible abundance of water; it still contained enough hydrogen when it formed, however, to create the oceans of water we see now, and more.

“Even a dry meteorite like an enstatite chondrite has enough water in it to [account for] all the Earth’s water,” he said. “I think that’s a pretty cool discovery.”