Earth’s water may not all have come from asteroids © NASA

Earth’s water may not all have come from asteroids

Earth’s water is believed to have arrived here on asteroids, but that may not be the full story.

Well, this has blown some of the current thinking out of the water! Asteroids typically get all the credit for bringing H2O to Earth, but a new study suggests they may have to share the limelight with another source – the solar nebula, the cloud of gas and dust that eventually coalesced into the Sun and the rest of our Solar System.

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Scientists from Arizona State University researching the chemical composition of Earth’s water have found that some of it doesn’t match the profile of the ice that arrived in asteroids. By checking the ratio of hydrogen to deuterium (a hydrogen atom containing an extra neutron) in water samples, the team could determine the water’s likely source.

Asteroid alert! © Getty Images

The ratio of hydrogen to deuterium in ocean water is a good match for asteroids, but oceans do not account for all of the Earth’s water. The rest of it is dissolved in rocks beneath the surface in the planet’s mantle, and there’s more deuterium in the mantle than can be accounted for water delivered by asteroids alone. The team thinks that as the Earth was forming, molten iron on the embryonic planet’s surface absorbed hydrogen from the solar nebula and drew it down towards the early Earth’s centre. The deuterium atoms that came along with that hydrogen weren’t absorbed, however: they remained in what became the mantle and were added to by subsequent asteroid impacts.

Dr Jun Wu, lead author of the study, says further clues to support this theory may be the presence of other elements buried within the mantle. “Besides the hydrogen that the embryonic planet captured, we expect it also caught some carbon, nitrogen and noble gases from the early solar nebula. These should have left some isotope traces in the chemistry of the deepest rocks.”

The finding fits neatly into current theories of how the Sun and planets formed, but also has implications for habitable planets beyond the Solar System. Many of these exoplanets are believed to have formed far from the zones where water-rich asteroids and other building blocks might have been encountered, but they could still have collected hydrogen gas from their stars’ solar nebulas in the same way Earth did.

This is an extract from issue 330 of BBC Focus magazine.

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