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The sun’s solar wind may have played a key part in delivering water to Earth, solving a vexing mystery about where our planet’s seas and oceans came from.

Studies of meteorites have found them to be surprisingly rich in water, suggesting that early in our planet’s history 4.6 billion years ago, incoming asteroids delivered water and allowed Earth to become the habitable world we see today. However, the composition of the water in meteorites doesn’t exactly match that of Earth’s: the extraterrestrial version contains more deuterium, a heavier form of hydrogen, indicating that there must have been another source of the water on our planet.

at the University of Glasgow, UK, and his colleagues think they have solved the issue. By studying a single grain of material returned from asteroid Itokawa by the Japanese spacecraft Hayabusa in 2010, they found evidence that the asteroid had been irradiated by particles in the solar wind, turning a small amount of each dust grain into water. “For every metre cubed of [asteroid material], you would get 20 litres of water,” says Daly.

Solar wind consists largely of hydrogen ions emitted by the sun, which combine with oxygen atoms in the asteroid rock to produce water. Previous research has shown that asteroids like Itokawa can contain a lot of water, but where that water would have come from wasn’t clear.

Our solar system is thought to have been abundant in dust in its early phases, so some of it could have been turned to water by the solar wind, before streaming down onto Earth’s surface after its formation. Crucially, this water contains less deuterium than that delivered by asteroids. “You could produce Earth’s oceans by mixing those two reservoirs together,” says Daly.

at the Natural History Museum in London says that while scientists have discussed the role of the solar wind in Earth’s water before, there has been little evidence that this process takes place. “This is real evidence that it’s happening,” he says. “It’s awesome. We can show that mixing that together with meteorites gives us a really nice match for Earth’s oceans.”

The method used to make the discovery involved studying individual atoms from the asteroid using a technique known as atom probe tomography, which could be useful in future studies. Daly hopes to use the same method to study grains from asteroid Ryugu returned by the Hayabusa 2 mission in 2020 to look for similar effects. “It’s going to be really interesting to see if we see the same thing,” he says.

The findings could have implications for space exploration too. “Every rocky surface will have small grains that have been irradiated by solar wind,” says Daly. “If we want to put up permanent human habitation facilities on other worlds, you could look at the [dust] as a way of producing water.”