During a seven-year round trip, the Japanese probe Hayabusa collected samples from the asteroid Itokawa and returned them to Earth in 2010. Now researchers from Arizona State University have discovered traces of water in those samples, adding weight to the idea that much of Earth's water may have come from asteroid impacts.

Although the sample has been extensively studied since its return, this marks the first time any team has specifically searched for signs of water. The Arizona State team studied five particles from the asteroid, using an instrument called the Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS).

In two of those five particles, the team spotted a mineral called pyroxene, which contains water in its crystal structure. That suggests that Itokawa is quite rich in water – after accounting for water that would have been lost through impacts and cosmic radiation, the researchers estimated the water content of Itokawa's minerals at between 698 and 988 parts per million (ppm).

"We found the samples we examined were enriched in water compared to the average for inner solar system objects," says Ziliang Jin, lead author of the study. "The minerals have hydrogen isotopic compositions that are indistinguishable from Earth."

According to the team, that makes asteroids like Itokawa (and the larger parent body that it's a fragment of) rich sources of water, supporting the long-standing theory that these space rocks helped deliver the vital liquid to the developing planets. In fact, up to half of the water in Earth's oceans today could have had these extraterrestrial origins.

While this is the first time scientists have found water in asteroid samples here on Earth, NASA's OSIRIS-REx probe recently beamed back news that it had detected water molecules in minerals on the asteroid Bennu.

And it may not be the last time, either. The follow-up mission Hayabusa 2 is currently out picking up samples from a space rock called Ryugu, and is due to return to Earth late next year.

The research was published in the journal Science Advances.

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