Where did Earth’s water come from?

Study sheds light on the origins of liquid that covers 71 per cent of the planet

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Space dust grains may have carried water to Earth as the planet formed, a new study has found.

For decades, scientists have been trying to learn the source of Earth’s oceans, which cover 71 per cent of the planet.

It was always believed that water-carrying space rocks, known as C-type asteroids, could have brought water 4.6 billion years ago.

Now, a study by an international team of scientists shows that there could be more than one source.

A team led by the University of Glasgow studied samples from Itokawa, a small S-type asteroid, collected by Japanese space probe Hayabusa, which returned to Earth in 2010.

A fragment of the Itokawa asteroid studied by the international team of scientists. Photo: University of Glasgow

Solar wind played a part

The findings, published in the Nature Astronomy journal, showed a significant amount of water was produced below the surface of dust-sized grains from Itokawa by space weathering – which occurs when charged particles from the Sun, known as solar wind, change the chemical composition of the grains to produce water molecules.

Research suggests that this water-rich dust would have rained down alongside the C-type asteroids and created the Earth’s oceans.

“As recently as a decade ago, the notion that solar wind irradiation is relevant to the origin of water in the solar system, much less relevant to Earth's oceans, would have been greeted with scepticism,” said Prof John Bradley of the University of Hawaii at Manoa, Honolulu, a co-author of the paper.

“By showing for the first time that water is produced in situ on the surface of an asteroid, our study builds on the accumulating body of evidence that the interaction of the solar wind with oxygen-rich dust grains does indeed produce water."

The team used a technique known as "atom probe tomography" to measure the atomic structure of the grains one atom at a time and detect individual water molecules.

Meteorites on their way to Earth breaking through the atmosphere. Water-carrying dust may have rained down on early Earth, alongside C-type asteroids, to create the planet's oceans. Photo: University of Glasgow

Prof Michelle Thompson, another co-author, said these kinds of measurements would not have been possible without the technology.

“It gives us an extraordinary insight into how tiny dust particles floating in space might help us balance the books on the isotopic composition of the Earth’s water and give us new clues to help solve the mystery of its origins,” she said.

Water was previously discovered on the Itokawa asteroid but the process of space weathering gives newer insights into Earth's oceans.

Discovery could help future space missions

The team believes the findings suggest a way by which space explorers could secure a water source even on dry planets.

It is expensive to launch water into space and scientists are hopeful that future astronauts will be able to produce water supplies on site when exploring the Moon or other planets one day.

“One of the problems of future human space exploration is how astronauts will find enough water to keep them alive and accomplish their tasks without carrying it with them on their journey,” co-author Prof Hope Ishii said.

“We think it’s reasonable to assume that the same space-weathering process which created the water on Itokawa will have occurred to one degree or another on many airless worlds like the Moon or the asteroid Vesta.

“That could mean that space explorers may well be able to process fresh supplies of water straight from the dust on the planet’s surface. It’s exciting to think that the processes which formed the planets could help to support human life as we reach out beyond Earth.”