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Solar winds from the Sun could have produced much of Earth's water by irradiating asteroids.
Reactions on the surface of these space rocks have been found to produce water, which could allow future astronauts to survive in space.
Though it may seem unlikely, the extremely hot atmosphere of the Sun could be responsible for filling the oceans with water.
An international team of scientists found that solar winds originating from our star changed the chemical makeup of asteroids orbiting it and causing them to eject oxygen. This oxygen then combined with hydrogen ions in the same solar winds and created water.
Some of this water is believed to have been brought to Earth by meteor strikes hundreds of millions of years ago, but a significant amount remains drifting around in space. In the future this water could be used by space explorers to replenish their supplies.
Professor John Bradley, a co-author of the paper, says, '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.
'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.
'It is directly relevant to the origin of water in planetary systems and possibly the isotopic composition of Earth's oceans.'
Dr Helena Bates, who researches water-bearing asteroids at the Museum and was not involved in the study, says, 'I think what is really special about this study is that they're using real samples from an asteroid that haven't been terrestrially altered. The problem with meteorites is that they have been sitting on Earth. Just by being exposed to oxygen and water vapor they change, and as a result evidence of the extra-terrestrial environment is lost.
'By using samples from an asteroid, we know exactly what they've been exposed to so we can remove those signals to give a better idea of what those conditions are like. I'm hoping in the future more studies are going to be looking into removing terrestrial contamination from their results.'
The findings of the study, led by the University of Glasgow, were published in Nature Astronomy.
Earth is an unusual planet in the solar system and as far as scientists are aware, the universe at large.
While water is relatively common across the universe it is normally found as ice or water vapour rather than as a liquid. Liquid water is only found on planets that are the right size to retain it, as well as being within the 'Goldilocks Zone' of a star where the conditions are right for it to stay as a liquid.
This means that having liquid water on the surface of a planet is rare. For 70% of the surface to be covered in water, as on Earth, is unheard of.
'There are a couple of theories as to how this water came to be on Earth,' Helena says, 'and the prevailing theory is that it has an extra-terrestrial source.
'At first, scientists thought comets were the ideal answer as they come from very far out in the solar system. This area is cooler so they contain water ice as part of their accreted material
'However, the evidence didn't stack up. The next solution was water-rich asteroids such as the carbonaceous chondrites, which we have a lot of on Earth.
'These rocks are made up of what are essentially clays that have been altered by the water and bound it within their structures. Their ratios are closer to that of Earth's water, but still not a perfect match.'
A potential alternative source is solar winds, which can cause a phenomenon known as space weathering when the hydrogen ions within it encounter objects in space.
Dr Luke Daly, the paper's lead author, explains, 'When those hydrogen ions hit an airless surface like an asteroid or a space-borne dust particle, they penetrate a few tens of nanometres below the surface where they can affect the chemical composition of the rock.
'Over time, the space weathering effect of the hydrogen ions can eject enough oxygen atoms from materials in the rock to create water trapped within minerals on the asteroid.'
The researchers used samples taken by the Hayabusa probe, which in 2005 collected samples from the asteroid Itokawa before returning its precious cargo to Earth in 2010. They compared these to materials provided by a range of universities.
The scientists found that the asteroid samples from parts of the rock which has been exposed to solar winds were more enriched with water compared to samples from parts of the asteroid which were not exposed. They estimated that every cubic metre of irradiated rock could contain up to 20 litres of water.
The samples taken by Hayabusa are thought to be representative of other asteroids, with experiments in the lab also mimicking the effect of solar wind to create water molecules. The scientists' findings suggest that water delivered to Earth by enriched asteroids may make up a significant amount of the oceans today.
But with many of these potentially water-rich asteroids still floating in space, while others are known to have collided with the Moon, it means that there could be a source of water for astronauts in the future.
Professor Hope Ishii, another co-author, says, '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.
'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.'