Water may have been delivered to Earth … by asteroid

Analysis of dust gathered from an asteroid may have cleared up one of science’s biggest mysteries.

Asteroid Itokawa, photographed by Japan’s Hayabusa spacecraft. Picture: University of Tokyo, JAXA

Analysis of dust gathered from an asteroid may have cleared up one of science’s biggest mysteries: where did Earth’s water come from?

The first search for signs of water in samples collected from an asteroid has found surprisingly high levels, scientists said yesterday. Enough water could have been ferried to Earth via one of the most common classes of object seen in the asteroid belt, which lies between Mars and Jupiter, to account for half of the oceans. Moreover, the water detected had the correct blend of deuterium — a heavy form of hydrogen — to match that found in our seas.

Competing theories have jostled for decades to explain how 70 per cent of our planet came to be covered with water: the molten mess that was our early world would have boiled off every drop, so it must have been added later.

Some scientists have argued that it was brought here by icy comets but the latest analysis supports the theory that we have asteroids to thank.

The Japanese space probe Hayabusa, which was launched in May 2003, embarked on an unprecedented mission to land on asteroid 25143 Itokawa, a 535m-long lump orbiting near Mars. Picture: Japanese Space Agency

The samples were gathered in 2010 by the Japanese space probe Hayabusa from Itokawa, a small S-type asteroid, one of the most common class of objects in the asteroid belt. It had been considered “nominally dry”.

“Until we proposed it, no one thought to look for water,” said Maitrayee Bose of Arizona State University, co-author of a study published yesterday in the journal Science Advances. “I’m happy to report that our hunch paid off.”

The researchers studied five particles, each about half the thickness of a human hair, collected from Itokawa. In two of them they found the mineral pyroxene, which can hold water in its crystal structure. It was possible that Itokawa’s turbulent history — it has been involved in collisions and exposed to very high temperatures over its lifetime — had left it dry, but analysis revealed that the samples were unexpectedly rich in water.

Itokawa is a peanut-shaped asteroid about 1,800ft (550m) long and 1,000ft wide. It circles the sun every 18 months, at its closest sweeping inside the Earth’s orbit; at its furthest, it travels out beyond Mars. It is a remnant of a parent body that was at least 12 miles wide.

“This is the first absolutely clear detection of water on an asteroid of this type,” said Barry Kellett, of the Rutherford Appleton Laboratory, who was not involved in the research. “And it’s a type of asteroid that is certainly in our neighbourhood.”

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