PLANETOLOGY : The memory of water. To find the origin of the oceans, look in outer space.

Earth - the quintessential blue planet - has not always been covered by water. Around 4.6 billion years ago, in the solar system's early years, the energetic young sun's radiation meant that the zone immediately surrounding it was hot and dry.

Earth, then coalescing from dust and gas in this region, thus began as a desiccated rock. How it subsequently acquired its oceans has long puzzled planetary scientists.

One possible source of Earth's water is carbonaceous [c-type] asteroids, the most common variety. But it cannot be the sole source, because water in chunks of these have landed as meteorites does not match the isotopic fingerprints of terrestrial water.

This fingerprint is the ratio of normal water [ H2O, made from hydrogen and oxygen ] to heavy water [ D2O and HDO, which both include deuterium, an isotope of hydrogen that has a neutron in its nucleus alongside the proton characteristic of every hydrogen atom]. Water from c-type asteroids has more deuterium in it than does terrestrial water.

Another possibility is comets, which are basically dirty snowballs that arrive from the outer solar system. A barrage of these a few hundred million years after Earth's formation would have done the job nicely.

But samples returned from comets by spacecraft suggest their isotopic fingerprint is even less Earthlike than that of C-type asteroids. So, as Luke Daly, a planetary geoscientist at the University of Glasgow, in Britain, observes : ''It basically means we need something else in our solar system, some other reservoir of water to be on the lighter side to balance the books.''

In their search for this reservoir, Dr Daly's team recently studied grains of silicate dust from another sample-return mission, to an asteroid called ITOKAWA.

This is an S-type stony body, with a composition different from that of C-types. The grains had been brought back by Hayabusa, a Japanese craft, in 2011.

Grains of this sort formed at the same time as Earth, and then spent the intervening billions orbiting the sun, occasionally gathering into tiny rocks or falling onto the surfaces of asteroids, such as Itokawa, to create a fine-grained regolith.

Most, though, have remained free-floating. Indeed, they are collectively visible at sunrise and sunset, in clear, dark skies, a faint glow known as the Zodiacal light.

Using a technique called atom-probe tomography, Dr Daly was able to examine the composition of the grains in his possession one atom at a time. He found, as he describes in this week's Nature Astronomy, that they contained a significant amount of water below their surfaces. That surprised him. What was intriguing, though, was his discovery's lack of deuterium.

Dr Daley reckons this water's existence can be explained by weathering of the space dust over billions of years by the solar wind, a stream of charged particles - mostly protons - that flows out into space from the sun.

When they hit a particle of space dust, these protons penetrate a few nanometres below the surface and change its chemical composition. In particular, if a proton knocks out one of the metal atoms in a silicate crystal lattice, it is then likely to bond with an adjacent oxygen atom to form a hydroxide ion  [OH].

Add a second proton, possibly billions of years later, and you have deuterium-free water. The result, in these samples, at least, was the equivalent of 20 litres of water for every cubic metre of rock.

Doing the sums, Dr Daly thinks about half of Earth's water derives from c-type asteroids, with an admixture of comets. The rest, which dilutes the deuterium in this, is the result of grains weathered space dust falling through Earth's atmosphere throughout the planet's history, burning up as they did so and freeing their microscopic aqueous payloads to rain down, literally, on the planet's surface.

This finding, moreover, also hints that water could accumulate anywhere in the solar system which the solar wind could reach - the surface of the Moon, for example, as well as asteroids.

That is good news for space explorers. In such places, the weathered dust could be a source of water for astronauts.

The World Students Society thanks author The Economist.


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