Oceans of the planet came from elsewhere

Understanding the origin of water on Earth, and the various possible explanations

Oceans of the planet came from elsewhere

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• The story: Earth is the beautiful blue planet. But this "blue" was not always there. Around 460 crore years ago, in the solar system’s early years, the energetic young sun’s radiation meant the zone immediately surrounding it was hot and dry. Earth was coalescing from dust and gas in this region, and began life as a desiccated rock. Scientists have worked hard to find how the oceans came about!
• Explanation: One source of Earth’s water may be carbonaceous (C-type) asteroids, the most common variety. But it cannot be the sole source, because water in chunks of these that have landed as meteorites does not match the isotopic fingerprint 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.
• Second explanation: Another chance is that comets, dirty snowballs that arrive from the outer solar system, kept hitting the Earth in the few hundred million years after Earth’s formation. Samples returned from comets by spacecraft show their isotopic fingerprint is even less Earthlike than that of C-type asteroids! So there is something else in the solar system, some other reservoir of water (on the lighter side), to explain this situation.

1. In their search, researchers 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.
2. Grains of this sort formed at the same time as Earth, and then spent the intervening billions of years 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.
3. Indeed, they are collectively visible at sunrise and sunset, in clear, dark skies, as a faint glow known as the Zodiacal light.

• New technique: Using a technique called atom-probe tomography, scientists examined the composition of the grains one atom at a time. They contained a significant amount of water just below their surfaces, which was surprising. The lack of deuterium too was! 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’s 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.
• It was a mix: Now researchers think that 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 of weathered space dust falling through Earth’s atmosphere throughout the planet’s history, burning up as they did so and freeing their microscopic water payloads to rain down on Earth! So water could accumulate anywhere in the solar system which the solar wind can reach (the surface of the Moon, or asteroids). That is good news for space explorers, as the weathered dust could be a source of water for astronauts.
• EXAM QUESTIONS: (1) Explain the origin of water on planet Earth. (2) What are the key constraints for future space travellers as they move through space, trying to colonise heavenly bodies like planets?

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