Water on Earth

Scientists believe that just after the Earth formed, it was very hot and dry. Theory also suggests that millions of water-rich comets and asteroids bombarded our planet around 3.8 billion years ago, neatly explaining why oceans later appeared.

What's more, the ratio of deuterium – or "heavy hydrogen" because it contains a neutron in addition to a proton – to hydrogen in our sea water matches the value found in water-rich asteroids, suggesting a common origin.

But Hidenori Genda and his colleague Masahiro Ikoma from the Tokyo Institute of Technology suggest another possibility. They say the Earth could have had a thick atmosphere of hydrogen, which reacted with oxides in the Earth's mantle to produce copious water.
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Evidence for the thick hydrogen shroud comes from the Earth's orbit. Its orbit, like those of Venus and Mars, is very circular now, but models suggest it started out more elongated. If the planets were still submerged in a thick, hydrogen-rich solar nebula after they formed, however, the thick gas might have damped out any elongation of the orbits.

If the water on Earth did form from a thick hydrogen atmosphere, however, it should have originally had a far lower value of the deuterium-to-hydrogen ratio than we see in sea water today. But Genda and Ikoma have got round this problem. Their calculations show that the ratio would have naturally drifted upwards over time.

Several effects would have contributed to this rise, including leakage of hydrogen into space. Energy from the Sun would have made most of the hydrogen escape, but the heavier deuterium would have escaped less easily, so it would have become more concentrated.

Also, chemical reactions favour the gradual exchange of hydrogen in water molecules for deuterium. Genda and Ikoma conclude from their calculations that that the oceans might well have been chemically manufactured right here on Earth.

Kathrin Altwegg, a comet expert from Bern University in Switzerland. "We might have to rethink theories of how much water the comets could have brought." She suspects the picture might be a complex one in which water came from chemical reactions on Earth as well as asteroids and comets.

But Altwegg says much more observational evidence is needed to clarify our hazy picture of the solar system's early history. Spacecraft missions need to investigate deuterium-to-hydrogen ratios on planets, moons and comets at various locations across the solar system, she says.

One intriguing clue could come from NASA's Phoenix Mars Lander, due to arrive on the Red Planet in May 2008. It aims to measure the deuterium-to-hydrogen ratio in Martian water ice for the first time.

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