London, Sept 26 : Oceans on Earth were ‘home-grown’ and formed because the young planet had a thick blanket of hydrogen, which reacted with oxides in the mantle to form lakes and seas, Japanese planetary scientists have said.

The dominant view has been that Earth got its share of water from moisture-rich asteroids and comets that rained down on the planet during its formative years.

Scientists believe that just after the Earth formed, it was very hot and dry. Theory 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’, so named because it contains a neutron in addition to a proton – to hydrogen in our seawater 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 believe otherwise.

“Water is essential for the origin and evolution of life. Why does water exist on Earth, where did it come from? These are fundamental questions for human beings,” said Genda.

The team said evidence for the thick hydrogen shroud came from the Earth's orbit.

Earth’s 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.

However, if the water on Earth did form from a thick hydrogen atmosphere, it should have originally had a far lower value of the deuterium-to-hydrogen ratio than we see in seawater today.

Genda and Ikoma have got round that problem as well.

Their calculations have shown that the ratio would have naturally drifted upwards over time.

The researchers said 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,” the researchers said in their study.

Genda and Ikoma conclude from their calculations that that the oceans might well have been chemically manufactured right here on Earth.

“It's only theoretical, but it's a good hypothesis and I think it's very interesting for future research. We might have to rethink theories of how much water the comets could have brought,” New Scientist quoted Kathrin Altwegg, a comet expert from Bern University in Switzerland, as saying.

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 the hazy picture of the solar system's early history.

Spacecraft missions would need to investigate deuterium-to-hydrogen ratios on planets, moons and comets at various locations across the solar system, she said. (With inputs from ANI)