Some people sustain that our oceans came from space, brought by water-filled asteroids and comets showering on a juvenile Earth, 3.8 billion years ago.

But this concept is challenged by Japanese planetary scientists, who point out that the oceans are a self production of the Earth, originating from a thick blanket of hydrogen, which oxidized with oxides from the Earth's mantle to form lakes and seas. "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 co-author Hidenori Genda from the Tokyo Institute of Technology.

Researchers believe in a very hot and dry young Earth, and the ratio of deuterium ("heavy hydrogen") isotope in the water matches the theory of the water-rich asteroids.

But the Japanese team comes with probes for its theory of the ancient thick atmosphere of hydrogen from the Earth's orbit. Current Earth orbit, similar to Venus and Mars, is circular now, but there are proofs that reveal it was once it more elongated. However, a thick, hydrogen-rich atmosphere would have buffered any elongation of the orbits.

The team also contradicts the idea according to which water forming from an early thick hydrogen atmosphere must have had a much lower value of the deuterium percentage than sea water nowadays has. Their calculations revealed that the percentage would have risen over time, especially due to the leakage of hydrogen into space. Lighter hydrogen escaped, but for the heavier deuterium, this proved more difficult, so its concentration grew up on Earth. Chemical reactions, too, led to the gradual replacement of hydrogen in water molecules for deuterium. "This is an interesting paper but in my opinion the results are not compelling", said comet expert Don Brownlee of the University of Washington in Seattle, US.

"A gas-rich nebula is not the only way to 'circularize' a planet's orbit. [And] as far as I know, there is no direct evidence for a large amount of free hydrogen . . . on the early Earth." he added. "We might have to rethink theories of how much water the comets could have brought. The picture might be a complex one in which water came from chemical reactions on Earth as well as asteroids and comets.", said Kathrin Altwegg, a comet expert from Bern University in Switzerland.

But "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 added.

NASA's Phoenix Mars Lander could offer interesting clues, once it arrives on the Red Planet in May 2008. "It will be really interesting once we analyze water on Mars. It would be funny if Mars did not get water in the same way as the Earth.", said Atlwegg.