Why water comes only on our …

The water that makes Earth a majestic blue marble was here from the time of our planet's birth, according to a <a href="http://www.sciencemag.org/content/346/6209/623">new study of ancient meteorites</a>, scientists reported Thursday.

Where do the oceans come from? The study headed by <a href="http://www.whoi.edu/profile.do?id=asarafian">Adam Sarafian</a> of the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Massachusetts, found that our seas may have arrived much earlier on our planet than previously thought.

The study pushes back the clock on the origin of Earth's water by hundreds of millions of years, to around 4.6 billion years ago, when all the worlds of the inner solar system were still forming.

Scientists had suspected that our planet formed dry, with high-energy impacts creating a molten surface on the infant Earth. Water came much later, went the thinking, thanks to collisions with wet comets and asteroids.

"Some people have argued that any water molecules that were present as the planets were forming would have evaporated or been blown off into space," said study co-author <a href="http://www.whoi.edu/science/GG/people/marschall/">Horst Marschall</a>, a geologist at WHOI.

For that reason, he said, scientists thought that "surface water as it exists on our planet today must have come much, much later—hundreds of millions of years later."