No one knows exactly how Earth acquired more than 300 million trillion gallons of water. The Earth would have been too hot during its formative years to have had oceans from the beginning.

Now, astronomers have found a hint by observing a distant space rock named 24 Themis. There, they found the first indication of water on an asteroid.

The prevailing theory had once been that comets brought water to the earth, said astronomer Josh Emery, of the University of Tennessee. But that idea fell out of favor when they realized that water in comets carries a different chemical fingerprint from Earth's water.

That left the possibility that our water came from asteroids - those hundreds of thousands of large celestial bodies (one kilometer across or larger) swirling around the sun - most of them in a belt between Mars and Jupiter.

Emery, along with astronomer Andrew Rivkin of Johns Hopkins University, scrutinized 24 Themis using a giant infrared-sensitive telescope on the high peak of Mauna Kea in Hawaii. They found not only the signature of frozen water but of organic molecules - the basic building blocks of life.

They announced their findings last week in the journal Nature.

Emery said ice shouldn't last on the surface of this asteroid for long, and therefore it must be getting replenished by some underground reservoir. The astronomers expect to learn more from a possible mission that will travel to an asteroid and return a sample.

The finding of water and organic molecules on 24 Themis may help explain how the ingredients for life came together in our solar system. And that understanding will help guide the search for life among the approximately 400 known solar systems sprinkled through the galaxy. - Faye Flam