Astronomers long believed that most of Earth's water came from outer space during a period of heavy bombardment by icy comets and asteroids, but a new study suggests Earth may have supplied its own water:
Our planet has always harbored water. The rubble that coalesced to form Earth contained trace amounts—tens to hundreds of parts per million—of the stuff. But scientists didn't believe that was enough to create today's oceans, and thus they looked to alien origins for our water supply. Geologist Linda Elkins-Tanton of the Massachusetts Institute of Technology in Cambridge didn't think researchers needed to look that far.
To make her case, she conducted a chemical and physical analysis of Earth's library of meteorites—a useful analogue for the building blocks of our planet. She then plugged the data into a computer simulation of early Earth-like planets. Her models show that a large percentage of the water in the molten rock would quickly form a steam atmosphere before cooling and condensing into an ocean. The process would take tens of millions of years, meaning that oceans were sloshing around on Earth by as early as 4.4 billion years ago. Even the scant amount of water in the mantle, which is much drier than the sand in the Sahara, should produce oceans hundreds of meters deep, Elkins-Tanton reports in an upcoming paper in Astrophysics and Space Science.