Where did Earth's oceans come from? Astronomers have long contended that icy comets and asteroids delivered the water for them during an epoch of heavy bombardment that ended about 3.9 billion years ago. But a new study suggests that Earth supplied its own water, leaching it from the rocks that formed the planet. The finding may help explain why life on Earth appeared so early, and it may indicate that other rocky worlds are also awash in vast seas.
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.
Astrobiologists have been continually surprised by how quickly life evolved on Earth—within 600 million years after the planet's formation, or about 3.9 billion years ago. Elkins-Tanton's findings may help explain why. "If water oceans were present shortly after the impact that formed the moon [some 4.45 billion years ago]," says Dirk Schulze-Makuch, an astrobiologist at Washington State University, Pullman, "much more time would be available for the evolution of life, and it would explain why life was already relatively complex when we find the first traces of it in the rock record."
Pin Chen, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, says Elkins-Tanton presents a compelling scientific story that oceans form very early in the history of a terrestrial-type planet. Chen notes that the work also supports the suggestion that early Mars had a wetter climate than it does today and thus might have supported life. So, too, might a number of Earth-like planets that astronomers are just beginning to discover, says Schulze-Makuch.
Even so, Max Bernstein, an astrochemist at NASA Headquarters in Washington, D.C., notes that Elkins-Tanton's models don't include the possibility that the huge asteroid and comet impacts prevalent during the formation of our solar system boiled off the water. "Just because there was an ocean early on," he says, "doesn't mean that it stuck around long enough for life." Elkins-Tanton counters that even a huge impact would not cause Earth-like planets to lose more than half of their oceans.