Ancient Earth Created Its Own Water from Deep Within the Mantle

Earth may be mostly covered by water, but where did it first come from? Scientists have long wondered whether water was created by our planet, or whether it arrived via icy comets. Now, they may have their answer.

While trying to understand the formation of early Earth, scientists have suggested that the planet was dry and inhospitable to life until icy comets pelted Earth and deposited water on its surface. Yet there is another theory; it's possible that our planet formed with entire oceans of water in its interior, and has been continuously supplying water to the surface with plate tectonics ever since.

Scientists have long known that the mantle of Earth contains some water, but exactly how much it contains has long remained a mystery. In addition, if some geological mechanism were supplying water to the surface, wouldn't the mantle have run out of water by now?

"When we look into the origins of water on Earth, what we're really asking is why are we so different than all the other planets?" said Wendy Panero, one of the researchers, in a news release. "In this solar system, Earth is unique because we have liquid water of the surface. We're also the only planet with active plate tectonics. Maybe this water in the mantle is key to plate tectonics, and that's part of what makes Earth habitable."

In order to see whether the mantle could be producing water, the researchers compressed different minerals that are common in the mantle and subjected them to high pressures and temperatures to simulate conditions in the deep Earth. They examined how the minerals' crystal structures changed as they were compressed, and then used that information to gauge the minerals' ability to store hydrogen. Then, the scientists used computer calculations to see what geochemical processes would be needed to allow these minerals to rise through the mantle and to the surface.

In the end, the researchers found that it's very likely that at least some of Earth's water comes from the mantle. In fact, it's possible that our planet may hold half as much water in its depths as is currently flowing in oceans on the surface. This water is continually cycled through the transition zone as a result of plate tectonics. That said, it's also likely that some of our planet's water also came from icy comets.

The findings were presented at the American Geophysical Union (AGU) meeting on Dec. 17.

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