Most scientists believe that icy comets seeded water on ancient Earth, but a new study suggests that it also came from inside the Earth itself.

A new study from researchers at The Ohio State University shows evidence that the Earth's plate tectonics pushed water from inside the planet's mantle to its surface.

Most researchers believe that early Earth did not have water, making it uninhabitable, but at some point, water came here and life developed. Part of that was due to comets hitting the planet and leaving water behind.

This new study suggests that the Earth made its own water, too, and that plate tectonics plays an important role in getting that water to the planet's surface.

"In this solar system, Earth is unique because we have liquid water on the surface," says Wendy Panero, associate professor of earth sciences at Ohio State. "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."

Panero and her fellow researchers worked with the idea that rocks, or minerals, contain hydrogen atoms trapped inside empty pockets and crystals. Minerals also have lots of oxygen, so when the hydrogen and oxygen combine, it makes water inside those rocks.

This process occurs inside the Earth's mantle, particularly in rock known as ringwoodite. A previous study this year on ringwoodite suggests that the Earth's mantle contains a deep reservoir of water because of this process.

In a lab, Ohio State researchers compressed minerals and used computer calculations to figure out how these rocks react in deep Earth and how they respond to plate tectonics. They discovered that these processes could easily carry water from the Earth's interior to the surface, creating our current oceans.

However, it would stand to reason that this supply of water should have run out by now. Researchers also worked with another set of calculations, this time with another water carrying mineral, garnet, which exists higher up in the Earth's mantle from the ringwoodite. In simulations, plate tectonics carried water from the garnet back down to the ringwoodite, preventing the supply there from drying up.

Researchers estimate that today's Earth probably has about half as much water in its interior as that of the oceans on the surface, basically about as much water as the Pacific Ocean. This water is constantly being recirculated, thanks to plate tectonics.

"If all of the Earth's water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years," says Panero. "But if mantle circulation is also part of the water cycle, the total cycle time for our planet's water has to be billions of years."

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