“In the geology community, biology community, and even in the astronomy community, they are all interested in the origins of life, and water is one of the most important key elements that has to be considered,” Dong said.

Researchers weren’t looking for signs of liquid water, but its chemical equivalent, oxygen and hydrogen atoms, which bond to the interior of the planet. They compiled all the data in the scientific literature they could find on minerals that hold these signs and used the figures to calculate how much water there could be in the Earth’s mantle, which makes up the bulk of the planet’s interior. That number is referred to as the planet’s mantle water storage capacity. It changes as the interior of the planet continues to cool.

The group calculated what that number could be today and how much could have been stored a few billion years ago to see how the number had changed. The capacity back then was significantly less.

Scientists then compared those numbers to geochemical estimates of how much water is in the mantle today. Analysis found that the actual water content today is likely higher than the maximum water capacity of the mantle a few billion years ago, meaning the water today wouldn’t have been able to fit in the mantle billions of years ago. This suggests the water was someplace else — on the world’s surface. According to the researchers’ calculations, the amount of water that could have gone down into the Earth’s mantle could potentially be as much as all the present-day oceans combined.

“There has been water falling into the Earth’s interior over time, which makes sense because with plate tectonics you have some of the plates on the Earth’s surface that subduct and go down into the interior and bring water down with them,” said Rebecca Fischer, the Clare Boothe Luce Assistant Professor of Earth and Planetary Sciences and the study’s other lead author. “There’s not really anywhere that water could come from besides the oceans on the surface, so that implies that the oceans had to have been larger in the past.”

The study isn’t the first to suggest Earth could have been a water world, but the researchers believe it to be the first offering quantitative evidence based on the water storage capacity of the mantle.

The researchers point out some caveats in the study, the main one being that data on the minerals used to determine the amount of water in the planet’s mantle is limited when it comes to its deeper parts, which go down thousands of kilometers.

In their next project, Dong and Fischer are looking toward Mars. They plan to use a similar model to determine the amount of water that could have been stored in its interior.

“Evidence seems to point out that the early Mars had a significant amount of water on its surface,” Dong said. “We want to investigate whether that surface water had some relations with the water that could possibly have been stored in its interior.”

This study was supported by the National Science Foundation, the European Research Council, and a James Mills Pierce Fellowship from the Graduate School of Arts and Sciences.

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