An ocean below Earth’s crust could be key to a habitable planet

A new model suggests “mantle rain” from inside Earth’s mantle ensures we will always have a surface ocean.
The deep water cycle is hidden under the Earth's crust. NASA

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Hidden inside the Earth—within the first several hundred kilometers below the crust—there is another ocean. It is, most likely, the largest ocean in the world. This water is not sloshing around in a big pool. No fish plumb its depths. In fact, this ocean is only water in the loosest sense: broken into its composite hydrogen and oxygen atoms and chemically bound to the surrounding rock, this ocean is in storage. Or, most of it is.

Denis Andrault and Nathalie Bolfan-Casanova, geoscientists at the University of Clermont Auvergne in France, have developed a new model that shows more of this water is in transit than previously thought. When the solid rock in the mantle—the layer of the planet between the crust and the core—becomes saturated with chemically dissociated water, it can transform into a water-rich molten slurry. When it does, it seeps back up toward the crust. The researchers call this mantle rain.

Much as the cycling of water between the atmosphere, glaciers, lakes, rivers, aquifers, and the ocean affects the level of the sea, the abundance of rain, and the frequency of drought, the exchange of water between the mantle and the surface also dictates the habitability of the Earth. Scientists already know that water can be dragged down to the mantle by subducting tectonic plates and brought back to the surface by things like volcanic eruptions, hydrothermal vents, and the creation of new crust at oceanic spreading centers. If this deep water cycle between the mantle and the surface is in balance, Earth’s sea level remains stable. If not, our planet could exist as anything from a singular global ocean to a desiccated world.

Earth’s habitability has benefited greatly from the fact that Earth’s sea levels have remained relatively stable over billions of years. According to previous studies of the mantle, however, it could have been very different. Estimates based on previously understood mechanics of the deep water cycle suggest that nearly twice as much water is carried into the mantle as is released back to the surface.

“There is a layer about 410 kilometers below the surface that can hold a lot of water,” says Andrault. The prevailing understanding says that water should stay there forever, he says. If that were the case, the Earth’s surface water would have slowly decreased, locked away in the mantle.

But that’s where mantle rain comes in.

In their study, Andrault and Bolfan-Casanova show that mantle rain could be enough to keep the deep water cycle in balance.

To discover mantle rain, the researchers looked at what happens when a subducting slab of rock and rock-bound water sinks deeper into the mantle. They found that as it descends, increasing temperatures and pressures cause the rocks to melt, releasing the water.

“The melt is like a slurry,” says Andrault. “Imagine a mushy mix of sand grains glued to each other with mud in between—the mud is the mantle rain.”

As more rocks melt, and as more water is liberated from the rock, this melt eventually becomes light enough that it begins to rise. As it does, the water bonds to minerals in the upper mantle and lowers their melting points, causing more melting that releases more water—and the cycle continues.

Andrault and Bolfan-Casanova’s model of mantle rain, says Yoshinori Miyazaki, an earth and planetary scientist at the California Institute of Technology who was not involved in the study, “shows there could be another way to transport water towards the surface in addition to the global-scale convection of the mantle itself.”

“Water generally doesn’t like to be in the rock phase,” Miyazaki says. “It will happily escape to the melt phase and percolate upwards.” Andrault says more work is needed to understand the extent to which water is escaping in this way.

The mantle rain model also suggests that there is currently one ocean mass in the upper mantle. “Together with the ocean on the surface,” says Andrault, “this ensures that there will always be water on Earth’s surface.”

“We still have a lot to learn about the deep water cycle,” says Miyazaki. “But one certain fact is that it has worked in an amazing way to keep Earth’s average sea level relatively constant over the past 500 million years, and probably longer, to sustain a habitable environment for life to continue.”