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

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This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at

Hidden within the Earth, a few hundred kilometers below the crust, is another ocean. It is most likely the biggest ocean in the world . This water is not swimming in a large pool. No fish plumb its depths. This ocean is actually water in the most loosest sense. It is composed of hydrogen and oxygen atoms that have been chemically bound to the rock surrounding it. Or at least most of it.

Denis Andrault, and Nathalie Bolfan–Casanova are geoscientists at University of Clermont Auvergne, France. They have created a new model that shows more of the water is in transit than was previously thought .. The mantle, the layer of planet between the core and the crust, can become saturated with chemically dissociated liquids. This can cause it to transform into a water-rich, molten slurry. It seeps back towards the crust when it does. This is what researchers call mantle rain.

Just as the cycle of water between the atmosphere and lakes, rivers and rivers affects the amount of rain and water frequency, so does the exchange of water between mantle and surface. Scientists know that water can be pulled down to the mantle through subducting tectonic plate and brought back to surface by things such as volcanic eruptions, hydrothermal vents and the creation new crust at oceanic spreading centres. Earth’s sea level will remain stable if the deep water cycle between the surface and the mantle is in balance. If it is not, the planet could be anything from a single global ocean to a desiccated one.

Earth’s habitability has benefited greatly from the fact that Earth’s sea levels have remained relatively stable over billions of years. However, previous research on the mantle suggests that it could have been quite different. Based on the mechanics of deep water cycles, it is estimated that almost twice as much water is carried into and released to the surface.

“There’s a layer that is about 410 kilometers beneath the surface that can hold a lot water,” says Andrault. He says that the prevailing belief is that water should remain there for all time. If this were true, the Earth’s surface water would have gradually decreased, locked away inside the mantle.

But here’s where mantle rainfall comes in.

In their study, Andrault & Bolfan-Casanova showed that mantle rainfall could be enough to maintain a balanced deep water cycle.

To discover mantle rainfall, researchers studied what happens when rock-bound water and a slab of rock subduct into the mantle. The researchers found that the water is released as the rock melts as it falls.

” The melt is like a liquid.” says Andrault. “Imagine a mixture of sand grains glued together with mud between. This is the mantle rainfall

As more rocks melt and more water is released from the rock, the melt eventually becomes lighter enough to cause it to rise. As it melts, water bonds to the minerals in the upper mantle, lowering their melting points. This causes more melting, which releases more water.

Andrault’s and Bolfan-Casanova’s models of mantle rainfall, Yoshinori Miyazaki, an earth-planetary scientist at California Institute of Technology, who was not part of the study, “shows that there could be another way of transporting water towards the surface in additional to the global-scale convection of mantle .”

“Water doesn’t like being in the rock phase,” Miyazaki states. “It will happily escape into the melt phase and percolate upwards.” Andrault said that more research is needed to determine the extent of water escaping this way.

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

” We still have much to learn about deep water cycles,” says Miyazaki. “But one thing is certain, it has managed to maintain Earth’s average sealevel relatively constant over the past 500million years, and possibly longer, to sustain a healthy environment for life to continue .”


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