Hidden in the Earth – in the first several hundred kilometers below the crust – there is another ocean. It is probably the largest ocean in the world. This water does not flow in a large pool. No fish sinks to its depths. In fact, this ocean is just water in the most relaxed sense: broken down into its complex hydrogen and oxygen atoms and chemically bonded 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 that 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 – is saturated with chemically separated water, it can turn into a water-rich melt. When it does, it flows back to the crust. Researchers call this cloak rain. As the cycle of water between the atmosphere, glaciers, lakes, rivers, aquifers, and oceans affects sea level, abundance of rain, and 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 the mantle by lowering tectonic plates and being brought back to the surface by things like volcanic eruptions, hydrothermal vents, and the formation of new crust in oceanic centers. If this deep water cycle between the mantle and the surface is in equilibrium, the Earth’s sea level remains constant. If not, our planet could exist as anything from a single global ocean to a dry world. Earth’s habitability has benefited greatly from the fact that Earth’s sea levels have remained relatively stable for billions of years. According to previous studies on the mantle, however, it could be very different. Estimates based on previously understood deep water cycle mechanics suggest that almost twice as much water is transported to 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 perception is that water should stay there forever, he says. If this were to happen, the Earth’s surface waters would have been slowly reduced, locked in the mantle. But there comes the mantle rain. In their study, Andrault and Bolfan-Casanova show that mantle rain could be enough to keep the deep water cycle in balance. To find out the mantle rain, the researchers looked at what happens when a slab of rock and water-bound rock sinks deeper into the mantle. They found that as it descended, rising temperatures and pressures caused the rocks to melt, releasing water. “Melt is like pulp,” says Andrault. “Imagine a porous mixture of grains of sand stuck together with mud in between – the mud is the rain of the mantle.” As more rocks melt, and as more water is released from the rock, this melt eventually becomes light enough to begin to rise. As it happens, water binds to minerals in the upper mantle and reduces their melting points, causing more melting to release more water – and the cycle continues. The model of the Andrault and Bolfan-Casanova mantle rain, says Yoshinori Miyazaki, an earth and planet scientist at the California Institute of Technology who did not participate in the study, “shows that there could be other way of transporting water to the surface than world-wide transfer of the mantle itself “. “Water generally does not like to be in the rock phase,” says Miyazaki. “It will happily escape the melting phase and infiltrate upwards.” Andrault says more work is needed to understand the extent to which water escapes this way. The mantle rain pattern also suggests that there is currently an ocean mass in the upper mantle. “Along with the ocean on the surface,” says Andrault, “this ensures that there will always be water on the Earth’s surface.” “We still have a lot to learn about the deep water cycle,” says Miyazaki. “But one thing is for sure, it worked amazingly well to keep the Earth’s sea level relatively stable over the last 500 million years, and possibly longer, to maintain a habitable environment for life to continue.”
