Research published this week by Graham Pearson of the University of Alberta and colleagues suggests that water—lots of it—is stored beneath Earth’s surface. At depths of 410 to 660 kilometers, deep within the mantle, water-rich minerals are present that contain up to 2.5 percent water by weight. It may not sound like much, but if that’s representative of the mantle layer, the total volume of water in the interior might be as much as in all of Earth’s oceans.
The Pearson group’s conclusion follows from their analysis of a diamond found in Brazil within a kimberlite pipe—a volcanic structure that can bring up rocks and minerals from deep within the mantle, where pressures are high enough to form diamonds. In one tiny piece of diamond the scientists found a mineral called ringwoodite, a variant of the common silicate mineral olivine, which they analyzed in detail using spectroscopic and X-ray diffraction analyses at the Arctic Resources Geochemistry Laboratory at the University of Alberta.
Why is this important for astrobiology? At these great depths, temperatures and pressures are high enough that we don’t expect to find microbial life. And the water in the mantle would be hydrated mineral water, meaning that it’s bound to the mineral and not directly available to organisms, even if pockets of lower temperatures do exist.
But this new result could help us understand how Earth’s oceans originated—whether by volcanic degassing from within, or from water delivered by comets. According to current thinking, our planet once had several times the amount of water it does today, due to ancient asteroid impacts that vaporized much of the early oceans. Some even argue that the oceans were completely vaporized by large impacts and almost all the water was lost to space. The new finding supports the idea that a large amount of the water currently present on the surface could have come from volcanic degassing from the interior (although infall by water-rich comets probably played a large role as well).
Earth’s diverse biosphere would be unthinkable without plate tectonics, the constant turning and churning of the planet’s crust that recycles nutrients essential for life. But in order for plate tectonics to work, convection cells within the mantle have to be active, and the deep subsurface has to be lubricated. That’s why we don’t see plate tectonics on Venus: its subsurface is bone-dry. The new results indicate that Earth’s mantle, as well as its crust, is water-rich, down to great depths, keeping Earth’s internal engine going, and supporting the rich biosphere that we see on the surface.