Water is essential to life on Earth, but it also plays a hidden role far below our feet. It helps the mantle, the thick rocky layer beneath the crust, move slowly over millions of years. This movement drives plate tectonics, which helps regulate the planet's climate over long periods of time. Scientists have long wondered exactly where this deep water is stored and how it got there in the first place. Some believe asteroids delivered water to Earth as the planet formed. Others think a dry mantle was hydrated later on.
A new study, published in Nature Geoscience, may have found part of the answer. Researchers believe the water sits near the boundary between the lower mantle and the liquid outer core, an area roughly 660 to 2,900 kilometres (373 to 1,802 miles) below the surface. This region contains strange patches known as "ultralow velocity zones", which have puzzled scientists for years.
Most minerals in the lower mantle, such as bridgmanite and ferropericlase, are thought to be dry. So the research team used diamond anvil cells, tiny devices that squeeze samples between diamond tips while lasers heat them, to copy the extreme pressure and heat found deep inside the Earth.
Under these conditions, they discovered two previously unknown minerals, called iron oxyhydroxides, which appear able to store large amounts of water. Remarkably, these minerals formed even when only tiny traces of water were present, less than 0.1 per cent in some tests.
The minerals are dense and stable under the harsh conditions of the deep mantle. This means that early in Earth's history, when a molten "magma ocean" cooled and hardened, these minerals may have formed and sunk towards the core.
The water may not stay locked away forever. As mantle material slowly rises and pressure drops, these minerals could break down, releasing water that eventually rises back towards the surface through volcanoes.
Geoscientist Alfred Wilson from the University of Leeds, who wrote a commentary on the study, said the new minerals "represent a breakthrough in the mystery of how the Earth obtained and retained its water." He added that many questions remain, including how much water the minerals actually hold and how long it takes for that water to reach the surface.
Mineral physicist Leonid Dubrovinsky from the University of Bayreuth said even very small amounts of hydrogen were enough to stabilise these highly hydrated iron compounds.
The research is published in Nature Geoscience.
