An experiment pushes matter to extreme conditions and reveals how two new minerals could hold huge reserves of water near the Earth’s core.
“Water is life” is not just a figure of speech: without liquid water, none of the life forms we know would exist on Earth. But its role does not stop at biology. Water is also the invisible lubricant that allows the Earth’s mantle – the thick layer of semi-solid rock between the crust and the core – to move, albeit very slowly. By hydrating deep rocks, water makes them more “sliding”, allowing the layers to slide over each other.
This movement fuels the tectonic cycle, the mechanism that over hundreds of millions of years contributes to regulating the movement of the earth’s plates, recycling rocks and gases through the mantle and, last but not least, the planet’s climate. “Liquid water is the key ingredient of the Earth’s habitability,” writes geoscientist Alfred Wilson, of the University of Leeds, in a commentary published on Nature Geoscience which accompanies a new study dedicated to the water reserves hidden in the depths of the planet.
Where did the water come from and where did it go?
If on the one hand we already knew about the presence of water in the depths of the Earth, on the other two questions remained open… “Where does the water trapped in the mantle come from?” Scientists have two main hypotheses. The first is that it was the asteroids, in the first convulsive millions of years of life of the Solar System, that brought water to Earth, which would have remained trapped in the planet since its formation. The second hypothesis is that the water arrived later, hydrating a mantle that was originally completely or almost dry.
The second question is: “Where is this water hidden within a mantle that extends for thousands of kilometers beneath our feet?” The new study attempts to fill this last gap, pointing to the border zone between the mantle and the Earth’s liquid outer core as the most likely location — a region that lies nearly 2,900 kilometers deep.
It is precisely in this band that seismic investigations have long identified strange anomalies called “ultra-low velocity zones”: portions of the mantle in which seismic waves slow down anomalously, a clue that the composition of the rock there is different from the rest of the mantle.
The lower mantle: an environment hostile to water
To orient yourself, it is useful to know that the Earth’s lower mantle extends approximately between 660 and 2,900 kilometers deep.
It is dominated by two minerals, bridgmanite and ferropericlase, both considered essentially “dry”, i.e. incapable of retaining significant quantities of water in their crystalline structure.
Other potentially hydrated minerals exist, but to remain stable at those pressures and temperatures they require very particular chemical compositions, or they simply decompose with the extreme heat that reigns in the deepest mantle, almost identical to that of the Sun’s surface. Faced with this apparent dead end, the research group decided to look elsewhere.
Diamonds, lasers and an unexpected discovery
To simulate the extreme conditions of the deep mantle — pressures of hundreds of thousands of atmospheres and temperatures of thousands of degrees — the researchers used laser-heated diamond anvil cells. These are instruments that squeeze samples just a fraction of a millimeter in size between two diamond tips as far apart as the thickness of a sheet of paper, while powerful laser beams hit them to reproduce the heat of the Earth’s depths.
In these extreme conditions, a surprise emerged: two compounds never observed before, both iron oxyhydroxides — that is, minerals that combine iron, oxygen and hydrogen — capable of trapping large quantities of water. It should be noted that this is, for now, a demonstration of possibilitynot of certainty, although they explain in an incredibly precise way where water can be trapped: the experiments prove that these mineral phases can form and be stable in deep mantle conditions, but they do not directly demonstrate that they are actually present inside the Earth at this time. “The identification of these iron oxyhydroxides is important because they are dense and apparently stable phases, capable of capturing and holding water under a wide range of lower mantle conditions,” Wilson writes in his commentary.
A significant detail is that these minerals are formed even when there is very little water available. In some experiments the starting material contained less than 0.1% water, yet this minimal amount was enough to stabilize the new crystalline phases. It is a crucial point, because the depths of the Earth should not be imagined as an underground ocean: any reserve of water present there must necessarily be “trapped” within mineral lattices, often in environments where free water is practically non-existent.
An elevator to the core
There is another element that makes this discovery interesting: both minerals are stable in the extreme conditions of the lower mantle and are decidedly denser than the surrounding rocks.
This means that as the ancient “basal magma ocean” — the immense sea of molten rock that covered the Earth in its earliest stages of life, billions of years ago — progressively cooled and solidified, these water-rich minerals may have formed first and then sunk, through sheer density, to the mantle-core boundary, where they still remain today.
But this imprisoned water does not necessarily remain so forever. The Earth’s mantle, despite being solid, moves in geological time through a phenomenon called convection: huge portions of rock slowly rise to the surface and others sink, in a cycle that lasts hundreds of millions of years. As the water-rich material is dragged upward by these movements, the pressure progressively decreases and at some point the minerals may become unstable, releasing the water they were holding and transferring it to other mineral phases in the mantle. From there, some of this water could rise to the surface through mantle plumes — columns of hot rock rising from the deep, at the origin of volcanic phenomena like those in Hawaii — and through volcanic activity in general.
It is worth remembering, regarding the difficulties of this type of research, that man has never been able to dig really deep to personally verify what lies underground: the deepest hole ever made, the famous super-deep Kola well in Russia, reached just over 12 kilometres, an infinitesimal fraction compared to the almost 2,900 kilometers of thickness of the mantle. For this reason, recreating the physical conditions of the deep underground in the laboratory remains the only way to understand what can really exist down there.
Questions still open
Despite the enthusiasm, many question marks remain, and Wilson himself calls the current picture “incomplete”. It is not yet clear exactly how much water these minerals can actually contain, nor what really happens when they reach the boundary between the core and the mantle. It also remains to be understood with what speed and ease the water trapped in these depths can eventually rise to the surface.
What this research suggests, however, is a fascinating idea: the Earth’s water cycle – what we study in school as rain, rivers, seas and evaporation – may not stop at the crust, but extend much deeper than previously thought, to the edges of the planet’s core.
