Earth’s largest water reservoir could be 1,800 miles beneath the surface

Source Article: Li, Y., Vočadlo, L., Sun, T., Brodholt, J.P., 2020. The Earth’s core as a reservoir of water. Nat. Geosci. 13, 453–458.

When Jules Verne first published his story “Journey to the Center of the Earth”, it was fantasy that one could sail across a vast ocean deep below the surface of the earth, but many became fascinated by what could exist thousands of miles beneath our feet. These fantasies remain even today as we are unable to study the innermost layers of our planet directly, leaving many questions that remain unanswered.

The vast ocean under the surface of the Earth imagined by Jules Verne. Illustration by Édouard Riou from “Journey to the Center of the Earth”. Source.

Each of the 4 major layers of the earth experience different amounts of heating or pressure and are made up of different materials. The crust is the uppermost layer, it is anywhere from 3-30 miles deep and composed of a variety of rocks. The next layer down is the mantle, which makes up 84% of earth’s volume. Most of that material is silicate rocks, and the deepest parts of the mantle are hot enough that the rocks become “plastic” and a bit stretchy.

The earth has four major layers: the crust, mantle, the inner and the outer cores. Source.

The inner and outer core are the innermost, and most mysterious, layers of the earth. Located 1,800 miles below the surface, we can’t study them directly. We do know that the conditions of these layers are probably a little extreme for any subterranean explorers, heated to around 9,000ºF and under extremely high pressure, anywhere from 330 to 360 GaPa, or 40 million psi.

The outer core layer is an impure mixture of liquid iron and other materials, while the inner core is nickel and iron under so much pressure that the melting point of the metals change and they are crystallized solid. 

One large secret surrounds the earth’s outer core, the material is 10% less dense than pure molten Iron should be at that temperature and pressure. Could it be some mystery material mixed in with the molten outer core? 

Not an ocean, but there could be water

When the earth was a young planet, just forming about 4.5 billion years ago, you can imagine that many different chunks of dust, rock, ice, and metal from the solar system collided together, eventually accumulating a structure of unconsolidated material.

The Earth was formed from unconsolidated gas, rocks, dust, ice, and metals. The process of planet formation can occur over thousands to millions of year. Image from NASA.

Before the earth’s core was formed 1-1.5 billion years ago, silicate rocks were mixed in with metallic iron, ice or water, and other alloys. Because metals are the densest material, the gravity of the earth pulled them to the center and eventually iron and nickel were separated from silicate rocks creating a distinct boundary layer between the core and the mantle.

However, it is unclear what happened to the water which was mixed in with all the other stuff. Did the water partition with the hot and dense core, or did it stick with the mantle? This has long been a mystery to scientists that study the earth’s inner layers, and although there are many theories, it has been difficult to test this question. 

But with recent advancements in laboratory and theoretical work, researchers from University College in London were able to come up with a well-informed guess how water may have behaved when the earth’s core was forming.

The researchers modeled the behavior of water subjected to high temperature and pressure conditions and tried to understand how the two materials present during the formation of earth’s core would affect the water. Based on those results, they speculated how water would divide itself between liquid iron core or plastic silicate mantle.  

They concluded that water would probably have preferred to stay with the molten iron layer, and they hypothesize that a large amount of water could have entered the molten core during formation. Based on these results, it is possible that even after the core was formed, any water that enters the deep mantle could partition into the core. 

This new study estimates that the earth’s core could contain at least 5 and up to 130 oceans worth of water.

Because studies of the core are theoretical, there are many pieces to this puzzle that are uncertain and so the numbers in this study are simply estimates. However, it is clear that if it was formed under the right conditions, the earth’s core could contain a large reservoir of water.

So although that water likely does not form a sea full of prehistoric marine reptiles, it does have cool implications for the history of our planet and the potential for planets to store life-giving water deep beneath their surfaces.

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Alina Spera

I am a second year PhD student at the University of Texas at El Paso. I study how climate change is impacting coastal Arctic biogeochemistry and ecology. When I'm not in the field or lab I'm probably running or hanging with my hamster!

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