A Journey to the center of the Earth

You may think that travelling to the center of the Earth is just science fiction. Impossible even? Yet, perhaps it is possible…

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Fig. 1: Jules Vernes novel, ‘A Journey to the Centre of the Earth’, source : Penguin.co.uk

Jules Verne’s famous novel from 1864, ‘Journey to the Center of the Earth’, has inspired many people to wonder what the center of our planet is like and, if we could ever go there, what might we find? Since then we have learned much about the inner workings of our planet but it hasn’t stopped science fiction writers or scientists from imagining some way of getting there. But first, let’s go over what we do know about the interior of the planet. The Earth has a radius of 6378.1 kilometers. The innermost 1,210 km kilometers is the solid inner core that is thought to be mostly composed of iron and has a temperature of around 5400°C. This is surrounded by the outer liquid core (2,260 km thick) with the same composition. The convecting liquid of the outer core drives the Earth’s magnetic field, which protects us from the solar wind. Beyond this is the mantle that stretches from around 35 km to 2,890 km. The outermost layer is the crust, which is approximately 35 km thick under the continents and 6 km in the oceans. So, how far can we get into the Earth to find out more?

Is it possible to drill there? In the blockbuster film ‘the core’, the Earth’s core stops spinning. In order to save mankind, a team of scientists drills down to the core by using a giant laser beam. In reality though, the deepest scientific drill hole in the world is in the Kola Peninsula, in Russia, and it only reached a depth of 12,289 meters. At the deepest point of the Kola drill core the temperature is a rather hot 180°C. Much hotter than this, and the drill bits no longer work. Drilling therefore seems unlikely to get us much further than scratching the surface.

This hasn’t stopped others from thinking of ways to travel deeper. David Stevenson from Cal Tech, USA, devised a method by which a grapefruit sized probe could be sent to the Earth’s core within a mass of molten iron weighing about 10^8-10^10 kg. This might sound like a lot of iron but it is approximately the amount of iron produced in the world in a week. The molten iron would sink through the ground by melting the surrounding rocks and by being much denser than everything around it. They calculated that such a mass of iron would take approximately one week to reach the core! Similarly, scientists from the UK and Russia calculated that a spherical capsule filled with radionuclides would produce enough heat to melt the rock in its way and be able to reach the core. No one has ever tried either method, so for now we have to use more indirect ways to learn about the deep Earth.  

One of these indirect ones is seismology, the study of how seismic waves created by earthquakes move through the planet. This has been the main technique used to determine the structure of the deep earth. Like ripples on water surface, seismic waves move through the planet after an earthquake. By using different stations all over the world, it is possible to detect these waves and how fast they have travelled as they passed through Earth’s interior layers. There are two types of waves; P waves (pressure waves) can travel through solids and liquids and S waves (secondary or shear waves) only travel through solids. We know that the outer core is liquid because S waves cannot pass through but P waves can. Waves are also refracted (change direction) when they hit layers of different density such as the core-mantle boundary. It is from these refractions that we can determine the location of different layers within the Earth’s interior.

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Fig. 2: The curved pathways of seismic waves propagation through the Earth. source: Physical geology, Steven Earle, fig. 9.8

Another indirect method uses an important mineral to expand our knowledge about the deep earth, diamond. The recipe for diamond is simple; take some carbon and apply lots of heat and pressure. In fact, as Dan Frost from the geological institute in Bayreuth, Germany found out, it is possible to create diamonds from peanut butter! When diamonds form, deep in the mantle, they often trap small fragments of other minerals that are present at those depths. Rapidly rising magmas from the mantle can then pick up these mineral carrying diamonds and bring them to the surface.

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Fig. 3: Photo of a garnet inclusion within a diamond crystal. Made by Stephen H. Richardson, University of Cape Town

The Juina diamonds, from Brazil have been found to contain mineral fragments that may originate from as deep as 1700 km below the surface. For geologists, diamonds are not only pretty to look at but give us a unique window into the inner structure of our planet.

Another use of diamonds is in experiments. A diamond anvil cell uses two diamonds as compressing pistons to locally generate very high pressures in the studied material (Justine’s post). This can generate pressures of up to 6 million times atmospheric pressure.  By creating experimental conditions at super-high pressures and temperatures comparable to the deep mantle and core, it is possible to investigate what minerals can form at different pressures.
Whilst we have not yet been able to directly travel to the Earth’s core, it is in theory possible. For the moment though we have to rely on diamonds, experiments and seismic studies.

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