A Journey to the Center of the Earth, to a Place Further than the Universe

“The surface temperature of Sun is about 5778 Kelvin”

Have you ever wondered what lies at the center of the planet you have spent the entirety of your life? If you are someone like Scott Kelly who thinks, “Well, I haven’t technically spent all my time here”, then here is a quick question: which is more difficult? Going to the outer space or reaching the center of the Earth? How much do we actually know of the place we call home, the only known observatory of our universe?

The lore and myths of the center of the Earth have been a topic of discussion in fiction, science, and beyond since time immemorial. Let us go back to 1692 AD. It was just 10 years after a mysterious unidentified comet flashed across the night sky. Astronomer Edmond Halley hasn’t yet identified the patterns related to this comet. Much before this, he proposed that “The Earth is hollow!” There have in fact been many over the years who believed in a hollow earth, much like flat-Earthers. Such a concept has been explored in science fiction, like the 1864 novel written by Jules Verne whose title you probably have heard of. (Didn’t you click on this because of that?) Surprisingly, there is even a website dedicated to this claim, https://www.ourhollowearth.com/ (Image: Movie Poster: Journey to the Center of the Earth

Today, we know for sure that Earth isn’t hollow. There is something inside, a place where no man has ever reached yet. Let us do some deep digging to find out what it is.

Technically, we are not going to do what Elon Musk did, but let’s dig deep into the information we have so far. Let me give you an ‘interesting company’ instead, if you are reading this stuck up in a traffic jam.

Into the Depths

Let us start our journey at the deepest place on the surface of Earth, the Mariana Trench in the Pacific Ocean. It is astonishing that even in this trench mankind has reached the deepest point, called the ‘Challenger Deep’which lies at a depth of around 10900 meters below the surface. With a pressure of about 1100 times more than that at sea level, at a dark place where even sunlight cannot reach, scientists have surprisingly identified life at the very bottom of the oceans.

But, I’ll have to rephrase my earlier sentence. The ‘Challenger Deep’ isn’t the deepest point on Earth as many might have believed.

This brings us to a hole drilled by humans in Russia which goes even deeper. Never heard of it? It goes by the name ‘Kola Super Deep Borehole’. People have literally dug it over a span of 19 years all the way down to the depth of 12262 meters making it the deepest point on Earth till date. They couldn’t dig further simply because the conditions were just too harsh with the temperature at the bottom reaching about 180 degree Celsius! That’s super hot. One thing is certain, as we go deeper, the temperature is increasing.

So, in the picture to the left, you can see a small sealed circle. Would you believe me if I told that it opens in a 12+ kilometer hole into the Earth? What you are looking at is actually the Kola Super Deep Borehole which has been sealed off in August 2012. If you are pondering over how deep that is, it is only about 0.192% of the distance to the center! That’s the maximum he who conquered the Moon could reach here on Earth. Everything below that level is explained only through various theories and hypotheses based on scientific data available to us.

Reaching the Underground Ocean

Had we continued digging, we would technically end up in a huge underground ocean that has water up to 3 times more than all the water of all the oceans on the surface combined together! Trust me, this is not taken from any science fiction novel.

How do we know this? Before I reveal the answer, a question. Where do you think all the water on our planet came from?

The research conducted by Northwestern University and The University of Mexico shows strong evidence of an underground water source as deep as 660 km. This beautiful info-graphic by Cath Levett illustrates the same.

The existence of Ringwoodite is the key to this puzzle. This material only forms under high pressure and harsh conditions deep beneath and guess what, it has water in it, as hydroxyl ions!

We are at 660 km below the sea level and still have 5711 km to travel. Back at school, we would have learnt that the Earth has a crust, mantle, and core; but that’s not all that is there. If you hadn’t realized it yet, we are already deep inside the mantle having surpassed the Mohorovi?i? discontinuity. (Shortened as Moho (at least now I can spell the word), this marks the transition from the crust to the mantle and exists at about 7–35 km depth)

Mantle: A place that makes 84% of Earth’s volume

Mantle is divided into 4 distinct layers;

1. Upper Mantle which includes the Lithosphere and the Asthenosphere
2. The Transition Zone (the 660 km mark where the underground water reserves are expected to exist)
3. Lower Mantle (up to 2891 Km in depth)
4. The Mysterious Core-Mantle Boundary

Towards the end of the lower mantle, we come across something quite unsettling. Before we go forward, a few things to tell:

Have you ever come across a Xenolith? In simple terms, it is a different type of rock within another rock. These special rocks give us insight into the structure of the mantle. More than often, these accessible xenoliths found in rocks originate from deep within the Earth and have made it to the surface. By analyzing their composition, we can understand the conditions that exist deep below.

Some Seismic Analysis

At the bottom of the lower mantle, we reach the core-mantle boundary (at 2981 km). What makes this special is that it acts as a boundary between the solid mantle and a liquid outer core. I repeat, the outer core is liquid!

Diving into the liquid metallic outer core, at about 2100 Km, we reach a point called the Gutenberg discontinuity. It’s time to turn on the Scientist Mode! To understand the significance of this, we must first know about the seismic waves, the body waves to be specific.

Seismic waves are just waves of ‘energy’ and can be broadly classified into surface waves (the ones that travel on the Earth’s surface) and body waves(the ones that travel through the Earth’s surface).

Body waves are further classified into Primary waves (or P-waves) and Secondary waves (or S-waves). A seismometer is a special sensitive instrument that can detect and record these seismic waves. Using these instruments, several things can be analyzed. Waves are generated because of vibrations, jerks or some disturbances within the Earth’s surface. We now know that P-waves are longitudinal in nature, meaning, the propagation of the wave is in the same direction as the displacement (or vibration) of the medium through which it travels. S-waves are found to be transverse in nature, meaning, the propagation of the wave is perpendicular to the direction of displacement of the medium. Transverse waves can hence only propagate through a rigid medium that can withhold the perpendicular displacements.

So, the conclusion to this whole analysis? Transverse waves (aka S-waves) cannot travel through liquids. But wait, did I just mention that our outer core is liquid in nature? Scientists immediately got hold of this argument and by simply analyzing the seismic waves, figured out the significance of the Gutenberg discontinuity. It essentially marks the boundary where the S-waves completely disappear (as they cannot travel beyond it), indicating liquid molten nature of the outer core, which also signifies why the P-waves decrease in velocity. That was how we were able to map the interior structure of Earth at such great depths.

Reaching the Final Frontier: The Inner Core

Traversing further into the outer core, we reach a place called the Bullen discontinuity where we once again meet with a remarkable distinction, the solid inner core. Yep! at the very center of the Earth lies a solid metallic inner core with a diameter of about 1220 Km. At this depth, the pressure is so huge that the iron and nickel present in the core can exist in a solid state in spite of the high temperature. How high is the temperature at this point by the way?

It is — wait for it — a whopping 5700 Kelvin! The very first line of this article which you may have thought was random, now starts to make some sense (scroll up and check if you have missed it!). Deep under your feet lies a hot ferocious metallic ball rivaling that of the Sun! How terrifying is that! These observations also help us understand the process by which planets are formed.

But wait, our journey isn’t just over yet. We now have a good idea of everything that exists up to the center of the earth, but something is amiss. No discussion about the core is complete without mentioning about the Earth’s magnetic field. Before we do that, here are a few questions to ponder.

"Questioning what may seem to be a very trivial observation can sometimes lead to revolutionary discoveries"

Have you ever wondered why the Earth rotates around its axis? Why do planets rotate?

Talking about rotation, did I mention that our inner core also, in fact, rotates in the east to west direction, that too at a rate faster than Earth’s rotation? Also, the outer liquid core rotates in the opposite direction (from west to east). Just take a moment to imagine these rotations and how complex the whole system is. These rotations are attributed to the magnetic field of the Earth.

From where does this magnetic field come from? Our best theory is from the core but there is a misconception here. Extreme heating of a material can have some interesting effects on it. It is time to introduce something called the Curie Point, which is essentially the maximum temperature up to which a certain material can retain its permanent magnetic properties. Above the Curie point, the material would lose all its magnetism. The Curie point of iron is about 1043 Kelvin. Isn’t the temperature at the core much much higher than that?

Clearly, the solid inner iron core is not the reason for the magnetic field. The origin of the magnetic field is explained using a more complicated dynamo theory. Let me end this technical jibber-jabber and get straight to the point.

If you are thinking that we have got it all figured out well, you are absolutely wrong. I have just scratched the surface of the tip of an iceberg. We literally know nothing but pretend as if we do! I’ll now just bombard you with some intriguing questions. Did you know that our magnetic field reverses it’s direction every few hundred thousand years? Mars doesn’t even have a magnetic field (as far as we know). How about that? Do you know about the T Tauri phase of the Sun that could have strongly influenced in generating the Earth’s magnetic field? There is too much to discuss and if I keep going, this article will literally become a textbook!

(Image: A computer simulation of Earth’s Magnetic Field (Just look at how complicated it is))

A few paragraphs earlier, when I mentioned that our journey isn’t over yet, I was actually referring to something else, something more bizarre.

Let’s Enter the Inner Inner Core:

A relatively recent research shows that the inner core itself has another layer called the inner inner core! Looks like the scientists are tired of naming all these layers at last.

(Image: CORE INCEPTION: The iron in the innermost part of Earth’s inner core (red) is oriented at a completely different angle (blue lines) compared with the rest of the inner core (orange). New research suggests that the innermost inner core actually formed billions of years earlier than previously thought, shortly after the planet’s formation. (PHOTO COURTESY: LACHINA PUBLISHING SERVICES))

The surprising fact about this region is that the iron crystals here are in east-west axis, unlike the other parts where they are oriented in north-south axis and we simply don’t know why.

Perhaps we can finally conclude that we have virtually reached the true center of the Earth.

One thing we can be certain about is that at the very center lies the most ancient point of our planet, a place that symbolizes the heart of a living planet. Maybe we will never reach it. It might even be better if we don’t try. To explore something without having a clear idea of the consequences is a risk. But, will we stop our pursuit before it is too late?

Maybe. Maybe not.

Here are some links to pursue your exploration further;

1. The Dynamo Theory: Origins of a Planet’s Magnetic Field
2. Why Are Venus And Uranus Spinning in The Wrong Direction?
3. Where is the Magnetic Field of Mars?
4. What is a T-Tauri Star?
5. An interesting article on geomagnetic reversal

Trivia behind the title of the article: For those who haven’t realized it yet, the title is a derived from a combination of a famous novel by Jules Verne (A Journey to the Center of the Earth) and a lesser know Japanese series by name, “A Place Further than the Universe” which follows the story of a group of young people who go on an expedition to Antarctica. One might wonder how the title might be misleading, but from my point of view, it as a whole signifies the fact that maybe, we will explore the stars and beyond someday but ironically we may never reach the center of our home planet, for it is just that difficult and catastrophic. It is indeed like a place further than the universe itself. If we do reach it, it is probably the end of humanity.

(This article was first published on the blog of Nakshatra, The Science and Astronomy Club of NITT, here)

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