comes to the earth, we’ve barely scratched the surface. Our planet’s core is a magnificent

mystery filled with secrets. It’s time to figure them out.

The Earth’s inner core is an extra hot solid ball with an approximate radius of 760 miles.

To put that into perspective, it’s just 30% smaller than the moon. But if we’ve

never been there, how did we find this out? Well, we’ve learned about the core by observing

the effects of gravity on objects on the surface of our planet. From there, it’s estimated

that the Earth’s mass is 5.6 sextillion tons. Get out your bathroom scale… no don’t.

The density of everything that lies on the surface is much lower than the core’s average

density. Scientists figured out that most of the Earth’s mass is located towards the

center of our planet. It’s estimated that more than 80% of the

core consists of one of the ten most common elements in our galaxy: iron. But, the iron

on the Earth’s surface is kind of limited. I know what you’re wondering, “How did

the iron make it all the way down to the core?” Well, there’s a simple explanation.

The heavy element somehow pushed itself – literally – towards the center of the Earth, and a

ton (pardon the pun) of research was done to figure out how. Most of the Earth’s surface

is made of rocks called silicates, and the molten iron had some difficulty passing through

them. To help you understand, think of how water struggles to get through a greasy surface.

But in 2013, Wendy Mao and her team from Stanford discovered a possible solution for how this

happened. They began an experiment to see how iron and silicate react when they’re

exposed to extreme pressure – like that in the core.

They used a diamond anvil cell to pinch the two substances under those conditions and

they achieved it. The pressure was 330 gigapascals, which is around 3.3 million times the atmospheric

pressure of our planet. The molten iron slowly squeezed through the silicate rocks, and they

had their answer. It took millions of years for the iron to reach the center, so it happened

at a snail’s pace. Since snails weren’t around back then, the iron had to guess how

fast to go. Well, now that we got that figured out, how

do we know what size the core is? That’s when seismology comes into play. During an

earthquake, shockwaves are spread through the planet. Seismologists study these vibrations

and try to read the reflections on the other side. It’s like Thor is hitting one side

of the planet with his hammer, and the seismologists are listening from the opposite end.

But these vibrations also take different routes. They go through various parts of the planet,

and that affects the sound they make at the end. Let’s take a small detour for a minute.

Seismology is quite an old scientific field. In the old days, when vibrations occurred,

scientists noticed that something was wrong with them. These vibrations were “S-Waves”,

and when they were supposed to show up on the other side, they just vanished.

At first, they thought that something was wrong with their equipment, and it just wasn’t

picking up the vibrations. But as science progressed, it turned out that these picky

“S-WAVES” could only go through solid material, and not liquid.

So something molten was present in the center of the Earth that was preventing the vibrations

from going through. So, they started digging into their data. They mapped out the paths

of the seismic waves and found that around 1,860 miles from the Earth’s surface, the

rocks transformed into a liquid. But there’s also an interesting fact in

the game. Inge Lehmann was a Danish seismologist (see,

there’s my Shakespeare tie-in), and in the 1930’s she discovered a new wave pattern.

First, we had the S-Waves that didn’t pass through liquid, but then there were also P-waves

that could travel through the core and appear on the opposite side of the planet. That was

when Inge came up with the theory that the core has two layers. The solid inner core,

which is around 3700 miles below the surface, and the molten outer core, which is around

1860 miles below our feet. When advanced seismographs were invented, her theory was confirmed, but

that took 40 years. So, now that we also have the structure figured

out, let’s talk about how hot the core is and why. We’ve already established that

we can’t put a thermometer down there to study the temperatures. So, scientists tried

to figure that out by creating the same crushing pressures in their labs.

Again, in 2013, a team of French researchers came up with the most accurate number that

we’ve had in years. They put pure iron through high pressure – almost higher than that

of the core, to come up with their findings: The temperature of the inner core is about

9800°F. While the melting point of pure iron is about 2,800°F, at the core, its melting

point is around 11,000° F. The fluctuation in those temperatures comes from factoring

in the extreme pressure the iron is exposed to at the core.

Also, other elements inside the core could be bringing the temperature down by approximately

400° F. But the reason it remains solid is because

of the slow cooling of the outer core and its compression. The inner core spins faster

than the Earth. That’s caused by the thermal activity inside our planet which creates the

magnetosphere. Oddly, it takes a ton of time for heat to leave the Earth. But I’ll get

into that in a bit. There are three main reasons why the earth

is still boiling. The first one is that the core has remained hot from the time our planet

was formed – roughly 4.5 billion years ago. Remember that number, because towards the

end I’ll explain how that happened. That heat hasn’t been lost yet. In fact, the

earth is only cooling down around 200°F every billion years.

Secondly, it generates heat from the friction of the dense materials as they move.

And the last reason it’s so hot is from the decay of radioactive elements. So, why

is this important? It makes it easy for scientists to understand how it affects the speed of

vibrations that go through the core. Remember the “P-Waves” I told you about

earlier? Well, these guys travel slower than they should while passing through the core.

This shows that there must be some other element in there that we haven’t figured out yet.

Nickel is one of them, but when scientists ran some tests with nickel, the P-waves didn’t

slow down enough. So, they started digging – metaphorically.

In 2015, a new study from Durham University came out. It claimed that 90% of the Earth’s

sulfur is in the core. So, maybe that could be the missing element. Around 4.5 billion

years ago, the Earth collided with a large planetary body that eventually tore apart

our planet and formed the moon. That incident left traces behind that led the studies in

a new direction. When the impact happened, the Earth’s mantle

melted, and some sulfur-rich liquid squeezed through the ruins and reformed it. Some of

it was probably lost in space, but the rest sunk to the core.

Scientists from Durham University confirmed that theory by measuring the isotope ratios

of elements in the mantle. They compared them to meteorites, which were possibly part of

the Earth’s original form. The problem was that there are so many different elements

in the mantle, it’s quite difficult to draw firm conclusions. So, they came up with another

idea. Copper is usually bound to sulfur. So, they

analyzed the copper from the Earth’s mantle and crust. Now, this was a 3-stage study done

in different labs, using state-of-the-art mass spectroscopes. Ya still with me here?

Good for you! They found that there was a teeny tiny difference in the copper ratios

between the Earth’s mantle samples and the meteorite samples. That confirmed the theory

that the Earth originally collided with another body, and most of its mantle just splattered

around space. We also know that the core consists of some sulfur.

Hopefully soon, we’ll be able to find out what the other trace elements are.

So to answer your final question, yes, the center of the earth is… hard-core! Yeah,

you were waiting for that one, weren’t you? Hey, if you learned something new today, then

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