You know, we’ve managed to go hundreds of thousands of miles into space, but when it
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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