Humans have been all over the Earth. We've conquered the
lands, flown through the air and dived to the deepest trenches in the ocean.
We've even been to the Moon. But we've never been to the planet's core.
We haven't even come close. The central point of the Earth
is over 6,000km down, and even the outermost part of the core is nearly 3,000
km below our feet. The deepest hole we've ever created on the surface is the
Kola Superdeep Borehole in Russia, and it only goes down a pitiful 12.3 km.
All the familiar events on Earth also happen close to the
surface. The lava that spews from volcanoes first melts just a few hundred
kilometres down. Even diamonds, which need extreme heat and pressure to form,
originate in rocks less than 500km deep.
What's down below all that is shrouded in mystery. It seems
unfathomable. And yet, we know a surprising amount about the core. We even have
some idea about how it formed billions of years ago – all without a single
physical sample. This is how the core was revealed.
One good way to start is to think about the mass of the
Earth, says Simon Redfern of the University of Cambridge in the UK.
We can estimate Earth's mass by observing the effect of the
planet's gravity on objects at the surface. It turns out that the mass of the
Earth is 5.9 sextillion tonnes: that's 59 followed by 20 zeroes.
There's no sign of anything that massive at the surface.
"The density of the material at the Earth's surface is
much lower than the
average density of the whole Earth, so that tells us
there's something much denser," says Redfern. "That's the first
thing."
Essentially, most of the Earth's mass must be located
towards the centre of the planet. The next step is to ask which heavy materials
make up the core.
The answer here is that it's almost certainly made mostly of
iron. The core is thought to be around 80% iron, though the exact figure is up
for debate.
The main evidence for this is the huge amount of iron in the
universe around us. It is one of the ten most common elements in our galaxy,
and is frequently found in meteorites.
Given how much there is of it, iron is much less common at
the surface of the Earth than we might expect. So the theory is that when Earth
formed 4.5 billion years ago, a lot of iron worked its way down to the core.
That's where most of the mass is, and it's where most of the
iron must be too. Iron is a relatively dense element under normal conditions,
and under the extreme pressure at the Earth's core it would be crushed to an
even higher density, so an iron core would account for all that missing mass.
But wait a minute. How did that iron get down there in the
first place?
The iron must have somehow gravitated – literally – towards
the centre of the Earth. But it's not immediately obvious how.
Most of the rest of the Earth is made up of rocks called
silicates, and molten iron struggles to travel through them. Rather like how
water on a greasy surface forms droplets, the iron clings to itself in little
reservoirs, refusing to spread out and flow.
A possible solution was discovered in 2013 by Wendy Mao of
Stanford University in California and her colleagues. They wondered what
happened when the iron and silicate were both exposed to extreme pressure, as
happens deep in the earth.
By pinching both substances extremely tightly using
diamonds, they were able to force molten iron through silicate.
"The pressure actually changes the properties of how
iron interacts with the silicate," says Mao. "At higher pressures a
'melt network' is formed."
This suggests the iron was gradually squeezed down through
the rocks of the Earth over millions of years, until it reached the core.
At this point you might be wondering how we know the size of
the core. What makes scientists think it begins 3000km down? There's a one-word
answer: seismology.
When an earthquake happens, it sends shockwaves throughout
the planet. Seismologists record these vibrations. It's as if we hit one side
of the planet with a gigantic hammer, and listened on the other side for the
noise.
"There was a Chilean earthquake in the 1960s that
generated a huge amount of data," says Redfern. "All the seismic
stations dotted all over the Earth recorded the arrival of the tremors from
that earthquake."
Depending on the route those vibrations take, they pass
through different bits of the Earth, and this affects how they
"sound" at the other end.
Early in the history of seismology, it was realised that
some vibrations were going missing. These "S-waves" were expected to
show up on one side of the Earth after originating on the other, but there was
no sign of them.
The reason for this was simple. S-waves can only reverberate
through solid material, and can't make it through liquid.
They must have come up against something molten in the
centre of the Earth. By mapping the S-waves' paths, it turned out that rocks
became liquid around 3000km down.
That suggested the entire core was molten. But seismology
had another surprise in store.
In the 1930s, a Danish seismologist named Inge Lehmann
noticed that another kind of waves, called P-waves, unexpectedly travelled
through the core and could be detected on the other side of the planet.
She came up with a surprising explanation: the core is
divided into two layers. The "inner" core, which begins around
5,000km down, was actually solid. It was only the "outer" core above
it that was molten.
Lehmann's idea was eventually confirmed in 1970, when more
sensitive seismographs found that P-waves really were travelling through the
core and, in some cases, being deflected off it at angles. Sure enough, they
still ended up on the other side of the planet.
It's not just earthquakes that sent useful shockwaves
through the Earth. In fact, seismology owes a lot of its success to the
development of nuclear weapons.
A nuclear detonation also creates waves in the ground, so
nations use seismology to listen out for weapons tests. During the Cold War
this was seen as hugely important, so seismologists like Lehmann got a lot of
encouragement.
Rival countries found out about each other's nuclear capabilities and along the way we learned more and more about the core of the Earth. Seismology is still used to detect nuclear detonations today.
We can now draw a rough picture of the Earth's structure.
There is a molten outer core, which begins roughly halfway to the planet's
centre, and within it is the solid inner core with a diameter of 1,220 km.
But there is a lot more to try and tease out, especially
about the inner core. For starters, how hot is it?
This turns out to be quite tricky to determine, and baffled
scientists until quite recently, says Lidunka Vočadlo of University College
London in the UK. We can't put a thermometer down there, so the only solution
is to create the correct crushing pressure in the lab.
In 2013 a team of French researchers produced the best
estimate to date. They subjected pure iron to pressures a little over half that
at the core, and extrapolated from there. They concluded that the melting point
of pure iron at core temperatures is around 6,230 °C. The presence of other
materials would bring the core's melting point down a bit, to around 6,000 °C.
But that's still as hot as the surface of the Sun.
A bit like a toasty jacket potato, Earth's core has stayed
warm thanks to heat retained from the formation of the planet. It also gets
heat from friction as denser materials shift around, as well as from the decay
of radioactive elements. Still, it is cooling by about 100 °C every billion
years.
Knowing the temperature is useful, because it affects the
speed at which vibrations travel through the core. That is handy, because there
is something odd about the vibrations.
P-waves travel unexpectedly slowly as they go through the
inner core – slower than they would if it was made of pure iron.
"Wave velocities that the seismologists measure in
earthquakes and whatnot are significantly lower [than] anything that we measure
in an experiment or calculate on a computer," says Vočadlo. "Nobody
as yet knows why that is."
That suggests there is another material in the mix.
It could well be another metal, called nickel. But
scientists have estimated how seismic waves would travel through an iron-nickel
alloy, and it doesn't quite fit the readings either.
Vočadlo and her colleagues are now considering whether there
might be other elements down there too, like sulphur and silicon. So far,
no-one has been able to come up with a theory for the inner core's composition
that satisfies everyone. It's a Cinderella problem: no shoe will quite fit.
Vočadlo is trying to simulate the materials of the inner
core on a computer. She hopes to find a combination of materials, temperatures
and pressures that would slow down the seismic waves by the right amount.
She says the secret might lie in the fact that the inner
core is nearly at its melting point. As a result, the precise properties of the
materials might be different from what they would be if they were safely solid.
That could explain why the seismic waves pass through more
slowly than expected.
"If that's the real effect, we would be able to
reconcile the mineral physics results with the seismological results,"
says Vocadlo. "People have not been able to do that yet."
There are plenty of riddles about the earth's core still to
solve. But without ever digging to those impossible depths, scientists have
figured out a great deal about what is happening thousands of kilometres
beneath us.
Those hidden processes in the depths of the Earth are
crucial to our daily lives, in a way many of us don't realise.
Earth has a powerful magnetic field, and that is all thanks
to the partially molten core. The constant movement of molten iron creates an electrical
current inside the planet, and that in turn generates a magnetic field that
reaches far out into space.
The magnetic field helps to shield us from harmful solar
radiation. If the core of the Earth wasn't the way it is, there would be no
magnetic field, and we would have all sorts of problems to contend with.
None of us will ever set eyes on the core, but it's good to
know it's there.
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