A number of recent studies have made surprising discoveries about the inner core of earth. While they won’t impact life up on the surface, they offer a cool window into the blue marble that we call home. Also: they offered a good excuse to brush up on our geology:)
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Researched by: Nirmal Bhansali & Aarthi Ramnath
Say hello to planet Earth
Origin story: Roughly 4.6 billion years ago, our solar system was nothing but a giant cloud of gas and dust. Then something happened—we’re not quite sure what. One theory is that some distant star collapsed—disturbing the cloud. It started to spin faster and faster—concentrating the gas and dust in the centre—creating helium which eventually resulted in the formation of our sun. FYI: this is called a solar nebula.
Say hello to Earth: While the sun gobbled up 99% of the matter in the nebula, the other 1% kept spinning and colliding into each other. Some of these clumps became big enough to develop their own gravitational pull and turned into planets. The process took a very long time. According to one estimate, “100 million years could pass between the formation of an object measuring 10 kilometres in diameter and an object the size of Earth.”
So first came this process of accretion. Then a protoplanet the size of Mars crashed into a very young Earth—and created our present planet plus the moon. Finally, Earth was bombarded with asteroids carrying water—which helped cool it down over a few hundred million years.
As Earth cooled, it became differentiated into layers. The denser, hotter material sank to the middle—while the lighter, cooler material remained at the surface. So we now look something like this:
The crust: where we live—extends 32 km below the surface of land—and 5 km below the ocean floor. It is divided into tectonic plates that are always moving. When they bang against each other, we get earthquakes. You can see them below:
The mantle: comes next. It is the largest and thickest layer. It has the consistency of caramel—and makes up 84% of our planet's total volume. Cool fact: diamonds are forged in the upper mantle—which is 150 to 200 km below the surface. They are brought to the surface when magma is churned up from the depths due to the movement of tectonic plates.
The core: is almost the size of Pluto. It lies almost 2,900 km below the surface. The temperatures match the surface of the sun—and is way above the melting point of iron. It is divided into the inner and outer core. The outer core is made up of liquid iron (Fe) and nickel (Ni)—known simply as NiFe. OTOH, the inner core is a dense ball made entirely of iron. However, unlike the outer core, it is solid due to the intense pressure—from the rest of the planet and its atmosphere—which prevents the iron from melting.
Fun fact to note: The core of our planet is 2.5 years younger than the surface. This is explained by Einstein’s theory of relativity:
It all comes down to the effects of "time dilation," that tricky little consequence of general relativity wherein gravity has the ability not only to warp space, but to distort time as well. Naturally, the deeper you are in a large object's gravity well, the greater the distortion of time experienced. GPS satellites even have to account and correct for it in their onboard clocks.
So the core experiences time more slowly—by about 0.0000000003 second. That’s really tiny but over billions of years it’s added up to 2.5 years.
Why the core matters: We are lucky that we have a red-hot outer core—unlike Mars whose core cooled down billions of years ago. That molten NiFe mixture is constantly swirling around—and acts like an “electric generator”—creating currents that produce a magnetic field:
The magnetic field loops out from the poles, trapping harmful solar energy a safe distance away so it can't strip away our atmosphere. This invaluable atmosphere keeps our planet insulated and wet, allowing life to thrive. And the fluid core is responsible for creating this invaluable shield.
What’s up with our inner core?
How we ‘see’ the core: The irony is that we can see faraway galaxies through our telescopes but we have to intuit what the inside of our own planet looks like. The method is similar to X-rays—which we use to map our body: “[T]he different densities of our muscles, organs, and bone mean that the X-rays travel through them (or get deflected by them) in different ways.” In the case of the core, we use seismic waves—generated by a strong earthquake—to do the same:
The only way to peer deep into Earth’s interior is to use earthquake waves like a scanner. By analysing how waves change as they travel through the planet, researchers can learn the properties of the materials they passed through. To do this with the innermost inner core requires a strong earthquake that occurs on one side of the planet and seismic instruments to pick up the waves on exactly the opposite side. Otherwise, the wave signal won’t pass directly through the heart of the core.
A slowing core: Our inner core—that iron ball—is caught between two opposing forces. The magnetic fields generated by the outer core make it spin. But the mantle and crust above exert a gravitational pull that slows it down. So it spins eastwards like the surface—but not at the same rate as the surface. It might spin faster or slower—and even pause. And after one of these peculiar pauses, it changes how it spins. New research suggests that we are on the cusp of such a shift:
In the early 1970s, relative to someone standing on Earth’s surface, the inner core was not spinning. From then, the inner core has gradually spun faster eastward, eventually overtaking the speed of rotation of Earth’s surface. Afterward, the inner core’s spin decelerated until its rotation appeared to have stopped at some point between 2009 and 2011.
In fact, now the inner core is starting to slowly spin westward—and is likely to do so with increasing speed. Then it will slow down and hit another pause around the 2040s.
Why does this matter: It won’t affect our daily lives—at best, it may tweak the length of our day by a teeny bit. But it tells us something about the planet we live on: “The inner core is like ‘a planet within a planet, so how it moves is obviously very important.’” Also, the inner core itself is “like a time capsule of Earth's history.”
Core within a core? In February, Australian scientists claimed to have discovered a core within the inner core. They claim this innermost core is 650 km wide and is made up of an iron-nickel alloy—but is different in structure from the rest of the inner core. But other experts think it isn’t significant: “I think it is an unnecessary splitting into multiple layers,”
Mountains ahoy! The latest discovery revealed mountains taller than Everest buried deep inside Earth. They are located in the border between the mantle and the inner core. Scientists think they may be remains of ancient ocean floors. Looping back to those tectonic plates, the places where they bump into each other are called ‘subduction zones’:
In these zones, one plate is pushed down under the other, causing it to enter the mantle. The new study suggests that parts of this subducted material may sink all the way down to the outer edge of the core-mantle boundary, where geophysical forces sculpt it into a kind of underground mountain range.
Why this matters: It isn’t exactly clear but scientists think they play a role in shaping those magnetic fields—which make life possible on Earth: “It’s not at all clear how the core generates this field, and every small piece matters in solving that puzzle.”
The bottomline: It’s funny how little we know about our own planet even as we dream of colonising new ones:)
Reading list
National Geographic provides the best and most detailed look into how the Earth was formed. Mashable has a good feature on the various fascinating aspects of our core. New York Times looks at why the core is slowing down while the Washington Post (splainer gift link) has more on the recent discovery of an inner core. For more on the mountains located deep beneath the Earth, check out Vice.
