How do we know what Earth's core looks like?

How can we be sure what the centre of the Earth looks like if we can't go down there?...
24 February 2023

Interview with 

James Jackson, University of Cambridge


"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.


I remain undeterred with my demonstrations because it’s not just important to know about the surface of the Earth, but also what’s going on inside of it too. Starting with the obvious question, how can we find out what the centre of the Earth is made up of…

Will - I've got these two Easter eggs here in front of me, and I want James to be involved in this as well because I've surprised James with this demonstration. He doesn't know what's going on here. So between the two of you, does anything about these two scream the processes going on inside the earth to you?

James - I guess they're both sort of roughly spherical and, well, that one looks like it's got it's hollow inside, which I'm pretty sure the earth isn't hollow inside. I might be wrong.

Will - So we have two eggs in front of us and my question to the pair of you is what can you tell me about the inside of either or both of these eggs just by looking at them?

Amelia - You can't really tell anything.

Will - James?

James - Well, yeah. Ignoring the picture of the egg on one of the boxes. <laugh>, nothing.

New Speaker - So if I were to put a challenge to you two as to, how would you be able to find out what might be inside without breaking the egg open? What would you do?

Amelia - Maybe give it a shake and have a look at what's inside?

Will - Maybe give it a shake?

James - She's good.

Will - She's good. So if you were to shake this egg here... <Silence> Nothing. Nothing. However, were you to shake this egg here <rattling>

Amelia -
It's not hollow.

Will - It's not hollow. So therefore, could we assume that by vibrating this 'egg earth' here, you could find out where the different densities inside the middle might be? You could find out what constitutes the center of this 'egg earth'?

Amelia - Potentially. Yeah.

James - Yeah. Seems plausible.

Will - Seems plausible. Would you say I'm onto something here?

James - Yeah. Who can tell us more?

Will - So glad you asked <laugh>.

We’ll call that a partial success, but obviously there’s only so much you can explain with a chocolate egg. And so I put the question again to James Jackson, just how could we know so much about the centre of our Earth?

James - How can we possibly know? Because you can't go down there. How we know is by looking at sound, how sound travels through the earth. So we can't go to the center of the earth, but sound can, and this is actually something you are familiar with. You see pictures all the time of mothers who go to hospital and they get ultrasound scans of the baby inside them. And this is just by sending sound through the body and the sound bounces off things. It goes around things and you look at how it goes through the inside of the body and that allows you to construct exactly what is inside there. And that is exactly the same technology that we use to look inside the earth. In fact, the geologist got there a long time before because the geologist worked all this out about 1920, which is long before anyone thought of using this to look inside the human body. But the science is exactly the same.

Will - There is a slight discrepancy in size between the entire earth and a human baby. How big do the vibrations involved have to be?

James - So if you are looking inside a human body, you are in a good position because you can completely cover the body with microphones. You can have little sound sources going, ping, ping, ping all the way around. You're in complete control of where the sound sources are and where the sound receivers are. The microphones on the earth we're not in such control of. The sound sources we use are earthquakes, natural earthquakes. So a decent sized earthquake will sens sound all the way through the earth. You can record it on the other side easily. But of course your sensors, your receivers are instruments called seismometers, which is a smart way of saying it's a microphone which picks up vibrations in the earth. But of course they're all on land cuz that's where we live. They're very few of them under the sea. And furthermore, the earthquakes are only in special places. The earthquakes are not everywhere, they're on the edges of these plates. And so we are much less in control of what's going on. We don't have earthquakes everywhere we would like them, and we don't have receivers everywhere we would like them. But we do have a lot of them and we do the best we can. But it means that we know some places in better detail than other places. That's all.

Will - And how does this work then? The vibrations pass through the earth? Do they get delayed in areas that are of different density?

James - Yes. The speed with which they travel depends on the density of the material and so that gives you some idea of what's going on. But the density of the material also depends on its temperature. So where it's hotter, they go slower, and where it's cooler, they go faster. So there are other tricks we can use to see exactly what the variations are inside the earth.

Will - When the vibration passes through areas of different density, how big is the delay between these different materials for you to be able to go 'ah, this is polymer A or substance B'?

James - It takes, for example, about 18 minutes for the sound to travel from here to the other side of the world. So if there's an earthquake in Britain it would take about 18 minutes for it to go to New Zealand. And the sort of delays you see on average are plus or minus, say 10 seconds. That sort of delay. It's quite easy to spot, 'Ooh, it's coming a bit earlier than it should, or it's a bit later than it should. That's because it's gone through something on the way, which is different'. And that may not seem much, but actually it's quite a lot. If you have thousands and thousands of different paths through the earth going between different earthquakes and different receivers, you can really put this together in three dimensions to see where that thing is, which is causing the delay or causing something to speed up.


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