Instant Genius - Earth’s inner core, with Dr Jessica Irving

Episode Date: March 6, 2023

A recent study carried out at Peking University has found that Earth’s inner core, a giant ball of iron located in the middle of the planet, is slowing down its rotation. But what exactly does this mean? We speak to Dr Jessica Irving, a seismologist based at the University of Bristol’s School of Earth Sciences. She tells us how scientists study the goings on deep inside Earth, what we can learn about the life cycles of planets and whether the news should be cause for alarm. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Visit name audio.com to learn more. Hello and welcome to Incident Genius, a bite-sized masterclass in podcast form. I'm Jason Goodyear, commissioning editor at BBC Science Focus magazine. Recently, a team of researchers based at Peking University found evidence that the Earth's inner core, a giant ball of iron located in the middle of the planet, is slowing down its rotation. But what exactly does this mean? We speak to Dr. Jessica Irving, a seismologist based on a seismologist based on the planet, at the University of Bristol School of Earth Sciences.

She tells us how scientists study the goings on deep inside Earth, what we can learn about the life cycles of planets, and whether the news should be caused for alarm. Okay, so there's a new study that's being carried out at the university in Peking that said that the rotation of the Earth's inner core may have stalled and potentially that it could go into reverse. So to the average non-geophysicist, such as myself, that brings up an absolute, you know, ton of questions.

So let's start with the basics. So what exactly is the composition of the earth and its core? And, you know, what does it look like if we cut it in half? That is a great place to start because what we can see as people, only the earth's outer layers, we're standing on the crust of the earth and we know that it's made of rocks. But when you go deeper into the earth, we've got this layered structure and we often talk about it being like lays of an onion. In fact, it's more complicated than that. So the crust of the earth is made of rocks, silicate materials.

And then we've got this huge expanse that we call the mantle. That's solid rock-like material, but it's rocks that are under high pressure and high temperature. So they're different to the rocks that you would find if you wandered out into a park. But we're rocks from the surface down to the bottom of the mantle. And beyond the bottom of the mantle, we get into the deepest regions of the earth, which are Earth's core. and there we leave the rocks behind and we enter a world that is made of iron.

So it's a world of metal. And that metal has ended up there because iron is heavy compared to rock. So that density contrast has put most of Earth's iron, although by no means all of it, into this big ball at the centre. So when we're talking about Earth's core, we're talking about a huge ball that's about half of Earth's radius made of metal.

And then we can split that core into two more distinct chunks. So we've got the outer core, which is made of molten metal or fluid metal. It's moving around quite fast. But in the middle of the earth, we've got this solid inner core. And that's what the paper you're talking about suggested might be doing something a bit unusual. The inner core has a radius, about a fifth of the Earth's radius. So it's small compared to our planet. It's not too different in size to something like Pluto. And it was actually discovered after Pluto was. So we knew about things way out in the solar system before we knew what was happening under our own feet. And that's the

layer of the earth that we're going to spend a while talking about today. Yeah, so that's really interesting. So how do we know this? How do we go about studying it? You know, as you say, it's something that we can't see. So we have a variety of different techniques to make what we call indirect observations. So we have no hole that has been dug that's going to help us out. The deepest hole that ever got Doug was a bit over 12 kilometers deep and we need to get down to the inner core we need to go down thousands and thousands of kilometers. We certainly have no samples. I'm a seismologist, and seismology is one of the best techniques

of looking out what's going on in Earth's deep interior. And the basic principle goes like this. An earthquake happens, and energy gets radiated. It's elastic energy. It's a sort of earthquake waves that come out. Those waves go right through the earth, and the different properties of the earth cause those waves to speed up or slow down and change direction.

And so we look at the earthquake waves on the far side of the earth, and we use those signals to work out what the middle of the earth must be made of. And the inner core was discovered by Inga Lehman in 1936, based on those sorts of seismic observations. So it's seismology that told us there was an inner core there in the first place. So there wasn't until 1936. Yeah, we're not even 100 years into knowing that we have this deepest layer of the earth, this inner core.

So this is a pretty new discovery. Yeah, if I get my history, right, that's more recent than things like quantum physics. Exactly. And that's because there's a whole rest of the planet in the way. So this is not the easiest place to study. And when we'll talk about the inner core moving relative to the rest of the Earth, this is still an area where scientists are still trying to work out what's going on. There are still some controversies. We don't have a completely settled story yet. But we're learning so much every year at the moment. So let's go into that, then. Let's go into the rotation. So what's happening? Let's have a look at the big picture. Obviously, everybody knows that the Earth's rotating on its own axis is it travels around the sun. But what else is going on? So when we think about Earth's rotation, we're thinking about our day, right? How long does a day take? And we're used to our 24 hours of a day.

And what we actually have with this idea that the inner core might be rotating is just that it's rotating at a slightly different speed to the rest of the earth. So it's not like it's going backwards completely. It's just that it's maybe moving at a slightly different pace. And so one relatively simple analogy might be that you and your friend, you go down to the swimming pool and you're going to swim some laps and you swim your laps and your friend is ever so slightly faster than you. So maybe by the end of an hour they've done one extra lap. It's not a very big speed difference, but if you add it up over a long enough time, you just get that one extra

lap. And so we're talking about very small differences in speed. And so one idea is that the inner core could be moving at a slightly different speed. Another idea is that just sometimes it's moving a bit faster and sometimes it's moving a little bit, it's slower than the mantle. So it's not that it's always going faster, it's just that the pacing is slightly different. And so to go back to that analogy, you and your friend, you've got a similar overall pace, but sometimes you're going a bit faster, sometimes they're going a bit faster. And you end up on average in that case doing the same thing. And we're still really trying to distinguish between those different cases because we've only got a limited number of years of scientific data to look back on. So there's a lot to unravel here.

Yeah, so what causes the rotation? Is it just the action of gravity? The rotation of the earth or the rotation of the inner core? Which one? Well, let's say both. Okay, so we're spinning around and we have this angular momentum. So we are a spinning ball that is spinning as it loops around the sun every year. The inner core has a couple of different forces acting on it, which could help to make it spin differently or could hinder it spinning differently. And so we're always talking about that just differential speed, that differential rotation, that slight difference to what we experience standing here on the surface.

The forces that might want to help it change a little bit are electromagnetic forces. So one of the most important things we haven't said yet about Earth's core is it's where our magnetic field comes from. So Earth's magnetic field is a geodynamo and that's generated by this molten iron moving in Earth's outer core. and that magnetic field extends way out beyond the core. It extends way out beyond the surface of the planet, and you can see it in space. And we're really fortunate to have that magnetic field there because it protects Earth.

But that's generated in the outer core, and it's possible that there are twisting or teroidal elements of that field, which might actually shove the inner core to twist it slightly. So there are these electromagnetic forces generated by the same processes that make our magnetic field, and they might cause a little bit of differential rotation of the inner. But that's not the only thing happening down there. There is another force as well,

and that would act to hinder rotation, and that's a gravitational force. So gravitational forces would say, actually the bottom of Earth's rocky layers, the bottom of the mantle, are a bit uneven, they're a bit variable, they've got slightly different compositions. And gravity would actually like,

where those uneven compositions are, to be locked relative to where any changes are in the inner core. So gravity would like things to stay lined up to keep them stable. And these electromagnetic forces would like to tug on the inner core a bit and make it rotate. And so we've got this huge battle at play right in the center of our planet between these different forces that are acting in different ways. So yeah, there's an awful lot going on with everything interacts with everything else, if that's correct, yeah? Completely. And so these are big puzzles. we're trying to solve them while looking through 5,000 kilometres of rock and metal that's in the way.

So they're kind of tricky to unravel. I see, yes. So let's have a look at this new paper then that we've seen that's come from China. So is this the first time this sort of thing has been observed or studied like with the slowing of the rotation or an alteration in its speed? So this isn't the first observation that's been made. The inner core moving at a slightly different relative speed was first suggested before we had any observational data. And the first observational data we had came in the mid-1990s that said, hey, the inner core, it might have an ever so slightly different speed to the rest of the rocky planet. And those results have been built on since the mid-90s and there have been multiple papers published just in the last year about this

from different groups across the world. It's not easy work. It needs to be done by very careful scientists. the very earliest studies were actually taking paper seismic records. So from the really old school pen drawing on a piece of paper when earthquake energy gets to your seismometer, they had to digitise those records and hope that the paper hadn't been distorted after 30 years of storage. So those are complicated records.

And so I think we'll talk in a minute about how these observations are made, but essentially you need old seismic records and newer seismic records. And different groups around the world have been comparing these and think that something deep in the earth has changed as time has passed if we look at older new seismic records. So it's not a brand new idea, but certainly in the last year there have been a few different groups who've all said, we really do think something is changing.

And it might not be changing in this steady fashion that says the inner core is always moving a bit faster. It might be coming and going. And so those are the sorts of stories that are being published at the moment. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. You said this place was steps from the water.

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every innovation in hi-fi design, technology and acoustic engineering, balancing craftsmanship and tradition with pioneering thinking. Name Audio pushes cutting-edge technology to ensure digital precision whilst sustaining Pratt, pace, rhythm and timing, the elusive quality that makes music feel alive and gives it emotional texture. Today, in partnership with French acoustic specialist's focal, name audio creates systems that deliver exceptional sound and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. Yeah, so you mentioned that. How are the observations made? You know, what's the sort of day-to-day work that goes into this sort of research? So these are primarily

seismological observations. And they're pretty challenging. and you can only do them in just the right set of circumstances. So what you want to do is try and work out if the inner core is moving faster or slower. You can't do that directly. So instead what seismologists do is they look at a record of an earthquake wave, which has gone from an earthquake right through the rocky mantle, right through the liquid outer core, into the inner core, and then has come all the way back out and gets detected on the far side of the planet.

And then they try and look for another earthquake that happened in nearly exactly the same place, as close as possible, that got detected by exactly the same seismometer some years later. And what happens when seismologists look at records like that is that they see that the bits of the paths of energy where the earthquake energy stays in the rocky layers of the earth, nothing changes. The wiggles on the seismogram look the same. but the wiggles corresponding with energy which has gone through the inner core look ever so slightly different. And they use those ever so slight differences in the inner core phases in the inner core waves and say, hey, these waves have been through the inner core, the wiggles you get on the far side of

the earth are slightly different. Something must have changed in the inner core or near it. So you make these relative measurements from earthquakes and you really hunt hard for earthquakes which are in the same place. But earthquakes aren't in exactly the same place each time. They're in slightly different places. And if you don't take that into account really carefully, you can come out with some really weird looking results. And scientists have been working on that problem a lot. There's a lot of research into, are these two earthquakes close enough to make this technique work? This pair might be, this pair might not be. And there's been a lot of scientific communication to try and get us into a place where we understand what's happening from the earthquake end of

things so we can look at the inner core end of things. Yeah, so that's really interesting. So you're sort of at the mercy of nature, I'm studying this in a way. I mean, do you use computer simulations and models and that sort of thing? So most of this research is at the mercy of nature, but there's actually one other sneaky seismological technique which one can use. It's not the best approach. It's not something that we'd want to do, but it's an outcome of the Cold War. So I said it was tricky to use earthquakes because you needed them to be in exactly the same place. But you know what's in exactly the same place? Nuclear tests, they're set off by humans.

They're set off in test sites. And those can actually be used as repeating seismic sources. So, Cold War, really difficult. Nuclear war, clearly a bad idea. But the nuclear test data from the 1970s is actually still used by scientists today because we can use energy from a nuclear test. that travels as earthquake waves through the earth, and you know where it got set off.

So we're either at the mercy of where earthquakes are or we're in this slightly worse situation where people are planning World War, but there's a happy side effect of the terrible World War idea. Yeah, that's really interesting. So let's have a look at this current paper, then, this current study. What exactly is it saying? You know, I did have a look at it,

but I'm afraid it's not my area of expertise and was a little bit high level for me. It's a pretty dense paper as nearly all of the papers that we write are. Essentially, the scientists here are looking at earthquakes in this case. They're not looking at nuclear test data. And they're saying that if they look at pairs of earthquakes, pairs of earthquakes from, say, the 90s seem to show some differences in what seems to be happening as seismic waves go through the inner core. But pairs of earthquakes that are maybe more recent don't seem to show differences between different years. And so, what the paper says is, oh, we used to see differences. We're not seeing any differences right now. So maybe what's changed is the inner core rotation is happening at a slightly different relative speed. So relative to the rocky bits of the earth, the inner core maybe is kind of stable right now. And maybe 30 years ago, the inner core was just moving an ever so slightly different speed to the rocky layers of the earth. And that's the big takeaway. It does have some side effects,

though, and it has effects when we think about things that maybe we're a bit more used to thinking about, like, how long a day is, because there are ideas that these changes in inner core rotation rate might, we're not sure, but they might be linked to ever so slight changes in the length of a day. And this is one of the conclusions that this paper draws. It looks like maybe the changes in inner core rotation that they think they see are related to changes that other scientists have observed in how long a day takes. So how big an effect could that have? So let's put this in context. This is not enough time to have an extra cup of tea in the morning. These are really, really tiny changes. We're talking about changes which are maybe a tenth of a millisecond in the

length of a day. So they're tiny, tiny fractions. Okay, so has the length of the day changed over the over the course of the Earth's history at all? Yes. The length of the day is always changing, and it's changing at short length scales and long length scales. So to put that into perspective, we get changes in the length of a day throughout the process of a year. We get changes in the length of a day

when we have an El Nino year compared to a L'Avnaemia year. So we've got these changes in the length of a day that we experience in modern life anyway. But there have also been huge changes in the length of a day over Earth's history. So if we go back to the end of the age of the dinosaurs, we're maybe talking about, you know, something like half an hour shorter day than we have now,

that sort of order of magnitude. And if we go back much further in history, days used to be shorter. That's well before human history ever took place. Humans have felt pretty similar lengths of days. But if we go back to earlier, then days used to be shorter and our days have steadily got slightly longer

over the history of our planet. So does this change in the rotation of the inner core effect, you mentioned that the Earth's magnetic field? Is that affected at all by the change in the rotation of the core? So it's likely actually a byproduct of what the magnetic field wants to do anyway. So it's not that the inner core is making the rest of the Earth do something. The inner core is kind of the recipient of being yanked on a bit by the magnetic field. So it's not that we have to worry about the inner core doing anything malevolent. It's just that we're able to try and understand a bit about what the magnetic field is actually.

doing by looking at the inner core. We don't have any ways to directly sense how the magnetic field is being generated in the center of the earth. We can only look with measurements at Earth's surface. So we can actually try and understand a little bit more about geodynamo physics by looking at the effects our geodino has on stuff that people like me can think about with seismology. So one thing that people have sort of picked up on, which is it almost sounds like the premise to a science fiction catastrophe movie, that the rotation is going to stall to such a point that it reverses. So that's sort of a two-parter.

So first off, is that possible? Is that happening? And second, what would that mean? So that's not happening. Nobody needs to panic. Bad Hollywood movies are amazing, but they're not reality. So what we're really thinking about here is just a slight change in the relative speed of the inner core. And to put the size of the inner core into context, it's less than 2% of Earth's mass.

So the rest of the planet is going to win. The rest of the planet has the rotation rate covered. It's got all our angular momentum. It knows it's in control. But these small changes in the relative motion are something we can observe and they'll help us understand a lot. But they're not a reason that we should be worried about our everyday life. They're going to help us unlock the secrets of the planet, which I think is really cool.

but they're not going to make me change my day-to-day schedule. Great. So that sort of brings me on to what I was going to say next then. So what exactly can we learn from these? I mean, obviously it sounds like very, as I say, like science fictiony and like we're just generally cool science. But, you know, what's the end goal of studying this, this type of thing? So there are a few different things that we want to learn.

And first of all, I want to be clear that there are different scientists studying this and some of them think that the inner core is changing its rotation rate slightly. Some of them think it's the very surface of the inner core that maybe changes and stuff in the middle of the inner core is just hanging about, doing its regular old thing. But what we're really understanding about are the dynamics of what happens inside a planet because we didn't always used to have an inner core. We actually don't know how old the inner core itself is. So I said that it's currently got a radius about the fifth of the earth.

but maybe if you go back half a billion years or a billion years, it wasn't even there at all. It's been slowly growing over many millions of years. And we don't really understand how old the inner core is. We don't really understand exactly how the inner core starting to grow could have changed our magnetic field, although we know it must have done. We're not sure whether we can see that or not in records of ancient magnetic field. So as scientist, we're still really trying to understand what goes on, in the middle of a planet. Earth is the planet we can study most easily, but scientists are interested

in what's happening in the core of Mars, for example. There are missions to think about the core of Mercury. And so we're at this place where we're really understanding how planets work. In terms of the more tangible context, we would really like to understand better how Earth's magnetic field works. We know that Earth's magnetic field flips. We know that isn't a process that happens like clockwork. It's a really complicated process. So if we can understand what the inner core is doing, we might be able to understand a bit more about the different sorts of magnetic field processes that happen to help us understand how our protective shield works, because it's there and we know it's there, but it's very, very difficult to simulate the physics that happens to make a global magnetic

field. And so any bit of data is really useful there. That was the University of Bristol's Dr Jessica Irving. Thank you for listening to this of Instant Genius. Brought to you from the team behind BBC Science Focus magazine. The current issue of BBC Science Focus is out now. Pick up a copy wherever you buy your favourite magazines or download a digital copy from your preferred app store.

You can of course also find us online at sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist vocal, name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist intended.

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