In Our Time - The Earth's Core

Episode Date: April 30, 2015

Melvyn Bragg and his guests discuss the Earth's Core. The inner core is an extremely dense, solid ball of iron and nickel, the size of the Moon, while the outer core is a flowing liquid, the size of Mars. Thanks to the magnetic fields produced within the core, life on Earth is possible. The magnetosphere protects the Earth from much of the Sun's radiation and the flow of particles which would otherwise strip away the atmosphere. The precise structure of the core and its properties have been fascinating scientists from the Renaissance. Recent seismographs show the picture is even more complex than we might have imagined, with suggestions that the core is spinning at a different speed and on a different axis from the surface.WithStephen Blundell Professor of Physics and Fellow of Mansfield College at the University of OxfordArwen Deuss Associate Professor in Seismology at Utrecht UniversityandSimon Redfern Professor of Mineral Physics at the University of CambridgeProducer: Simon Tillotson.

Thank you for downloading this episode of In Our Time, for more details about In Our Time, and for our terms of use, please go to BBC.co.com.uk slash Radio 4. I hope you enjoy the program. Hello, it's said that we know more about the composition of the sun, 90 million miles away, than we do about the core of our own planet Earth. It's only in the last eight years the scientists have moved towards a view that Earth has a solid inner core of mainly iron, the size of the Moon, which is then surrounded by a large, liquid outer core, an ocean of molten metal, again mainly iron. The edge of this core begins 3,000 kilometres beneath our feet.

Our going understanding of the core has helped explain the creation of the Earth's magnetic field, which shields this planet from the worst of the cosmic rays, so making life, as we know it, possible. With me to discuss the Earth's core are Stephen Blondell, Professor of Physics and Fellow of Mansfield College at the University of Oxford. Owen Deuss, Associate Professor in Seismology at Eutrecht University and Simon Redfern, Professor of Mineral Physics and Fellow of Jesus College at the University of Cambridge. Stephen Mondell, let's start with the scientific journey to the centre of the earth

in the 19th century when people began to be interested in it. What did they know about it then? Well, in 1798, Henry Cavendish had measured the density of Earth. He'd done this by looking at the gravitational force between lead spheres, and this had calibrated the strength of gravity. And he worked out that the density of Earth was about five and a half times the density of water. Now, why that was significant is that the density of rocks is only about two and a half to three times the density of water.

And so if the Earth is just made of rocks, there's something that's gone wrong with the calculation. And it looks like to compensate that, you need a much more dense core in the Earth. And the density of iron is about eight times the density of water. iron is a very common material, and so it seemed potentially logical that the Earth could have a much more dense core than the density of Earth, the density of the surrounding rocks.

Was he following the Great Banner and idea of curiosity, or was he doing it because there was a purpose? He was going to find something worth, which could be put to use, for instance. Well, I think what he was doing was following Newton's theory of gravity and trying to test it, essentially to try and calibrate it, and then also what came from this in France, again, right at the end of the 18th century, just before the 19th century, Pierre Simon de Laplace, 1796, what he noticed was all of the planets and the solar system were going around the sun in the same sense.

And also most of the planets, Earth included, and the Sun also rotate in that same sense. And he thought this was consistent with the idea that all the planets, including Earth and the Sun, had condensed from a large gas cloud. so therefore they were made from similar material all collapsing under gravitational attraction. And one of the things that happens when things attract gravitationally is an enormous amounts of energy is released. And you can do the calculation. Laplace did this,

and so did later scientists, and worked out the amount of energy that would be in the Earth and also in the Sun would be enough to keep the inside very hot. What led scientists to think that the Earth was, entirely liquid except for the crust of the surface? Well, this heat that you get from gravitational collapse is enough to warm the thing in the first place. Also, if you look at volcanic eruptions,

it looks like there's a liquid centre to the earth. This looks reasonable. Rocks on the surface of the earth look like they've been melted. Also, it was noticed that as you go down deep into a mine, the temperature increases. So this is all consistent with the interior of the earth being hot. and again if you extrapolate this rate of increase in heat as you go down deep into a mine, every kilometer you go down, the temperature goes up by 30 degrees Celsius.

So it was reasonable to expect that if you go down deep enough, the rock would be molten. And so this was a dominant idea in the 19th century. There began to be in the late 19th century, early 20th century, fantasies or fantastical literature about what the centre of the earth was like. Can you give us some idea of that? Yeah, so Giorven writing in 1864, in his voyage to the centre of Earth, actually mirrors these scientific discoveries. So he talks about the work of Ponson and Davy and other 19th century scientists.

And one of the debates was, is the centre of the earth hot? And of course, for Giorven's story, he concludes that it isn't because then, of course, you can walk down to the centre very conveniently for the fiction. But of course, most scientists in the 19th century thought that the inside of the earth was hot, and the dominant feeling was that it was molten. Lord Kelvin had problems with this, mainly because he was worried about tides. And so the tides, the effect of the moon, causes the oceans to rise and fall. If the centre of the earth is liquid, then the same thing ought to happen.

And therefore, you might worry that as the earth's crust, which is floating on an entirely liquid earth, goes up and down, you wouldn't notice the ocean tides. He also worried that the earth would wobble as it rotated. And Kelvin was very fond of a demonstration that listeners can do at home if you take a hard-boiled air, and a raw egg and you spin them, the raw egg wobbles when you spin it. And this is because it's got a liquid centre. And Kelvin said, well, if the earth has an entirely liquid centre, then it's going to wobble as it rotates, and we don't see that.

So he assumed, and from various other arguments, he assumed that the earth was as rigid as steel, he said, all the way down to the centre. Simon Redfern, what's the current understanding of the structure of the earth? Because it was in the 20th century that's what we might call a more scientific, concentrated study began to get underway? Well, to understand what's inside the earth,

the first thing, the obvious thing that you can say is that we can't look directly there. So the deepest that you can look is in boreholes that go about 10 kilometres. 10 kilometres, and a long way to go. How much further is to go to the centre? The centre is over 6,000 kilometres away. So information

that we get about what's inside the earth come from two areas of science. There's from physical sciences, from physics, we see the passage of seismic waves through the Earth, which give a huge amount of information. And from chemistry, models have been developed of how the planet's chemistry fits together. And so that's really the starting point. And as Stephen was saying, thinking back to the original formation of the Earth,

the condensation from the pre-solar nebula, gives ideas about what the chemistry of the Earth is. Can you give the listeners an idea of the various letters say, well I'll say, and you can correct it, concentric circles to make up the earth's earth. So looking at what the chemistry should be and what the density is, the rocks at the surface are much less dense than the total...

That's the crust, we're talking about it. Yeah, than the total density of the earth. And the crust is this thin veneer on the outside of the earth. And then there are concentric spheres, rather like onion skins going down. and for the first 2,900 kilometres, the first 3,000 kilometres or so, the earth is composed principally of magnesium silicon and oxygen. And as you go deeper, the pressure increases.

So pressure is the main variable that's changing. And the weight of the rock above changes the way the atoms fit together. And so the density of the minerals changes, the mineral structures change. and as the mineral structures change, the density of the rock changes and the velocity, for example, of seismic waves change. So that's the way that those observations were made. And so within the mantle, this silicate outer part of the earth, there are down to about 400 kilometres and then 600 kilometres of various changes that occur. And then there seems to be a range beyond there to about 3,000 kilometres where not much happens,

At the very base of the mantle, things get interesting again, and then it suddenly changes into this liquid iron outer core. And the change there is almost as significant as, or probably as significant as the change between the ground that we're standing on and the atmosphere above us. So there's huge change in physical properties. The outer core is very runny, as runny as the water in the cup here.

And then you go down to the centre of the earth, and just in the last thousand kilometres before you get to the middle, it appears to go solid again. How did you people, scientists, did you say it was iron? Well, that's coming back to this idea of the chemistry. Yeah. So the Earth formed a little over four and a half billion years ago, at the same time as the sun and the other planets,

there's condensation from this disk that's surrounding the early sun. So as the sun condenses to form a star, other bodies, asthmids, and planets start to condense and form. And looking at the chemistry of that, the sun and all of the material that we find from that area, meteorites, we see that we can estimate the chemistry of the Earth, because all of these bodies have a similar ratio of elements within their compositions. And you look at the composition of the rocks that we think we have in the mantle and in the crust,

and they're light in iron. So the iron, which is denser, appears to have, in the early stages of formation of the earth, travel down to the bottom. So by gravity, it's segregated. And this is a process of planetary differentiation where you separate a planet out into the silicate rocky part

and the metallic iron part in the middle. How is the inner core discovered? Well, what we need to do to really discover the layers in the earth is we need to use seismology. So what we do, is we use seismic waves that are generated by earthquakes and they travel through our planet and just as light they illuminates the different layers. And what was discovered in 1906 by Oldham was that there is a fluid core.

And what this fluid core does, it creates a shadow zone. There's a whole area where no seismic waves arrive after an earthquake, a big shadow. And what then happened a few years later in 1936. there was a lady called Inge Lehman and she discovered a wave inside the shadow zone. It's very unusual. She was a female seismologist in 1936, very unusual for women

to actually be doing science and she discovered this strange wave that was quite small and there was no other explanation for this wave in the shadow zone. And there had to be another layer inside this fluid core and so a solid inner core was discovered. So how did she

discover this wave that seems to have been new and eluded most people until then? She was a very careful seismograms, she was very carefully studying seismograms and she just realized that there was this part of a seismogram that should be quiet. Can you just explain to the listeners that it would help me as well?

Exactly what a seismogram is. A seismogram is a recording. If you have an earthquake, then we have seismometers all over the servers of the earth and they record the waves that come from an earthquake and they show nothing more than just wiggles, movements up and down, and those are just the shaking of the earth at that location

where there is an earthquake. Can you tell how deep they go? Because the deepest we can drill is about 10, 12 kilometres. There's a long way to go to that of 6,000 kilometres. So what, how deeply, where do they come from and how do you know where they come from? So earthquakes, there are earthquakes that happen just a few kilometres deep in the earth, but the deepest earthquake is about 650 kilometers. But the waves that come from these earthquakes, they can travel all the way through the center of the earth. So it's really our only way to see through the earth

and make pictures of the deep part of our planet. If you want an analogy, it's very similar to making a brain scan where you have lots of receivers on your head and then you get a nice, colorful picture of what your brain looks like, and we basically do the same thing for the earth, but then with seismic waves. It's been discussed a bit by Simon but can you say something else about it?

What proof is that that the inner core is solid? Now, to prove the inner core is solid is complicated and it's something that I worked on and what you need to do is there are two type of waves that travel through the earth. One type of these waves will travel through any material, solid and fluid materials. But there's another type of wave which is very special.

It can only travel through a solid material. So if you want to prove that the inner core is solid, you want to observe this wave because if it's solid, this wave will exist. If the inner core would be fluid, this wave would not exist. But you can't get through the liquid, but that's the problem. Yes, you're right. So this wave is tiny because we'll have to travel as this wave that can go through any material through the liquid outer core.

Then I will have to change the boundary between the outer and the inner core to this we call shear wave which can only travel through solids. But to leave the inner core, we'll have to change back again to a wave that can actually pressure if they can travel through fluids. So it loses energy all the time and it's a tiny wave, but as a student, I looked for this wave and I managed to find it. It's tiny. You won't easily see it, but you need

to combine a lot of data and then suddenly it is this tiny, tiny wave will become visible. How long do you take you? It took me about half a year to make diverse observation and then another year to prove you're right. But if it doesn't travel through liquids, how could you get held of it at all?

So you don't actually observe the, so you observe a wave that was a pressure wave which can travel through fluids. It's a pressure wave, if it starts at the earthquake at the surface, it will be a pressure wave in the mantle and in the outer core. And at the boundary between the inner and the outer core, it changes to be this sheer wave which can only travel. So it changes its nature. And it changes nature completely. And it becomes much slower when it's a sheer wave. So that's how you can identify the waves at a completely different time from a wave that would not have been a shear wave. So it's got a few characteristics that say if it's arise with these characteristics,

it must be this wave that has travelled the inner core to prove that it's solid. Stephen Blondell, can you tell us a bit more about the Earth's core? How hot is it? Why is it as hot as you're going to tell as it is? Well, we think the centre of the Earth's core is about 5,000 degrees centigrade, but it's quite a hard thing to estimate. How do you compare with, how does that compare with the sun? Well, it's getting on for the temperature of the surface of the sun, which is of the order of 6,000.

So it really is getting up there. But the important thing is how do we know this? And Arwin's just described about seismology, which is the only way we can look inside the earth. Seismic waves are bouncing around inside the earth, as Arwen's described, and they are refracting almost like beams of light every time they hit an interface. And from reconstructing this,

essentially what you can do is work out the speed of sound at each point inside the earth. And the speed of these waves, which is essentially a sound wave, a compressional wave, tells you about the elastic properties of the material that you're going through. What are elastic properties? How stretchy, how rigid it is, and it also tells you about the density.

So something you can do fairly well is to estimate the density at each layer. And from that, you can work out using the force of gravity what the pressure is as you get closer to the centre of the Earth. So we know, for example, that the pressure at the centre of the Earth is more than 3 million times the pressure at the surface of the Earth,

at three million times air pressure. Now, then we have to then use our physical understanding of the composition of the rocks to work out the temperature profile. And so what you've essentially got is a whole series of chains of inference. So although we've got the density and the pressure reasonably well determined,

the temperature is something that's a much harder to nail down and to pin down exactly, because it really does depend on us knowing exactly what the composition is at every depth. And as Simon explained, we can't see directly. Simon Redford, when did the solid in the core

form? Well, the... So we start with the four and a half billion years. We start four and a half billion years ago. The solar system. So the earth, in the early stages of the earth, as was mentioned earlier, it was completely liquid.

So the whole planet was completely molten and very hot because this gravitational, the formation of the earth by gravitational accretion of asteroids and planetesimals, generates a huge amount of energy. So you've got both gravitational energy

being released as the planet forms and also additional energy from radioactive decay and from the segregation of this core to the centre. So there's all sorts of energy sources that are heating up the planet. Initially, the core, the liquid,

iron core, iron rich core, would have been very hot, hotter than it is now, and would have remained hot above the crystallization temperature of iron at the conditions of the centre of the earth, which is over 3 million atmospheres. And so over time, the planet has started to cool. And as it cools eventually, at the centre of the earth, the highest pressure point,

we pass over the crystallisation temperature, the freezing temperature of iron, and iron starts to freeze at the centre of the earth, and you get a crystal of iron right in the middle that starts to grow. I'm a bit worried about freezing when it's that hot. Well, freezing we think of as being zero for water, but any liquid will go through a transition between the molten state and the solid state to the solid state.

I'm sure worried by freezing, but you better go on. The freezing point of iron is more like 6,000 degrees at the center of the earth. And it was. And so iron has started to crystallize from the centre outwards. And this has been happening. Estimates put it somewhere around a billion years. So a billion to 800 million years ago that this started form. And it's been growing ever since.

It's growing today at a rate of about half a millimeter a year. So it's growing very, very slowly as the core slowly cools. And as it grows, what happens, is it actually changes the chemistry of the core as well because the solid inner core is more pure than the liquid it's forming from. Rather like an iceberg floating on the sea, an iceberg is pure water, and the sea is salty. So in the centre of the earth, the solid inner core is much purer composition

than the outer core that it's growing from. Arwen Doiss, why does it take less time for seismic waves to travel through the earth from north to south and from east to west. Yes, that's a very, very interesting observation. And what is basically happening is that the inner core is it's a crystal. And how we see this is

if you would have an earthquake on the North Pole and a seismometer on the South Pole and you would recall to a wave that's trails from north to side, it will arrive about five seconds faster than if you had the seismometer and an earthquake on the equator. And it would have traveled a similar path but from east to west.

and what that means is that the waves just have a faster speed in a north-south direction and in the east-west direction and that tells us about the properties of the inner core. It's a bit like if you are running... Just a second. Do the waves have a faster speed or is there passage more simple one way than the other? Yes, this is indeed a good way to look at it. It's like if you were running the hurdles.

If you have to go and jump over all the hurdles, you go slower because you need to jump all the time. and if you were allowed in the opt to run kind of along the hurdles then you would just go in the long line and you wouldn't have to jump over them you would go faster you would reach the same distance much faster so what are the hurdles east to west then the hurdles are boundaries between the crystals and it depends on the shape of the crystalline

the waves that travel from east to west just have to jump over more of these boundaries and they just have a different speed in that direction Simon you want to come in it also comes into the possible atomic structure of what the iron is in the center of the earth. And this is a big question that's unresolved at the moment. The problem with iron is that it can take many atomic structural forms. So we know if we look at steel here in the foundry, you can change the properties of steel by heating it and quenching it

and going through various chemical compositional changes. And similarly, the iron in the center of the earth, the structure of those crystals are inherently potentially different in different orientations. So one possibility is that there are boundaries within the structure of the crystals are all oriented in one particular direction. And if they are all oriented in one particular direction, then these changes in velocity that Arwen's talking about could be related to the changes in the crystal structure.

Can I go back to your own? Does this difference in the time, what does that tell you? I mean, you know one slow and the other. It tells you that, okay. We've talked about boundaries and hurdles, okay? What else does it tell you? Well, it tells us something, we need something to align these crystals. They don't just grow like that.

We need a mechanism, and this is one of another unanswered questions that we're just wondering, why would that be? Do they grow? Because it's in the same direction as the rotation axis of the Earth and also the same direction as our magnetic field. do they align with the magnetic field or is there a flow in the inner core? Is the inner core slowly changing,

moving the crystals in one direction or is it related to how the solidification of the inner core happens? We don't know, but if we can know this, then we've learned a lot about how the deepest part of our planet is working and the whole evolution of our planet, how it may come to be the way it is today.

On the magnetic field question, Stephen Blondale, what were the early theories about the creation of that field? Well, from the 12th century, it was known that the compass needle points north, and the question was, why does it do that? And there were various theories that were going around,

is there perhaps a magnetic island somewhere north of here that's attracting the compass? Is it possibly the pole star? The pole star is north, and so maybe that's causing the compass to point north. That's why we have the phrase

lodestar. The lodestar is the pole star because it makes the lodestone, which is the old-fashioned name for a magnetite, a magnet that sits in your compass and points north, maybe the compass is attracted to this load style. And it was William Gilbert in 1600. He wrote a book called De Magnete.

He was physician to Queen Elizabeth I, first, and his spare time he was working on magnetism. And he did a whole series of experiments on magnetism to try and understand what magnetism is. And he was particularly fascinated by the magnetism of the earth. and what he did was he made a spherical sample, with a lave, he made a spherical loadstone, a crystal of magnetite that was spherical in shape,

as a kind of model earth. And then he took his compass needle and went all the way around this sphere and noticed that the way the compass needle pointed mimicked the kind of behaviour that is noticed by mariners when they're sailing around different parts of the earth. And so he concluded from this that the earth, in fact, is the source of magnetic field,

and he therefore came to. to the conclusion that the earth is a giant crystal of magnetite, it's a giant magnet. And so that was an important breakthrough to at least realise that it was the earth that's the source of the magnetic field. Where he was wrong is in thinking that the earth is just a giant ferromagnet,

as we'd call it using the modern lingo, because of the extraordinarily high temperatures in the centre of the earth, that would destroy permanent magnetism. Can we take that on, Sam, right from, to the present day? What's the view of the magnetic field now? and what does it do? So the views

evolve from Gilbert's ideas partly from observations of the magnetic field of the sun and at the beginning of the 20th century people noticed that when there were solar flares there were changes in the spectroscopy of the sun that could be associated with magnetism

and so people started to look at magnetism generated by fluids and then in the mid-20th century century, Teddy Bullard developed the idea that the magnetic field is actually generated in the outer core, the liquid part of Earth's core. So this liquid iron, because it's moving and is able to carry electrical current as it moves generates a magnetic field. And the idea currently for the Earth's magnetic field is that it's a dynamo, which is rather like a motor. the fluid iron is moving and it's moving in spiral sort of trajectories

within the outer core and that's because of a Coriolis force so in the same way as when your water goes down the plug hole it starts to spin so the liquid in the outer core starts to spin due to the rotation of the earth and the fact it's also convecting it's also moving itself

in a rather turbulent manner and to do that you need some heat so heat beyond the simple pressure heat that's generated as you go down so as you go down it gets hotter simply because the pressure goes up in the same way as you go up a mountain it gets colder so as you go down into the earth it gets hotter due to the pressure but with additional heat you get turbulence and it's these turbulent movements of the liquid iron

that generates a magnetic field and it self-sustains through the dissipation of this heat Owen, Ours, first of all, can you just say, what is the importance of the impact of the magnetic field? And can you tell us about the variations in it? Well, the magnetic field is very important because it protects us against cosmic radiation. So that's one really... How does it do that? It just creates a shield, which will just deflect the cosmic rays from the sun to actually reach us at the service of it.

So it protects us. So it goes up there and there's a... You would see the radiation kind of go right in the earth and not actually reach us. So that's one really important reason why we need the magnetic field. And what, for example, happens is the magnetic field flips over time. What we now have is a compass pointing north. But we noticed in the past our compass would have pointed not through the North Pole, but to our current South Pole.

And this is because the magnetic field flips. And this has happened with intervals of several millions of years, and we can see that in our geological paths. And that's, of course, very important, because when you have one of those flips, the magnetic field becomes weaker, when the pulse change, and then we would have less of this cosmic protection from cosmic radiation.

So Stephen Nondell, this suggests that the intercourse is quite active. Can you tell us a bit more about that? Well, the magnetic field is jiggling around. We know this because magnetic north, at the moment is marching through northern Canada at a rate of about 60 kilometres a year. And if you follow it back... Can you just say that again in a different way?

So the North Pole, the magnetic North Pole, is not stationary. It moves a lot. It moves a lot every day because of changes in the ionosphere over a period of years. You can see the actual position moving. And this has been known right back since Gilbert's time. Gilbert was worried about this. Edmund Halley. was one of the first people to actually do a survey of the magnetic field across the earth

and look at the corrections that sailors needed to include in their charts. And he also realized that you'd need to monitor this over a period of time because the magnetism of the earth is changing. In fact, you can even notice it in modern life, because if you go to an airport, you'll see that there's a number at the end of a runway. And that number tells you the orientation of that runway with respect to magnetic north. And every now and then they have to repaint them. So Stansted was done a few years ago, Manchester, Prestwick, Vancouver,

they've all been done in the last few years. It's worth working out which one's being done before you catch a plan. Absolutely. They often have to close the airports at night to repaint the numbers. I think they did it at Heathrow in the 1980s. So this is because these runways are labelled according to Magnetic North and Magnetic North changes. Simon Redfern, as well as the inner core, they're supposed to be an innermost core. Right. Right, well, it's over to you.

So, yeah, the story gets more complicated because this inner core that we have itself has structure. So this sphere... How big is this inner core? The inner core is about the size of the moon. It's a radius of just over 1,000 kilometres. And seismologists, such as Arwen, have made observations of the passage of these vibrational waves, these seismic waves,

that pass through the inner core. And they vary themselves in different directions. We've heard they travel faster north-south than they do east-west. But also, you can begin to pick out how they vary through the inner core. And some of the latest evidence suggests that the innermost part of the inner core itself is slightly different to the outer reaches of the inner core. And that's interesting because it might tell us something. So as it were on the same planet, but inside the middle of that planet.

Yeah, spheres within spheres. So what's going on in the innermost core? That's a good question. Yeah. So, I mean, I think here we're reaching the points where we're looking at scientific problems that have not been sold yet. What do you think is happening? What appears to be the case is that there's a strong alignment of crystals in the inner core. That alignment may not be quite so strong in the outer reaches of the inner core.

And some of the latest results have suggested actually the alignment might not even be north. south, which is kind of quite counterintuitive. Having a north-south alignment is aligned with the rotation of the earth, with the magnetic field of the earth, which are all the forces on the formation of the crystals in the inner core. So there are questions that are being raised at the moment that I think still require further study to work out. But if this is the case that the innermost inner core has some different structure to the outer parts, then that implies that maybe 500 million years ago, something changed in the potentially the magnetic structure of the earth that changed the way the inner

core was growing. What do you think that change was, I think that change was now, I don't really know, but it could be a change and this is related to the iron crystals making up the inner core. It could just be a different way of how the crystal is built, a different way of putting all these iron atoms together and building a structure out of it. So that would be one way. It could also be something quite drastic, changed in the earth. Maybe the way

the thermal evolution of the earth was happening, it might have changed. We don't know yet because it's difficult to look back into the past and actually know what's happening. It might be related to the magnetic field. We know that there are periods of time where the magnetic field changes a lot

where it changes every few million years and there are periods of time which are kind of quiet times of hundreds of millions of years, but the magnetic field is very stable. Maybe there was a time the of the pulse or if it was more or less stable than now and that changed it the way the inner core solidifying

and the innermost part came to be. So you can't see a sort of path of scientific inevitability about it? You just say it might happen a bit quiet and then it wakes up one morning and away we go. Yes so this is because it's flow of fluid. The flow in the

outer core it's what we call kind of a non-linear. So we can't really make any predictions. We can't even generate it in our When people try and make computer calculations of how the magnetic field is generated, they need to use really, really big approximations. We're not even close to what are the real characteristics of our magnetic fields.

So we have no full understanding of what's happening there. So this is the excitement of what we're doing and the excitement of having these three people here where I'm a seismologist looking at seismic wave. Stephen's talking about the magnetic fields. And Simon is a mineral physicist. We can only really discover what's happening in the innermost part of our earth by combining these three different fields. And then we hope we've all got our own three or four theories.

We hope only one theory will fit all our three different fields. And maybe then we get a step closer. But the inner court, so far, we can't go there. We can't travel there like Jules. Yeah, it's very difficult. I asked this at the beginning, Stephen Lundell, and I got an answer from Simon, but I'm going to ask it again, really. It will seem odd to many listeners.

And to me, really, that we're, there we are, foraging away, billions of miles in the distance, and we know quite a lot about the sun and just been around Mercury several times and crashed onto it and so on. And yet that on which we live is very, very little noun.

What's the difficulty? The difficulty is essentially that we can't see what's going on. The great thing with astrophysics is that you can point a telescope out to some distant star, supernova, gas cloud. And you can look at the light and that light gives you a spectral fingerprint, which allows you to work out the chemical structure

of the material that's giving out the light. It can tell you also about whether the object is receding from you or coming towards you because of the Doppler effect. So we can study these things which are light-years away and get a lot of spectroscopic evidence about it that tells us a lot. The problem with the earth is that we can't see through it. And therefore we've only got these seismological probes to really work out what's going on.

That for Simon Redfern, do you think it's a question of instruments, developing instruments, and been able to get them through somehow? Well, you know, you can go into the realms of science fiction and the Hollywood view of the core where you develop some sort of inner spacecraft, but that's really completely unfeasible. But in the lab, you can recreate the conditions of the air. You can inco. Yes, but only in microscopic quantities. So the way to do it is you have to squeeze rock.

and iron between diamonds. Why is it got to be diamonds? Diamonds are the strongest material that we have. So if you want to... Diamonds can withstand the pressures of the inner core and still remain solid if you do a good experiment. Very often they break. So these experiments, diamonds are consumable in the experiment.

And you squash the sample between two diamond crystals, use the tips of diamonds to generate very, very high pressures. and you can reach the pressures of the centre of the earth that way. But then getting the temperature as well is a big problem. And to get the temperature of the centre of the centre of the centre of it, so to get to 6,000 degrees, you then have to, it's typically done by firing a laser through the diamond. And again, the diamond helps you here because it transmits the laser light,

and then the sample absorbs that laser heat. And by doing these experiments, you can begin to look at properties of iron. For example, it's recently been measured, the melting, point of iron. So you can observe iron melting and from that infer what the temperature of the inner core is. But the number of labs around the world that are doing this are very few and the experiments themselves are very, very difficult and being able to reproduce the experiments and be certain that the results you're seeing are not due to some artefact in the experiments is very tricky. Do you want to comment on it? Are they doing that at Eutrecht?

No, no, no. There's a big lap in Japan where they're doing that and they have tons of of money to actually, yes, do that and crush lots of diamonds. Sorry, so are we doing it here? There are experiments that are going on of this type in the UK and in Europe. And to do it, you actually need very intense x-rays to be able to measure what's going on inside this cell in a sample that may be only a micron, less than a hair's width across. So there are experiments that are carried out at the UK synchrotron, which

confusingly is called diamond and a synchrotron also these are x-ray sources, very high intensity x-ray sources in Grenoble in France where these sorts of experiments are also taking place. Arwen, it's been thought that the inner or innermost core, I don't know which one I'm talking about here, you'll tell me, rotates

in a different way, rotates differently to the rest of yours. Yeah, this is a very interesting idea and it relates to the magnetic field because you have these flows in the outer core. they generate a little force on the inner core which just pushes it to rotates a little bit faster than the rest of our planet or at least there are models of the magnetic field

that predict that this may be happening and this could be up to a few degrees per year faster this means that if it be one degree per year faster than in 360 years the inner core would have done one extra rotation than the rest of our planet now these are interesting ideas and seismologists have been looking for this

and something that we investigated a couple of years ago is that we discovered that the inner core is actually separated into two different halves, two hemispheres. It's as if the inocor is a piece of fruit. You could slice it into two, and you have these two different halves, which are totally different. And we can actually image the boundary

between these two different halves. And what we saw was that that boundary was slowly changing eastward as a function of depth. And we could infer from that that this is because the inner core is slowly rotating, but not one degree per year, but we thought more, one degree per million years. So much slower, but much more in line with most recent predictions

of the magnetic field. So it's kind of exciting that the inner core is almost a planet on its own in the centre of our planet with different regions. You know, these hemispheres are like continents and oceans we have at the surface of Europe to totally different parts and that we can use it to infer things about the faster rotation of the inner core, which will also tell us about the magnetic field. So we seem to be in a rather goldilocks zone here, Stephen Blandall, although we were by one accident or design, depending where you come from, or another, this complex system delivers a place where we live on now.

Is that a good thing or a bad thing? We'll leave that for another programme. But never mind. What could be slightly different to make the whole thing collapse? Well, we get a view of that by looking at the planets in the solar system. And some of them have dynamos like the Earth, which produces a nice magnetic field, and others don't. So, for example, Mars,

is quite a small planet. It's about a tenth of the mass of the Earth. And that means it's cooled much more than our planet. And so it's got a solid core. And therefore it can't sustain a dynamo. There isn't fluid to go around. So Mars has a very small magnetic field. Venus is about the same size as Earth, but it spins very slowly. And you need rotational, you need rotation to sustain the dynamo. And so Venus also isn't very magnetic. But other planets like Jupiter, the gas giants, they've all got very large dynamos. And in fact, if you look at the magnetic field of Jupiter on the sky, not that you can do that with your eyes,

you see Jupiter as a tiny dot, but its magnetic field would extend to the size of the moon. Yes, it's interesting that we think that Mars might not have a fluid core because its magnetic field has gone. But, of course, we will only know that for sure if we ever sent a seismometer to Mars, and we haven't put any seismometers there yet. So that's really the future,

putting seismometers on the planets and knowing if these theories are true. what do you think what do you think you might be consequences of people like now like to know I do so you're doing all this and it's fascinating and

I'd keep curiosities the driving force good good and good again but what might come of it that would affect the way we live well I think people are interested in the way planets form that are habitable and it's clear that the magnetic field

that we have on earth is linked to the fact that we have a habitable planet when you look at these other planets with low magnetic fields and no magnetic fields, their atmosphere has been stripped away by the cosmic wind. So the magnetic field actually protects us and has allowed Earth to develop an atmosphere that we can breathe. So fundamental questions about the origins of life,

why we have life on Earth, why there may be life on exoplanets on other stars or not, are all linked to how a planet works. And the core of the Earth is clearly an important part of the way this planet operates. Well, thank you all very much, Simon Redfern, Owen Doiss and Stephen Blondell. Next week I'll be talking about the Bengali poet and Polymouth Tagore

thought by Yates and Ezra Pound to be a very great poet indeed, and thank you very much for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. Well, thank you very much. Now then. What did we miss? Well, we didn't talk about the reason of variations we've seen

the magnetic field and what I think is interesting is how that relates to the surface of the earth because we've only been talking about the core but if you look at

the service of the earth the ring of fire around the Pacific where we have all the volcanoes and all the earthquakes North, South America, Indonesia, Japan

what we have there is plates subducting into the mantle cold material and that goes all the way down to the core mental boundary and really they're making a ring of every cold

material at a graveyard of subducting cold slaps that extracts more heat out of the core there the flow gets faster there and we get stronger magnetic field. We see that. That's been seen in shiplocks even of the magnetic field. But on the north and South America and on the Europe and Africa, there are stronger regions of the magnetic field.

And I think it's interesting that we're not just talking about this core. It relates to right the surface of the earth. It's all influencing each other. Yeah, it's how the interface between the mantle and the core controls what goes on in the core because it all depends on the heat flow out of the core. And the mantle is that boundary where whatever is going on at this. core mantle boundary, which is a sudden jump in temperature, as well as in the chemistry of what's there. So you go from a liquid metal that's a conductor to an insulator that's both a thermal

insulator and an electrical insulator in the rocky part of the planet. The base of that rocky part of the planet, the base of the mantle seems to be playing a big role in what goes on in the outer core where the magnetic field's been made. Are you anywhere near saying, well, this might enable us to do this or that. A lot of the work, people in Allied fields are probably in yours, are probably in yours, are doing, out of proper scientific curiosity, end up as sort of running the world, really. Is there anything?

Well, I mean, just to bring back one issue that we had in the program a little bit about predictability. So a very similar problem, an allergist problem in a way, is weather prediction. Because with weather, you have a small fluid layer, which is actually quite thin compared with the size of the Earth. It's on a rotating planet, which means if you have a pressure gradient, rather than the air going from high pressure to low pressure,

it goes around in cyclones and anticyclones. It's a non-linear problem, and as we all know, predicting the weather over any decent interval is very difficult. Now, the problem that you've got with thinking about the dynamics inside the core, the outer core in particular, you've got a problem that involves fluid dynamics, magnetic fields. No wonder we have difficulty in these kind of simulations. that you were talking about, Arwin,

in terms of predicting what's going to happen, when will the magnetic field of the earth flip next? You spoke about if the three of you, if your three disciplines, your three disciplines would have to come together in order to solve this. Is there any indication that this is going on at the universities? Oh, it is, yes, yes. We all go to similar conferences.

I see Simon a lot, and we actually have conferences which just studied the Earth's Deep Interior, and they are known for having all these different disciplines coming together. What I like to say is in the 60s, we were totally at a loss what was explaining the movement of the tectonic plates and then suddenly plate tectonics was discovered and everything fell into place. I think we're at that point now with the core, that we have all this data and observations

and we just don't know yet how to put it all together. But I expect within the next 10 years, a similar theory to plate tectonics, but for the core will come and suddenly it will all fall into place and you understand what's happening. Yeah, and I think my own interest is actually in the theory of magnetism and understanding magnetism is a property that has all kinds of applications in everyday life. And the kind of experiments that Simon was talking about under high pressure

tell us about the basic physics of magnetism. So that's useful for telling us about the Earth's core, but it's also for understanding magnetism, designing new materials that will have technological applications. Yeah, so the link between material science and mineral physics, for example, is very strong. The technologies that mineral physicists are using to replicate what goes on inside the Earth and even a bit larger planets is the same technology that people are using

to try and develop new materials that are super hard, so alternatives for diamond that are cheaper to make, or even to measure what's going on, for example, inside an explosive device. So a lot of the people who've looked at very high pressures have actually done so from the perspective of... Here's a producer Simon Tillerson to tell us that we don't need another programme.

We really need a cup of tea or coffee. There are many more radio four. arts and discussion programs to download for free. Find these on the website at BBC.com.uk slash radio four.