Under Our Feet: What's Inside Earth?

We're digging into the science under our feet.....
16 June 2020
Presented by Chris Smith, Katie Haylor
Production by Katie Haylor.

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This week, put on your hard hat and steel capped boots: we’re journeying to the centre of the earth! From the soil at the surface to the magnetic molten iron at the core, we’re delving into what’s going on inside our planet. Plus in the news - How llamas can help us to combat Covid-19, archaeology on a quad bike, and Einstein is proved right again in a test of extreme gravity...

In this episode

Llama head

01:01 - Covid 19: antibodies and llamas

Could members of the camel family help scientists find a Covid-19 treatment?

Covid 19: antibodies and llamas
Daniel Wrapp, University of Texas at Austin

While we wait for a vaccine for Covid 19, it's possible that members of the camel family could be coming to our rescue with a treatment. Scientists in America have isolated a class of antibodies from a llama vaccinated with a coronavirus. These antibodies are called nanobodies - they're like a miniature version of one of our human antibodies, and they work the same way, but they may even - on account of their small size - be able to access and neutralise parts of the virus that chunkier antibodies can't get to. Daniel Wrapp is at the University of Texas at Austin…

Daniel - We started this work in 2016 when there was no active coronavirus outbreak. And we were trying to understand the mechanisms that allowed the coronaviruses to enter into host cells and infect us. To do that, we vaccinated a llama named Winter with proteins from coronaviruses that had emerged into the human population previously.

Chris - Now, what were you hoping to learn by doing this? And why did you pick on Winter the llama?

Daniel - When we started, there were no, uh, approved vaccines or therapeutics for any coronaviruses. And in fact, that's still the case. So we were hoping that we were able to elicit antibodies from this llama, that bound to portions of the coronavirus that are critical for its function and prevent it from entering into host cells. And the reason why we picked a llama is because llamas are known to produce a specialised class of antibodies that humans aren't capable of producing. Those antibodies are about half the size of the antibodies that you and I would produce. And because of that smaller size, they're able to bind to small crevices and pockets that otherwise wouldn't be accessible to larger antibodies.

Chris - Why do llamas do this then?

Daniel - Good question. We think it's sort of just an evolutionary quirk that arose, and because it was beneficial for the llamas to be able to fight off infections, it stuck around. Sharks do a similar thing, but we think they're probably two distinct evolutionary events that just spontaneously arose.

Chris - And what about animals that look a bit like llamas? Because alpacas look like mini llamas. Do they do that as well?

Daniel - They do, yes. It's a, it's a group called camelids which includes llamas, camels, alpacas, and a couple other organisms.

Chris - And your idea then was we make them make these mini antibodies that might be endowed with the ability to reach parts of the virus that other antibodies might not. A bit like the sort of antibody equivalent of Heineken beer and what, then you could use those therapeutically, or use them to understand the virus better? What was your motivation?

Daniel - Yeah, we were trying to sort of use them to understand the virus better and if they were capable of neutralising the virus, then there would be potential therapeutic candidates. We found one mini antibody, or it's sometimes called the nanobody, that was capable of very potently, neutralising a virus that caused a Middle Eastern respiratory syndrome outbreak in 2012. And one that was capable of neutralising the virus that caused severe acute respiratory syndrome coronavirus in 2002.

Chris - How do you go then from one of these nanobodies, mini antibodies in a llama, to something that will help a patient who's infected with coronavirus of the COVID-19 type today?

Daniel - Fortunately, the antibodies that we isolated from the llama were actually so potent that we didn't have to change them much at all. Theoretically, we could take those antibodies, administer them directly to a patient, and they would be able to fight off COVID-19 more easily with the help of that treatment.

Chris - Where would you get the antibodies from though? Would you have to basically turn Winter the llama into a blood donor?

Daniel - Using this llama that we vaccinated, we identified the gene that encodes for the exact antibody that we're interested in. And now that we have that genetic information, we can put it into cells in a dish growing in the laboratory, and we can scale that up for bulk production.

Chris - And have you actually tried this against the new coronavirus? Because you've tested it against the original SARS, you've tested it against this Middle Eastern respiratory syndrome virus, have you actually challenged an individual or an animal with the new coronavirus and then shown that these mini antibodies work?

Daniel - So we haven't tested this in an animal yet, but we have done it using just cells in a dish in the laboratory. Uh, and we did see protection against the COVID-19 coronavirus. So we're optimistic that when we move into hamsters, which is going to be our first test organism, that we'll see the same effect.

Chris - And do you know how it's working?

Daniel - Yes. So the way that it works is it binds to the portion of the coronavirus that attaches to our host cells. So basically the antibody is preventing that attachment from taking place and therefore the virus can't infect.

Chris - Now, given that the llama can make antibodies, is that not just easier, since we're going to have to put these antibodies into people to treat them, is it not just easier to do to people what you did to the llama and make people make normal human antibodies?

Daniel - Yeah, the vaccination would be ideal, but as sort of a stop gap until we have an approved vaccine, this will be useful for people who have already come into contact with the virus, because you have to be administered a vaccine probably about two months before you would ever come into contact with coronavirus. So a vaccine won't be useful for people who are already infected, And there's also some interest in administering it prophylactically to the elderly, who aren't always able to raise an effective immune response upon vaccination.

Chris - And you think you could make enough of these.

Daniel - Yeah, we've scaled up production and we get very good yields. So we're optimistic about being able to produce sufficient quantities.

Pipette

Modelling early human development
Alfonso Martinez Arias, University of Cambridge

In order for us to be talking to you today, and for you to be listening, an incredibly complex cascade of events took place during our mothers’ pregnancies. There’s so much still to be understood about human development, and this week news is out of a model that could give scientists much better insights into how we come into being. Alfonso Martinez Arias from Cambridge University is one of the scientists behind a new model, which they say will allow scientists to probe the processes of early human embryonic development in the lab for the first time. They’re modelling an early developmental stage called gastrulation, where different parts of the body start to form. This is poorly understood in humans owing to restrictions around growing embryos for research beyond 14 days. So what exactly is this model, and what can we learn from it? Alfonso spoke to Katie Haylor...

Alfonso - What we have achieved on the plate is a simulation of the process of gastrulation. Now this is a mouthful of a word for many of the people listening, but as a biologist said, the most important moment in your life is not when you're born or when you get married or when you die. It's when you undergo this process of gastrulation. So before explaining what it is that we have achieved, it's important to get the perspective of what this process is. During these magical event, the ball of cells that results from the fertilisation of the egg, it transforms itself in a very origami sort of way into the outline of an organism, with the seat of the head at one end and behind it, the heart and the gut and the muscles and the bones. This is a process that happens deeply inside the uterus. It has never been observed in the lab, in reality. And we have managed to reproduce it in the lab.

Katie - How have you done this then, if it's so difficult to do?

Alfonso - One of the things that has opened up the way for these kinds of experiments is embryonic stem cells. Embryonic stem cells are cells derived from early embryos that can be kept in the lab indefinitely and can be enticed to differentiate into different cell types of an organism. But they had never been put together or attempted to build the rudiments of an organism with that. We found the way to entice them, if you wish, to come together in a way that they are able to undergo this process in the dish and outline the organism, the little human being. I should point out that at these earliest stages, the human embryo is about one or two millimeters long, which is the kinds of object that we obtained with the cells.

Katie - Wow. Why hasn't this been done before though? Can you tell us a bit about the ethics around this kind of work?

Alfonso - Yeah, it is important to highlight that we know a lot about this process in many, many animals and even in mice. But it has never been achieved in humans. And the reason for that it has to do with something that is called the 14 day rule, which was set up during the studies of in-vitro fertilisation, when people realised that you could grow early embryos in a dish, but then that raises the point of how long could you keep these embryos grown in the dish? A committee was set up, run by Mary Warnock, and in that committee, they decided that the point at which you could never grow embryos longer in the dish was day 14. Why? Because that's the day at which this magic process of gastrulation happens. In any case, this process takes place very deeply inside the uterus. And therefore has never been observed before. These days, there are people growing embryos in the lab, but they have technical difficulties to get them beyond this day 14. We have managed to find a way which does not exactly reproduce the early events of the embryo, but it produces the process of gastrulation. And it yields the structure of these early embryos.

Katie - I see. So would it be fair to say that this model wouldn't result in an embryo, if you were to continue the model?

Alfonso - That is indeed very, very important. There are two features of our system that are present in embryos and that we have managed to avoid in the way we treat the cells. One is the appearance of a brain. That doesn't mean that the system does not have a nervous system. There is more to the nervous system than the brain. And we have the seeds for the neurons in our system. And it also lacks a set of cells that allow the embryo to attach to the mother. Therefore, the system that we have would never develop into a full embryo at the moment.

Katie - I see. And just briefly, what are you hoping to learn? Or you're hoping the field will learn this model?

Alfonso - There are three things. One - as a scientist, this is the first time that we can actually see how gastrulation happens in a human embryo and compare it to other animals. And this is one of the things that we do in the paper. Second - most importantly, there are many birth defects that probably have the roots in the process of gastrulation and that has never been able to be studied. With the advent of these IPS cells, induced pluripotent stem cells, you can make now embryonic stem cells from patients with these diseases. And we could try to model them, to copy them and ask how do they produce, how do they go through this process of gastrulation? Finally, I think it would be a very good system for drug screening of substances that people might want to test for pregnancies, and how would they affect the earliest stages of human development.

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.

14:09 - Einstein and extreme gravity

Even under extremes of gravity, general relativity works correctly...

Einstein and extreme gravity
Ben McAllister

In 2015, theoretical physicist Albert Einstein’s work arguably faced one of its toughest tests when researchers went looking for the gravitational waves predicted by his theory of general relativity. Einstein passed with flying colours. And this week he’s got another feather in his cap, because scientists have proved right another prediction: that even under extremes of gravity, like that around massive stars, general relativity works correctly and explains how gravity behaves. To fill us in, here’s physicist Ben McAllister…

Ben - In the late 16th century, Italian physicist Galileo climbed the Leaning Tower of Pisa and dropped two rocks of different masses. He measured how long it took them to hit the ground. To most people's surprise, despite having different masses, the rocks hit the ground at the same time. This was one of the earliest observations of what we now call 'the equivalence principle'. The principle roughly states that things accelerate the same way in a gravitational field, regardless of their mass or what they're made of. So, whether we're talking about a falling rock or a falling meteor, they should both obey the rules identically. But when this pronouncement was made, we didn't know about any of the more exotic things out there in the universe, like black holes or neutron stars. These are cases of extreme gravity, with gravitational fields so strong and complex that they end up interacting with themselves in ways Galileo could never have even imagined when he dropped his stones.

Over the centuries, we eventually developed a theory of gravity which did allow us to comprehend those gravitationally-complex extreme cases: Einstein's theory of general relativity. This new theory of gravity suggested a possible extension of the equivalence principle, which we call 'the strong equivalence principle', and which states that even gravitational gargantuas like neutron stars should follow the rules and behave like Galileo's rocks. Nevertheless, this stronger version of the principle was initially just a theory and had yet to be tested. Fortunately, the universe gives us nice laboratories to do these tests. And recently a group of astronomers from around the world have used a radio telescope to perform the cosmic equivalent of Galileo's Pisa experiment.

They measured the motion of a fast- spinning neutron star known as a pulsar. This was being orbited by a white dwarf star much like the moon orbits the earth. The pulsar and its white dwarf companion were in turn both orbiting another star, much like the way the earth and the moon both orbit the sun. By monitoring the light signals coming to earth from the spinning pulsar, and looking at how the arrival times of those light signals differed when the pulsar moved away from and towards Earth, the scientists were able to make extremely precise measurements of the motion of the stars. And what they found was that the equivalence principle seems to hold, even for gravitationally-complex bodies like neutron stars. The pulsar and the white dwarf star, despite having different masses and different compositions, appeared to both be accelerating towards the star they were orbiting at the same rate. Much like Galileo's different rocks falling towards the earth.

So for now it looks like our old pal Einstein was right yet again. Fortunately we didn't need to go lobbing rocks off of tall buildings to get to the bottom of it this time.

Ancient Rome

Archaeology - no digging required
Martin Millett, University of Cambridge

The typical image of an archaeologist is somebody with a trowel down a hole, digging away, unearthing the past. Well it turns out that’s a bit old hat these days, because all you need is a quad bike with a fancy electronic gizmo on the back. The device is called a ground penetrating radar and, using one, scientists have mapped out the ruins of a Roman town in Italy, without ever having to pick up a trowel once! The town is Falerii Novi. It’s about 50 kilometres north of Rome and was founded over two thousand, two hundred years ago. All that’s on the site these days is the old town wall and a few sheep and it’s a protected monument, so you can’t dig there. Adam Murphy has been speaking to archaeologist Martin Millett, and he’s been explaining how the radar on the quad bike produced clear images of the entire town beneath the ground...

Martin - We've known about the site for a long time. It has had work done on it in the past; since the 18th Century, actually. We chose it for our work because of two things. One: it's greenfield site; in other words, the Roman town has gone and it's just fields now, so it's easy to access for experimental work. It's an historically important city anyway. The other point that makes it important from the point of view of this study is that, about 25 years ago, we did a magnetic survey of the whole site. By having that dataset, as well as the one that we collected now, we've got two geophysical datasets that could be directly compared. And that's very helpful for understanding how the techniques work and how they're complimentary.

Adam - Can you tell us how you went about looking for the things there that are under the ground?

Martin - Well it's relatively straightforward: we're using ground penetrating radar. So you have a radar antenna that sends a pulse of radio signal into the ground. And as the pulse of energy passes through changes in density in the ground - it hits a wall or something - you get an echo back; and the echo time is proportional to the depth. So at a basic level, you could just drag a single antenna across the site. What we're doing here, and what's new about this, is doing it over a very large area and at very high intensity. The solution for that is, rather than having a single radar antenna, you have a whole array of them; and our Belgian colleagues have put together 16 antennae that are dragged behind a quad bike. What that means is that you know the setup of the array, and with satellite surveying, you can tell exactly where each sensor is at any one time; and that means that you can collect a mass of radar data by gently dragging the array across the field.

Adam - How much data do you end up with at the other end of something like this?

Martin - Well, from this site it's somewhere around 28 billion data points.

Adam - And then how do you go about beginning to analyse 28 billion data points?

Martin - Well again, at the simple level, each single pulse gives you a series of reflections back. And it's a question of crunching the data to pull out the things that are coming back at the same nanosecond in time, so that you can differentiate what's going on at different depths beneath the surface. And then effectively you use an image processing software to turn that into a series of grayscale images that give you what's going on at individual depths; and there are various ways you can then play with that in three dimensional visualisation.

Adam - What kind of things have you found under the ground here?

Martin - We get the whole city plan. That's what we've been after. And that's of course made up of different buildings and different streets and structures. The beauty of this is that if you look at the images, they're very crisp, because we're collecting data at six and a quarter centimetre intervals; so you can see very small features, 20 to 30 centimetres across, under two metres of soil.

Adam - And you said you could see the individual buildings; what kind of buildings were they, there, that you've seen?

Martin - There... an array of big houses, and the more spectacular buildings. We've got very good evidence on the theatre, that was previously known, but we can see its structure very clearly now. We've got a completely hitherto unknown temple, and a big bath building, and a market building; and this very curious big monument up near the North gate, which is a large courtyard structure with colonnades on three sides, opened onto the street, but we don't know what it is! There's quite a lot of that sort of detective work to do on the images we've got at the moment.

Hands holding soil

Unearthing the soil's microbiome
Jacob Malone, John Innes Centre, Norwich

Before we start tunneling into the planet though, let’s first consider what’s directly beneath our feet. Across most of the Earth’s surface, that’s soil, which sustains the plants and trees we see growing everywhere. But invisible to the naked eye, and buried within the soil itself, is an enormous community of microbes, without which most things could not grow. Jake Malone is from the John Innes Centre in Norwich, and he spoke to Katie Haylor...

Jacob - I guess soil is a mix of two main things. One of these is minerals: things like clay or sand or stones. But the other very important part is decomposed organic matter. So it's mainly plant matter which has decayed. But what we're really interested in - or at least my lab is really interested in - within the soil are all of the different microorganisms that live there. As well as tiny insects and worms and things like that, you'll also find thousands of different species of fungus and bacteria. They're all interacting and fighting with one another and communicating with one another. And that's what gives soil its really special characteristics.

Katie - The soil has a microbiome, just like we have a microbiome then?

Jacob - Yeah, that's true. There are thousands of different species of fungi, and bacteria, and other microorganisms which live in the soil. And they're all competing with one another, and communicating, and interacting with other things in the soil like plants. So what you'll find is soil pathogens, for example, fungi and bacteria that will try and kill plants; but you'll also find bacteria that will fight them off, or fungi that will fight them off, and try and protect the plants in order to benefit from having it in the soil. And you find some interesting types of bugs, like symbiotic bacteria such as rhizobia or arbuscular mycorrhizal fungi, which are a type of fungus which provides nutrients to plants exchange for carbon, which comes from the plant roots.

Katie - So it's a trade-off, then? There's a relationship going on here.

Jacob - Absolutely, yeah. What you find is that most plants will secrete a large amount of their fixed carbon - so the carbon that they take from the air as carbon dioxide - they secrete that into the surrounding soil. And the reason they do this is to attract these microorganisms, which will benefit them. In the case of the symbiotic microorganisms like rhizobium or these mycorrhizal fungi, they will be providing something that the plants can't get easily, such as fixed nitrogen or fixed phosphorus, things like that. And in return they will be taking sugars directly from the plant roots. The plants, I should say, don't normally want to do this. They'll do it if it's advantageous to them. But if you have a farming system and you supply, for example, a lot of nitrogen or lots of phosphorus to the soil artificially, then the plants will actually suppress the relationship with the microorganism, and they'll say, "okay, I don't want to do this, it's costly for me, and I don't need to because I have this fertiliser". So the plants are quite clever in that respect.

Katie - In general, how are our soils doing? Do we have enough soil to grow the amount of food that we need?

Jacob - That's a good question. We don't have enough soil in Britain. About... I should say about 70% of the soil in Britain is currently used for agriculture.

Katie - Wow. 70% - that sounds like quite a lot.

Jacob - It is a lot, yeah. Apart from some areas of upland and some forests, and about 10% of built over land, most of England certainly is given over to farmland. That doesn't provide enough food to feed everyone; and I should also say that, certainly in recent years, the quality of soil has been degrading. The reason for this is intensification of agriculture. And this has led to things like the compaction of soil, from driving large tractors and other vehicles over it; soil erosion from removing hedgerows and other landscape features that prevent erosion; and also the contamination of soil, for example with microplastics or with chemicals. The addition of things like phosphorus and nitrogen to the soil has also degraded it to some extent.

Katie - What can be done about that? Because it doesn't really sound that sustainable. Is that a fair thing to say?

Jacob - I think it is a fair thing to say. Although there is a lot we can do. I think one of the absolute best things we could do is more sustainable land management going forward. So for example, if you plant trees around the sides of fields, it's been shown to reduce erosion and also to increase crop yields within those fields. And a lot of what we're doing - a lot of what we're trying to study at John Innes and elsewhere - is how we can intelligently manage and sustainably manage farming practices going forward. And I tend to be an optimist. I think we're learning a huge amount. And I think people are a lot more aware these days of the importance of soil, and the importance of keeping and managing soil sustainably.

Katie - So on that then... how do scientists like yourself study the microbes in the soil in order for us to make - if we're talking about, say, crops - as much food as sustainably as possible, where do the microbes come in?

Jacob - Well we're studying microbes in lots of different contexts. For example we might look at what microbes are present in a field, or we might look at what do those microbes do. We can look at what proportion of... for example, what proportion of a particular type of microbe is present; and what does it do, what molecules does it produce, is it good for a plant or bad for a plant. And we can also study - and we are studying - how different plants interact with these microbes. So what molecules are they putting out to attract to the bugs, which bugs do they attract, how important is this, and how does that interact with different soil types to lead to sustained plant growth.

Crack in earth

How do we know the crust and mantle exist?
Marijan Herak, University of Zagreb

Despite the fact that the Earth is 4500 million years old, and we’ve lived on it for thousands of years, it’s only in the last century or so that we’ve had any more than a vague notion of what actually lies inside our planet. We now know that there are several layers inside the planet, the outermost one being a relatively thin crust, and below that is the mantle; this is a rocky layer that extends 3000km down to the outer edge of the core. The reason we know this is thanks to earthquakes. These send waves of vibrations through the planet. Some of these waves shake from side to side, while others compress the thing in front of them. And, critically, these waves change their paths in predictable ways when they meet the boundaries between different layers. The first person to realise that this could reveal what lies inside the Earth was Croatian mathematician Andrija Mohorovičić. At University of Zagreb, where he worked, they have many of his original notebooks and records in which he proved where the crust must stop and the mantle begins. Geologist Marijan Herak showed Chris Smith around...

Marijan - We are in the Mohorovičić Memorial Rooms in the building of the Department of Geophysics faculty of science in Zagreb. In this room, we tried to collect everything we have from Professor Mohorovičić, who is one of the founding fathers of seismology. Universally Mohorovičić is famous for his discovery of the crust-mantle boundary, which is today known as the Mohorovičić discontinuity. When Mohorovičić published the discovery in a paper in 1910, this proved that the earth is best described as being made of shells. So the uppermost of those shells is today known as the crust. And he conclusively proved that the crust exists, that it has the depth of about 50 kilometres, the average is about 33 kilometres, and that the waves and the properties of the earth abruptly change at such discontinuities.

Chris - Why was that so groundbreaking at the time?

Marijan - Because by proving that the properties of the earth do not change continuously, he added an important piece of information into the common knowledge. He established ways to see into the depths of the earth, by observations at the surface of the earth. And this was one of the goals of seismologists, he postulated it to continue where the geologists stops. And he realised that by modern instruments, a scientist is given a kind of binoculars with which he can look into the greatest depths of the earth.

Chris - How did he do it?

Marijan - He was a bit lucky that soon after he installed the modern seismographs in Zagreb, an earthquake occurred not far from here in the Valley of the Kupa river. He recorded it beautifully and was able to collect the seismograms from all over Europe.

Chris - Other people were also doing similar measurements in different parts of Europe, and so he was able to bring the same records of the same event together.

Marijan - Yes, seismology was in its infancy. The seismographs had just gotten good enough to record faithfully the movements of the earth during earthquakes. Of course, this was more complicated than today. There were no emails or scanners or anything like that. He had to obtain copies, or in some instances, the originals were sent to him by post.

Chris - Because you've got an example of one on the table here in front of us. And these are long pieces of paper or card, which have a scratched sort of signature, which the stylus of the seismograph has scratched into the surface as it's moved in response to the vibration. So he would have been receiving things like that from across Europe.

Marijan - Yes. Things like that or photographic copies. So he analysed those records and realised that some things were unexpected. There were four waves observed instead of two.

Chris - Just explain these different waves for a second. So when you have an earthquake, what are the two sorts of waves that should be coming out of that earthquake then?

Marijan - So when the earthquake occurs, two waves radiate from the source. One of those are the longitudinal waves and those waves, which are like sound waves, the particles oscillate in the direction of the wave spreading. The other type is the transversal wave which has particles oscillating perpendicularly to the direction of the spreading of the wave.

Chris - So if I had a slinky spring and I pulled some of the coils towards me and let them go, and they would ping away, the wave would travel along that spring in the direction the spring was stretched out. Whereas if I sort of wiggled the spring from side to side, that would be like a transverse wave. Those would be the two sorts of waves that you should get from an earthquake. And you're saying he actually saw not just those two, but when he made his recordings, it was clear there were four waves.

Marijan - In some stations, yes. This was funny in some stations, you'll see what you expect, two waves, then you'll see four. And then again, you see two.

Chris - Were they the same waves except they were delayed or something, were they arriving at different times?

Marijan - He knew that only two waves can exist. So he had to come to the conclusion that this interval of distances, and those waves were not four different ways, but two pairs of waves of which one came directly from the source, and the other pair were initially going down and then they were refracted to unknown depths. And then refracted back to the earth, after some distance traveled.

Chris - So one of the waves goes straight through the earth, the other goes deep into the earth and hits this boundary that we now know exists between the upper part of the earth, the crust and the mantle, which is deeper. And that bends the waves back up towards the surface, and that was his breakthrough.

Marijan - Yes, in a nutshell.

Chris - That must have been an incredible amount of maths for him to work that out.

Marijan - Yes. It was really difficult for him to understand. But once he understood it, then all he had to do was find the model to invert the observations on the surface for the properties of the earth. The only thing to do is to find the model of the velocities and the depths of the discontinuity and to match the observations. And he did it and we called such problems inverse problems. So like your doctor when you go to have a CT or X-rays, by observing things on the surface of the body, you conclude about the composition of the interior of the body. The math, of course, without computers, without anything, and I must say, without any help, he did it alone. And we have stacks of papers in his handwriting. He worked with logarithms to seven decimals and he made it so difficult for himself because he wanted to have it very realistic. So some of the assumptions we even do not use today, he used them. This was a breakthrough.

Globe

39:26 - Musing over the mantle

How do we study the layers of the earth?

Musing over the mantle
Huw Davies, Cardiff University

Since the time of Mohorovičić we’ve learned a lot more about the Earth’s interior. Geologist Huw Davies from Cardiff University is setting up a project to learn a lot more about the mantle, and he spoke to Katie Haylor, explaining firstly what the crust is.

Huw - Well, the crust is a silicate rock, basically rock. And yeah, primarily, has a slightly higher level of silica than maybe some other rocks.

Katie - And if it were possible, I don't think it is at the moment, to drill down into the mantle from the crust. What would that look like? What's the environment like?

Huw - So the mineralogy would change. So the composition would change, simply. So basically the crust really results from melts produced from the mantle. So elements that like to be in melts have ended up being in our crust. So one thing we would see is clearly slightly different minerals as we got into the mantle, and that's what leads to this different seismic velocity we just heard about, but also we will be getting hotter. And that's part of the reason why it's actually very difficult to drill into the mantle because it becomes a very hot environment. And basically our drills can't sustain that very well.

Katie - How far down has anybody got?

Huw - So there's two aspects of that. So the deepest hole is in the Kola Peninsula in Russia, and they've drilled 12 kilometres, so that's in continental crust. But there the reason they've been able to drill so deep is because it's a very, very cold part of the earth. And actually that's in the continental crust and actually to get to the mantle they probably have to drill another 30 odd kilometres. So where we've drilled closer to the mantle is in the oceanic crust, which is a lot thinner, but there we've only managed to drill about a kilometre or two, and we would need to drill another four or five kilometres to reach the mantle. So the only places where we've seen the mantle is where tectonic processes have brought it up to the surface, or we've had little fragments brought up by volcanic eruptions.

Katie - I'm glad you mentioned that because where do tectonic plates come into this? How do we end up with these really dramatic geological events like earthquakes and volcanoes?

Huw - Well it's kind of strange in a sense. The surface of the earth as we discovered sort of, well, we sort of got to the argument sort of in the fifties, but really was won in the sixties, of the surface of the earth is broken up into these tectonic plates. So basically large regions don't suffer much deformation. And then the movement is all occurring at the boundary of these plates and that can occur in three different ways. They can be moving side by side, like in the San Andres Fault. They can be approaching each other, converging, like in Japan where one plate goes under another. Or they can be diverging separately in like in the mid-ocean ridges deep in the ocean where we get magmas.

Katie - Can you break that down a little bit in terms of what leads to quakes and what leads to volcanoes?

Huw - So when the plates rub against each other, in the case of side by side, so the plates are basically rubbing against each other. And the earthquake is because you get a stick-slip mechanism. So the plates kind of stick, but the forces of the mantle are moving the plates, and then at some point, the fault can't sustain the force any longer and then it jumps and that would be the earthquake event. And basically the movement will have caught up with the movement in the mantle, and then it repeats itself. But the biggest earthquakes of all are in the convergence zone. So these are where the plate goes down beneath each other, for example, in Japan, in Indonesia and beneath South America, for example, and it's the same idea, basically, again, the plate sticks and then it moves suddenly to catch up with the forces that was always wanting to pull it down.

Katie - So in a volcano, we get an eruption and magma. What's going on there compared to what we were just talking about?

Huw - Most magmatism occurs beneath your ocean floor, where the plates are moving apart. So we get hot rock coming close to the surface. And interestingly, as it gets closer and closer to the surface, the pressure on it gets less. And interestingly, the melting point of rocks gets less as the pressure decreases. So at some point the rocks are hot enough, but not under enough pressure. So they melt and they produce in fact, the oceanic crust, but those typically are beneath the ocean floor so we don't see them very much. But the case where the plates are colliding against each other, we also get volcanoes. And these are the dramatic volcanoes, like in Japan, in the Andes, in Indonesia. And in this case, what we've got is the ocean floor, which is as a converging plate, is jumping stick-slip, as earthquakes, gets down into the interior, and the water of the ocean that has penetrated into the crust - in fact, when it formed the lavas, when it was spreading way back at the ridge - so all this gets carried down and the water gets carried down inside the earth, and then it gets released, and water also reduces the melting point of the mantle, and that's how we get melts in that part of the world. And that's why actually those magmas tend to be the most explosive because that's actually that water forming bubbles as the pressure's released exactly the same idea as we get with a Coke bottle or something, when we open the top and loosen the pressure. And then the third scenario is where we get just purely hot rock coming up towards the surface, just the heat sort of melts the rock. And that's a place like Hawaii is an example of that region.

Katie - I'm pretty sure you said magmatism, and magmatism-magnetism, I guess you gotta be a bit careful between those two!

Huw - Yes. Magmatism coming from magma, magnetism of course is to do with magnets yes.

Katie - How much do we understand about the mantle then? Where does the knowledge gap exist? Because you're about to start a pretty big project on the mantle, is that right?

Huw - Yes. So we understand a fair bit given that it actually isn't something we can really get our hands on very easily. And a lot of it comes from seismology, and we just heard earlier about how seismology helped to tell us the difference between the crust and the mantle. Well today, we kind of know the onion-peel structure of the earth very well, all the way down to the core-mantle boundary, but we can take that further and we can now do CAT scanning type ideas using the seismic waves. So we have some sense of the 3D structure. Resolution is relatively poor. We have some sense of the down-going movement, where the plates go down, but the bit that's less well understood is, clearly if a thing goes down something has to come back up and it's the up-flow that we don't understand as well. And that's going to be the primary focus of our project.

Katie - So how do you go about trying to look at that then?

Huw - The centre of the project we can say will be a model of the dynamics. So the inside of the mantle creeps sort of at the same rate as our fingernails grow. So we don't see them when we look at them, but we know they've grown after a few months. So that's the same thing for the mantle. We can model it on a geological timescale like a flowing liquid. So we'll have this model and we'll apply the plate pushing histories that we know to the surface. And then it will make predictions for example of how the structure would be at present day, which we can compare with the seismology. And we'll also have looking at chemistry of the magmas that come out and they'll give us different constraints. And we will also look at magnetic signatures of rocks at the earth surface, which will tell us something about the old magnetic field, which tells us about the core, which the mantle sort of puts a constraint on. So we'll bring this model of how the earth's moved, and then we'll be constraining it with all these other different techniques and hopefully understanding how the Earth's flown.

North east south west

What's going on in Earth's core?
Claire Nichols, University of Cambridge

We’ve cracked open the crust and mused over the mantle, but now let’s get to the core of the matter. What’s actually at the centre of the earth? Claire Nichols is a paleomagnetist formerly of Cambridge University and now working at MIT, and she spoke to Chris Smith...

Claire - We only indirectly know what's in the core, but there are lots of lines of evidence that tell us that it's predominantly made of the element iron.

Chris - How do you know that?

Claire - Two main ways. One, we know from the mass of rocks that we see at the surface of the Earth, that what's in the interior must be a lot denser. And the other reason we know is because we know it must be conductive because it's generating Earth's magnetic field.

Chris - When you say the density gives this away, is that because we have some inkling as to how much the Earth "weighs", in inverted commas. And so given that if you know what proportion of it is the lighter rocks on the outside, the heavier stuff sinks, we're inferring, there must be really heavy stuff in the middle?

Claire - Right. Exactly.

Chris - Now, in terms of the heat that's down there, I mentioned at the top of the program that it's about 6,000 odd degrees, isn't it, but where's all that heat coming from. Because the Earth's four and a half thousand million years old, it's been around a long time. You'd have thought it would have cooled down by now. So why is the Earth still so hot? Why is the core still so hot?

Claire - Yeah. So all of that heat, as you say, it does come actually from the very, very beginning of the planet's formation, billions of years ago. And the reason it's still so hot in the middle is because there's just so much crust and mantle that all that heat has to be extracted through. So it's like the core is wearing the thickest winter coat of all time. So it is cooling down, but slowly.

Chris - And as it cools down, does it harden because right at the very centre, it is solid, isn't it? And it's the outer part of the core that's the liquid bit.

Claire - Yeah. Yeah. So we think that it started to solidify right in the middle and the inner solid core is now growing through time.

Chris - And do we have any insights into how that is spawning the magnetic field that we have?

Claire - Yeah. So what we think is happening today, is that the solid inner core is ejecting other elements. So light elements, like things like oxygen and sulfur into that liquid part of the core, and that's driving really, really vigorous convection.

Chris - I'm just picturing this. Therefore, I've got right in the centre of the Earth, I've got the inner core. And that's the bit you're saying has gone hard, that solid. There's stuff coming out of that into the liquid bit that surrounds it, and that's spinning, and that spinning is in some way creating this magnetic effect.

Claire - Yeah, exactly. So you can kind of think of this liquid part as like a lava lamp. So things are mixing around and because it's made of a conductive material, so something that electricity can travel through, by moving that charge around, that's actually generating the magnetic field.

Chris - Obviously you're trying to work out how something that is 6,000 kilometres below our feet is working. How do you do that?

Claire - So what we look at is rocks that form on the surface of Earth, very conveniently when they cool. So like lava flows coming out of volcanoes. They trap a record of the magnetic field at the surface. So we can look at what the magnetic field is doing today, and what it's doing back in time. And that tells us about what the core is doing.

Chris - Oh, right. So when the stuff spawns as liquid magma from within the Earth, before it goes hard, it can move in any direction. But because it's got stuff in it that is susceptible to the Earth's magnetic field, it will sort of line up with whatever the field is doing at that time.

Claire - Yeah. So there's just those little magnetic blobs in those lava flows. And as those little blobs cool down, they will align with the direction of the magnetic field.

Chris - How do you know what way they're pointing? So if I hand you a pebble, how do you know that the pebble was orientated in a certain direction relative to the Earth's magnetic field then?

Claire - So well, a pebble is tricky because we don't know how it was oriented on the surface, but if we go to, let's say a cliff face or something that we know it's original orientation, then we can take oriented samples of that to a laboratory. And then we can measure the direction of the magnetic field in that sample very, very accurately. And then we can use that to tell us about the ancient magnetic field.

Chris - And because you can date the rocks, you know how old it was, and therefore what the magnetism was doing in rocks that age. So you can wind the clock back.

Claire - Yeah exactly.

Chris - And if you do that then, what do we learn about the Earth's magnetic field through time?

Claire - So we have learned that we've had a magnetic field for billions of years, and also that the magnetic field wobbles around. So it's not perfectly North and South all of the time. And sometimes it even flips. So it's actually very dynamic.

Chris - When did it last flip round?

Claire - So it last flipped hundreds of thousands of years ago. So it doesn't happen very often.

Chris - Do we have any insights into the consequences of that? Because obviously hundreds of thousands of years, there were our human ancestors walking around on the Earth at that time. So presumably when this happens, it's not terribly catastrophic for life as we know it.

Claire - No. So we don't think it is. So actually there's no evidence going back in time of humans or fossils being made extinct by a reversal, but one thing it will affect is technology. So things like your mobile phone will not work very well during a reversal.

Chris - Why?

Claire - It's because our magnetic field is shielding our planet from cosmic radiation. So basically radiation coming in from the Sun. And if our magnetic field is flipping, it's much weaker and that means we get a lot more radiation and that will interfere with satellites and all sorts of things for technology.

Chris - And do you know how quickly these flips happen? Can we see evidence of the field collapsing and then reestablishing in the new direction? And does that happen really quickly, or does it happen geologically really quickly? Meaning over thousands of years.

Claire - Yeah, exactly. So it happens geologically quickly. So a reversal would take well beyond our lifetimes. But in the rocks it looks instantaneous.

Chris - And in terms of actually what's causing this flip, can we work this one out or do we just have to say, well, it's something to do with some convulsion in the core at some point, with the movement of things, and it causes this to happen. Do we have an idea as to why this does what it does?

Claire - So it's still a bit of a mystery. We know it happens on a fairly regular timescale, but we're not entirely sure what's driving it. But it's basically, it's indicating to us that the flows within the core are quite complicated and something must trigger a change in their behavior.

Chris - And as more of the core hardens, which is happening with time, does that mean that the frequency of this happening may change too?

Claire - It might do. Yeah. That's something that we'll have to look for evidence of, as and when it happens.

Chris - Let's hope it's not too soon. I like my mobile. It's bad enough, the signal where I live already. And one last question, Claire, because obviously Mars is quite a similar size to the Earth, but Mars doesn't seem to have a magnetic field anymore. Many people blame the absence of a magnetic field for the fact that it is now a prune of a planet with almost no water left, where previously it was a Waterworld. So why has Mars lost its magnetic field, and we haven't?

Claire - That's a good question. Partly it's probably because Mars is much smaller than the Earth. So it cools down much, much quicker. But we also think that something a bit weird happened that made the Martian magnetic field switch off so early. So that's something else that scientists are actively looking at today.

A golf ball on a green.

QotW: Why don't we have dimpled cars?

Adam Murphy has been finding out...

Darren - Golf balls are dimpled to disrupt the air around the ball. As far as I can gather, this reduces their drag and allows them to fly further than they would if they were perfectly round. Why do we not see dimpled cars, aircraft, and trains? If this effect is so effective for golf balls, why not use it on Formula 1 cars, for instance?

Adam - And why not, you can drive a golf ball and a car after all. So, what makes one good for dimpling and the other not. Sam Grimshaw, from the Whittle Jet Engine lab at the University of Cambridge is here to take a swing at the answer

Sam G - To answer this question I need you to picture a stream of air flowing past an object. We call the messy flow behind the object a wake, just like you see behind a boat. The drag on the object is related to the size and shape of this wake.

Now if we zoom in close to the object’s surface, we see that friction slows the air; this region of slower flow is called a Boundary Layer.

If the overall flow is relatively slow and the object small, then the viscous nature of the air makes the boundary layer flow smooth. This flow struggles to follow a curved surface, so for a sphere, the air leaves the surface about half way around, producing a large wake and lots of drag.

Adam - Imagine dragging your hand through some water. If you go with your palm first, you’re going to make a big wake, and you’re going to feel a lot of push back. But if you use the thinner edge of your hand you’re going to chop through the water a lot easier, and make a much smaller wake. So because a ball isn’t a sleek, streamlined shape, the air can’t follow the ball all the way around and it makes a big giant wake behind it, pulling it back...if only there was some way to mess that big wake up...

Sam G - For fast flow past a large object, the boundary layer becomes churned up, or turbulent. This type of flow follows the surface of the sphere further around, giving a smaller wake and reduced drag. 

A golf ball, which is small but fast, is delicately balanced between these two behaviours. A smooth golf ball tends to have that smooth kind of boundary layer which gives high drag. However, dimples disturb the flow enough to make the boundary layer turbulent, reducing the drag and allowing you to hit the ball further. For a car or train, which are quite fast and very large compared to a golf ball, the boundary layer is turbulent anyway so dimples have no effect.

Adam - Shame, I’d like to see a dimpled F1 car, personally.

Thanks to Sam for that answer, and also to evan_au, and Janus on the forum, who came to similar conclusions. Join us next time, when we answer this question, from a different Sam...

Sam - If identical twin brothers marry identical twin sisters, and each of those couples has a child, will those two children be like twins?

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