Daniel and Jorge Explain the UniverseDaniel talks to Prof. Sabine Stanley about what's happening inside planets, and how that helps us understand what's out there.
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When we set out to understand the universe, we usually start by looking up. After all, that's where the best views are of the glittery cosmos stretched across billions of miles. We wonder, are we alone? Is there anyone up there looking back at us? But what if the best way to find answers to questions about what's up there? Is actually to look down under our feet. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I desperately want to know who's out there in the universe and if they are wondering the same things we are. And Welcome to the podcast Daniel and Jorge Explain the Universe, in which we do just that, wonder about the nature of the universe and try to explain all of it to you. Regular listeners the podcast know that I am desperate to understand the nature of the universe, how it all works, and to explain all of that knowledge and all of that confusion to you. One of the deepest questions we wrestle with on the pod is not just about the universe, but kind of about ourselves. How weird? Are we? Are there more like us out there in the universe? Or are we alone? How rare and special is the Earth anyway? Are we one of a kind out of a trillion planets? Or are we one of many rocky balls covered in Curious Life. We're frustratingly limited by what we can learn about distant planets, though we're doing our best. But something we can do right now is drill deeper into our own planet, understand the forces that shaped it, and whether those are finely balanced in a rare way or naturally in harmony, in a way we'll find everywhere in the universe. So today on the podcast, we'll be answering the question what's hidden inside planets? And to help me explore this fascinating topic, I'm pleasing to be speaking to Professor Sabina Stanley, author of a very recent book of that same title. All right, well, then it's my great pleasure to introduce the podcast Professor Sabina Stanley. She's the Bloomberg Distinguished Professor of Planetary Physics at Johns Hopkins University, where she focuses on magnetic fields and other geophysical elements as a means of studying the interiors of planets, moons, and asteroids. She's an Alfred Peaceloan Research Fellow and has also received the William Gilbert Award of the American Geophysical Union. Sabina, Welcome to the podcast, and thank you for coming to talk to us.
Thanks so much for having me.
So. One thing we always wonder about as we look out into the night sky is all the other planets that are out there. Of course, we can't study many of them up close, and so often on this podcast we've tried to dig into what's under our feet, the mysteries that are right here in our Earth. And so I really enjoyed your recent book, What's Hidden Inside Planets, and I'd love to talk to you about what's in our planet. Could you start us off by taking us sort of on a brief tour of like what is under our feet, layer by layer, all the way down to the core.
Yeah, absolutely, great question.
So I think it's interesting to note that when you start on the surface, as you go deeper and deeper, stuff gets kind of weirder and weirder and much more different than what we're.
Used to on the surface. So we start on the crust.
This is where we live, This is where all the stuff happens that we're used to. Crust can vary in thickness five about you know, five kilometers depth to almost one hundred klmeters depth. But under that you get to the mantle that's also still mostly rocky, the type of rocks that are rich in magnesium and silicates, but still what we would recognize as rocks. So about half the radius of the earth are those rocks. It goes down about two thousand miles deep.
So what distinguishes then between the crust and the mantle. Is it like how squeezed they are and how much they flow? Is it a different kind.
Of rock, great question.
Yeah, it's a little bit different kind of rock. So essentially the crust layer of the earth. I sometimes referred to it as like the scum of the earth. So it's kind of like, you know, like when you're making a soup and you're boiling your broth and you've got all that light, floaty stuff that comes to the top. So the stuff that's the most buoyant when you have certain heat, thermal reactions and chemical reactions happening with rocks near the surface, all of that percolates up to the top and that ends up becoming the crust, and then sort of the stuff underneath might be less scummy, less you know, it's been less reworked, and it's sort of more kind of pristine rock.
I see, we're going to get started very quickly with the food analogies.
Yeah, I'm sorry, it's just going to be how it goes.
It's going to be food involved in almost every analogy I make here.
Are you a big fan of soup or you a cook at home?
So I'm a terrible cook, but I grew up in a restaurant family, so I've been around sort of good food my whole life.
All right, Well, then let's do our best to at least use tasty food analogies. I don't want anyone to think that the earth is like a disgusting bowl of soup. Maybe it's like, you know, bubbling hot cocoa, and this is that delicious film that forms on top.
I love that so much you don't even know.
So that's amazing, all right. So the crust is the sort of coolest part that floats to the top, and underneath that it's still rock, but it's able to flow. How do we visualize that? I mean, it's not like liquid lava that's flowing on the surface. This is still like solid rock, but it's flowing. How does solid rock flow? Is something I've always tried to visualize and failed.
Yeah, so the answer to that question is very slowly.
Right.
So, yes, it's solid, but it's still deformable, right, And I think we have experience with different types of solids in our everyday life, and that some are more deformable than other.
Right, Like you might have clay.
Clay is solid, but you can still deform it, whereas a metal also is kind of deformable. But then you have some rocks that are really like a diamond, really hard to deform.
But the rocks in the.
Mantle, they are solid, but they can be deformed. And if they can be deformed, then they start being influenced by the forces like gravity such that you can get them to flow.
I see, all right, So we have the crust and we have the mantle, both of which are still really rock. Take us down below.
That, right, So then you get down about halfway through the Earth, and you only hit a very big boundary, complete change of environment. Now you're at the iron core. So the inner half of the planet about it's mostly made of iron. There's a little bit of nickel mixed in there, and about ten percent of some sort of lighter elements that we have a whole sort of platter of possibilities for but we don't actually know what they are. And that makes up the core. The core has two parts to it. The outer port is liquid. It can flow very easily, much faster timescales than the mantle, and it's really important for us because that's where our magnetic field is generated in that liquid iron core. Then below that, the innermost thirteen hundred kilometers of our planet is a solid iron core.
And so what distinguishes then the mantle, which can flow but is a solid not a liquid, from the outer core, which can flow but is a liquid and not a solid, Like, is there really a distinction here? Are we just putting labels on things?
When we study fluid dynamics, we talk a lot about there being a spectrum of fluids. Right, nothing's ever purely a solid that are purely a fluid. It's all about the time scales. So the mantle, for example, if you want to talk about how materials flow in the mantle, a parcel at the bottom of the mantle could take hundreds of millions of years to make it to the top of the mantle, whereas a parcel at the bottom of the core could take a couple of years to get the top of the core. So it's a very different timescale of the flow. You could actually see changes in material in the core flowing, but.
There's also like a boundary. It's not like there's a smooth, very gradual transition. There's like a line. You can say, this is the core and this is the mantle.
Yeah, and that happens because mantle, rocks, and iron in the core have very different densities, and at one time in the past in our planet, it was mostly molten and so the heaviest stuff, when you have a bunch of stuff mixed together, the heaviest stuff's going to sink to the bottom. And so that's what happened in Earth. All the iron, most of the iron sunk to the center of the Earth and made up.
The core like the big chunks in a stew or something exactly.
Yes, I like it.
So the reason that there's a boundary there and like a transition, and rather than just like a smooth gradation from more liquid to less liquid, that reflects like the phase transitions and materials. Is that right, the way that like ice turns solid at some moment and doesn't just like gradually become more and more solid.
I would say that's more representative of what kind of happens at the inner core outer core boundary, so where the iron becomes solid, but above that it's more kind of like a maybe you go with an oil and water type thing. You've got two materials with very different density and very different properties, so it's really hard to mix them.
Wonderful and tell us about how we know about this. I was reading in your book this really exciting description of the mantle race, basically like a parallel to the space race, but into the Earth. Tell us about our humanity's efforts to like literally tunnel to the center of the Earth.
Yeah, So if you imagine you want to figure out what's inside the Earth, right, your first instinct might be, hey, why don't we dig down as far as we can and actually sample it?
Right?
And it's a great instinct. Unfortunately, it's incredibly challenging to do. And that's because pressure increases so fast as you go deeper inside the planet, and so do temperatures.
So as you can imagine, humans.
Don't like really high pressures and temperatures, neither does equipment. And the farthest we've been able to dig with sort of a really concerted effort to do so, right, Like this was something on the scale of moonshot to the Moon in the late sixties. This is something very similar to that, and you could get only down to about eight miles in depth and the radius of the Earth you're talking about four thousand miles, so tiny, tiny scrape of the surface by going down that deep. Equipment does not like high pressures and temperatures.
But how do you even get eight miles deep? I mean, I remember digging in my backyard with a shovel, wondering how far I could get, and it's not very far. How do you get eight miles down?
This is like high tech technology kind of stuff. It's at the limits of what we can do for drilling that we do now to drill for resources, etc. So it's a lot of fancy equipment and challenges that are over met that way.
So we can't dig and we can't drill.
But that's okay because there are other ways we can figure out what's going on deeper inside the earth.
Yeah, so tell us about some of those ways. You were talking in the book about diamonds, how we can use diamonds to give us little snapshots of what's inside the planet.
Yeah.
So, you know, it would be great if we could dig down, but when it'd also be great if the stuff down there came to us. And that's really what happens with diamonds. Diamonds are produced deeper inside the Earth and then they come up to the surface, usually in volcanic vent things known as kimber like pipes and those diamonds. You know, Jewelers love diamonds when they're as pure as possible. Geologists love diamonds when they're as impure as possible. So sometimes diamonds, when they form, they can enclose a little capsule of some of the material where they formed inside them, right, So you might get a little bit of garnet in the diamond, or a little bit of something that was created deeper in the earth, and when it brings it up, because diamonds so strong, it actually keeps the material in its like pristine form. So you really have this like sample from the interior of the Earth come to the surface for us to investigate. So that's a great way, and we've used that, for example, to figure out that there is actually water deeper inside the Earth because we've found water inside diamond inclusions.
It's fascinating to me though, that this thing that you make in a higher pressure environment, when you bring it up to low pressure, it doesn't explode. Is that just because of the incredible structure of diamond.
Yeah, when they say diamonds are forever, that's technically not true, right, They just have a really really long lifetime before they revert back to their carbon phase. So yeah, it's just a great property of diamond.
So is it's sort of like you know, you put a pan of brownies in the oven and it changes into something else, and when you take it out, cool it down, it doesn't revert back into batter.
That's an excellent way of thinking about it.
Yeah, okay, and so then what have we learned from these diamond samples, Like what's inside these diamonds that we didn't realize other than water?
Yeah, I think water is the big thing.
Sometimes it's a lot about sort of the smaller amounts of elements that we don't know about, right, how much of a particular kind of silicon down there is sulfur down there, these kinds of questions, and those all just help us understand and what the building blocks of Earth were they when Earth formed, and what the geochemistry the kind of chemical reactions that can occur as material descends into the Earth. That's really where we get that information. But even with diamonds, right, we're talking about the outermost layers of the mantle, right, diamonds. We don't get diamonds, say from the core mantle boundary or anywhere deeper than that. So we can't use the diamonds to learn about the deeper parts.
Is that because diamonds aren't made deeper or because diamonds from that far down just don't make it up to the surface.
Mostly the latter.
I think also, like if you get carbon down there, yeah, it doesn't necessarily join into making diamond at that depth.
All right, So there aren't like huge diamonds buried deep in the Earth that we are.
Not on Earth. Not on Earth.
Well, that was my whole motivation for digging so deeply when I was a kid, fantasizing about revealing some you know, boulder sized diamond. All right. So diamonds give us one sample of what else can we do? What about gravity? What about just studying like the variation in Earth's gravity as we you know, orbit to play or look around it. What does that tell us about what's inside the Earth?
The way I like to think about it is, you know, if you want to figure out what's going on inside the Earth, try and make an analogy to a human body. Right, if you have an ache and you go to your doctor and you're like, this hurts, Hopefully they're not. Their first kind of instinct is not to drill a hole in you to figure that out, Right, There are ways that they can use different fields and different scans to figure out what's wrong with your insides.
And we can do the same thing for the inside of the Earth.
So we can scan gravity, as you mentioned, that's one, magnetic fields is another one, and we can also determine properties of waves that travel through the Earth from earthquakes through seismology, So we can use all these scanning techniques to figure out what's going on deeper inside the Earth.
So what do you mean by using gravity? Is it just like measuring the variations of gravity so that we understand how the Earth is not a perfect sphere or how the Earth is not homogeneous in density? What is it we're learning?
Yeah, great question. So, yeah, it really is the fact that both isn't a perfect sphere and has some inhomogeneous material below it. Right, So if you were walking around with a grivi emitter that could measure gravity and it was really really good, and you walked around, you would get slightly different values everywhere you walk, and that would be determined by the mass directly under your feet, and so we can use that information. We have spacecraft that are but the Earth that measure Earth's gravity feel to really high precision, and we can use that to figure out what is the distribution of density inside the Earth. And that kind of allows us to kind of image what's going on. Where's the denser stuff in the Earth, where's the lighter stuff, And we can actually see things like convection cells in the mantle and plumes of magma coming up for volcanoes, things like this.
But gravity is such a weak force. How do you identify these variations in density with such an incredibly weak force. They must be pretty big effects.
They aren't.
They are very very tiny effect. We're just really good at measuring them.
So a gravi emitter, you mentioned this might seem like a weird object or listeners, but I guess like my bathroom scale is a gravi emitter. If I walked around the Earth with my bathroom scale, I would measure different weights before and after lunch, of course, but also if I didn't need anything, or are you use their reference mass, then I guess that would measure different accelerations due to gravity.
Yeah.
Absolutely, And if you were someone on the surface taking gravity measurements, that's exactly the kind of instrument you would use. Interestingly, once we get into orbiting around a planet like Earth to take measurements, we use a completely different technique. We basically use the fact that if we have a spacecraft in orbit around the Earth, we know it's in orbit around the Earth, and its orbital speed and altitude is completely determined by the mass of the planet.
So we can use things.
Like two spacecraft just slightly at different locations from each other, kind of moving around, and we can use the distance between the two spacecraft as like a proxy for how much g is right where they are, how much the gravity is right where they are. So that's actually how it's done in practice with spacecraft.
Wow, that's incredible. And how sensitive are they? I mean like one part in one thousand, one part in a million, Yeah.
One part in a million. That's where we're getting to.
Wow, So they can really tell if I've eaten lunch. Some spacecraft up there can tell that the mass of the Earth has changed.
One thing they're actually used for. So the great satellite which orbited Earth for about ten years. One of their main applications was to follow water flow on the surface, so you could see, for example, when water was filling reservoirs underground reservoirs in certain parts of the country or different countries, if you wanted to see are we going to have a drought, are we in a rainstorm season, or what's the water situation going on here?
So we can even use gravity to tract climate change.
Wow, that sounds like modern day divining rods. But you're actually using science to find the water underground. That's incredible, all right, So gravity's one way to do it. You also mentioned seismic probes. These are like waves inside the earth. How do we use that to see what's going on?
So every time there's an earthquake, it's like sort of something kind of punched the inside of the Earth at some point and it causes the Earth to ring. It causes waves to travel through the interior of the Earth and on the surface of the Earth. If we put out a bunch of instruments that can kind of measure the shaking, so seismographs, then we can figure out a few things about the earthquake waves we can figure out when they arrive at different locations around the planet, and how big the waves are.
The amplitude of the waves and the speed.
The timing of when the waves arrive is completely directly related to the material properties that the waves traveled through. So for example, we can figure out the density of material that a wave traveled safe from. Let's say an earthquake happens in California and the wave travels up to Seattle in Washington. You can use that to figure out kind of what's the material just under the surface there, Whereas if you try to go across the globe to another part on the other side, the waves might travel through the entire planet, and we could actually sample the material in the core, for example. So you can use all those different measurements. The more locations you have on the Earth for these seismic measurements to be made, the more you can kind of discern what is the lateral structure of the interior of the Earth.
And we're really talking about sound waves, right, These are pressure waves in the rock, and so we can think about how denser materials have sound travel faster, and less dense materials sound travels lower. So you're measuring the density of the material by measure in the speed of sound. But again, these are rocks that are like pushing on each other, right, Like sound waves through rock is a very weird thing to think about.
Yeah, absolutely, So there are the sound waves that The other type of wave that goes through are these sheer waves. So those are kind of more like waves you'd experience in a fluid, let's say, or not in fluid, sorry, waves that you would experience if you try to kind of bend peanut butter or something like that. Right, So there's multiple kinds of waves, and some of them are very diagnostic of what's going on in certain types of materials.
Well, I never thought we'd be talking about peanut butter waves, but here we are. So when did we get this picture? Like, what is the first technique that really gave us a view of the inside of the Earth. Was it the seismographs or is it something else?
That's a good question.
It's not like there was a moment where suddenly we had this picture of the Earth. I think we developed our understanding to higher and higher precision as time went on, Right, I think early studies of gravity, going back to Newton, let's say, was able to tell us this is the mass of the Earth, and then you could take, for example, samples of crustal rocks and figure out what they're density was and infer hey, there must be a lot more mass deeper in the center. So that was kind of first order information you might get so through both seismology. So early nineteen hundreds was when we were doing some really great seismology figuring out things like, oh, look we have a core. Right, that was where the core was first discovered. The inner core was discovered in the early nineteen hundreds. The first sort of real profile of density through the Earth happened, I think it was in the seventies with what was called the Preliminary Reference Earth Model, which really used a whole bunch of seismic data to really kind of do an inverse problem and figure out, here's what the seismic wave speed and the density has to be at every depth in the in sort of like a onon d Earth.
So that was a big step forward there too.
But at the same time gravity was being used and so we were getting pictures from different types of information.
But it's really only a few hundred years that we've had any sort of reasonable idea of what's under our feet. And it sounds like only the last few decades, maybe fifty years, that we've had any sort of detailed picture of what's actually inside our own planet. It's incredible how long we can remain ignorant about really basic science about our own lives.
Yeah, when I talk to people, I tell them geophysics is really modern physics because all of the stuff we're doing now is all stuff that's happened sort of in the last sixty seventy years.
So I like to think of it as a modern physics.
Approach, right, And now we've extended this frontier too other planets. We've talked in the podcast before about the Insight mission, and I think you worked on that measuring Mars quakes to see what's inside Mars. Did the same principles apply there?
Yes, absolutely, So.
The amazing thing with the Insite mission is brought a seismometer and that seismometer had to be placed onto the surface of Mars so that it could measure the ground shaking essentially, and it worked, like it was just amazing that it worked. But it was a very interesting experience because for most of the mission, and especially in the beginning, all the Mars quakes we were seeing were quite weak. We were looking for the big one, right, We're looking for the big Mars quake, because the bigger the quake, the more ways we'll travel through the deeper parts of Mars. And so we really wanted to study or I really wanted to study the core, and for that we needed some big Mars quakes. And they really didn't happen for the first few years, and then right near when the mission was about to end, we suddenly had a few.
So that was really amazing to get that data at the end. So Mars kind of kept us hoping for a while and then finally delivered.
And before you landed on Mars with this seismometer, did you have much reason to expect that there were Mars quakes? Or it could be that Mars was totally silent.
I mean it could have been we didn't have any direct evidence from marsquakes. But my geologist friends who are used to looking at say, tectonic features on the surface, looking at things like where are the cracks in the surface, where are the mountains, they would have told me to expect Mars quakes because.
They see movements geologically.
They see movements on the surface. But also, luckily, we kind of have our own source of Mars quakes. In a way, when meteors hit planets, they crash into them. They're kind of like a hammer that's smashing into the bell, right, And so a lot of the marsquakes we measured were actually caused by meteors that hit Mars as opposed to just tectonic activity happening in the interior.
Well, it's terrifle to me or feel a little conflicted the geologists are rooting for quakes and rooting for like big impacts because they're like, oh, yay data.
Yes exactly, I will I mean, as a funny story on the mission, we did at one point, so the Insight mission was on the surface when the Perseverance rover was planning to land, and we did kind of do a calculation where if the landing didn't go so well, would be able to.
Detect away from that. Luckily that didn't happen. We had a very nice landing.
Yeah, the congratulations on your landing. Too bad we didn't get some cool data though from your explosion of your huge project. Oh my gosh, all right, this is really fun and I want to hear a lot more about what's going on inside our planet. But first, let's take a quick break. With big wireless providers, what you see is never what you get. Somewhere between the store and your first month's bill, the price you thought you were paying magically skyrockets. With mint Mobile, you'll never have to worry about Gotcha's ever again. And Mint Mobile says fifteen dollars a month for a three month plan, they really mean it. I've used mint Mobile and the call quality is always so crisp and so clear. I can recommend it to you. So say bye bye to your overpriced wireless plans, jaw dropping monthly bills and unexpected overages. You can use your own phone with any Mint Mobile plan and bring your phone number along with your existing contacts. So dit your overpriced wireless with mint mobiles deal and get three months a premium wireless service for fifteen bucks a month. To get this new customer offer and your new three month premium wireless plan for just fifteen bucks a month, go to mintmobile dot com slash universe. That's mintmobile dot com slash universe. Cut your wireless bill to fifteen bucks a month at mintmobile dot com slash Universe. Forty five dollars upfront payment required equivalent to fifteen dollars per month new customers on first three month plan only speeds slower about forty gigabytes On unlimited plan. Additional taxi speeds and restrictions apply. See mint mobile for details.
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Okay, we're back. We're talking to Professor Sabina Stanley, author of the book What's Hidden in Side Planets about what's inside our planet. You mentioned earlier that it was amazing that Insight worked. Is that just because it's hard to land stuff on Mars and operate a robot on another planet or was there something particularly challenging about a seismometer on another planet.
Yeah, Insight had a lot of firsts. I would say, right, it wasn't the first lander. We've had other landers on the surface, but this was the first time we were going to take equipment that was stored on top of the lander and actually physically move it to put it on the surface. So there were lots of ways that could have gone wrong. Right, This lander had this arm type device that had to pick up the seismometer on the lander and move it onto the surface, So that required tons of work to get that to just work properly. Then it had to put a wind shield on top of the seismometer to make sure that we didn't measure a bunch of wind basically because wind also shakes schismometers. Then you know, the seismometer wasn't the only instrument on insight. There was also a thermal probe what we called the mole, which was supposed to down about ten meters and take temperature measurements at depth, which would have told us about the heat flow coming out of Mars. Again, this was going to be the first time anything like this was tried, and unfortunately we couldn't get the mold to dig deeper than about tens of centimeters. The properties of the soil soils kind of word we use, but the properties of the sand on Mars were not as we expected, and just the device couldn't actually use friction to dig down deeper and deeper.
So that was a struggle.
And we actually the insight engineering team that worked on this and the scientists that worked on this, you know, I wasn't part of this. It was just amazing the things that they tried, and in the end we actually did get some good science out of it. We measured more sort of the thermal properties at the upper part of the crust as opposed to deeper down. But it was just amazing to see how much they tried to work on doing this first digging on here. You know, we talked about digging on the Earth is hard. Now imagine digging on another planet without humans, and it's even harder wonderful.
And then what are the plans for the future. Is NASA planning to dig into the surfaces of any other objects in the Solar System or put seismometers on any other surfaces, So.
I think seismometers is definitely something that's going to go. So there is a big push right now to send spacecraft back to the Moon so that we can better understand our closest celestial body, let's say. And so there is a mission that will involve putting a seismometer, putting more seismometers on the Moon. We already have some seismometers on the Moon that we're turned off a while ago for budgetary reasons, right, So it'll be great to get seismology again on the Moon. But for me, the most exciting is that an upcoming mission that's planned to go to Titan, which is a moon of Saturn, is actually going to have a seismometer on it as well. So it'll be interesting to see what we can learn about the interior of Titan.
And what do we know right now about the interior of Titan, And how could we know anything about it just from like looking at a few photons that happen to reflect off of it.
So Titan is one of my favorite places. So it's really exciting to think about what they're going to see. So Titan's a unique place. First, well, it's the only other planetary body in the Solar System that has a nitrogen based atmosphere that's thick like the Earth's right, So earth Is atmosphere is mostly nitrogen, and the surface pressure on Titan is about one and a half bars, so one and a half Earth atmospheres. But the cool thing about Titan is that it's a small planet and so it has very little mass and so its gravity is really low. So if you were to go to Titan and put some cardboard on your arms and flap them, you would be able to fly on Titan because you have ideal buoyancy situation there. You've got thick atmosphere, low gravity, so it's really easy to fly there.
So in contrast, like they had the helicopter on Mars, that was a real challenge because the atmosphere was thin and the helicopter needs atmosphere exactly exactly.
So the Dragonfly mission, which is going to Titan should get there in the mid twenty thirties. It is going to involve a dual quad copter. So this thing has basically eight rotors and this to me, because I'm Canadian, it looks like a skidew or a snowmobile because it has these sled track treks underneath it. But it's basically going to fly around land somewhere, do a bunch of science, then take off again, look for a new location, scout somewhat, then fly to a new location, land again. And so it's going to be able to do ground local science right at an individual location for a bunch of locations over the surface. And that's really the challenge in planetary science is this kind of combination of get lots of data from lots of different places, really locally, really close to the surface.
So that's going to be a very exciting mission.
We'd have to ask you about that. Is that going to be self directed? Is it going to decide on its own where to go? Or is it going to wait for signals for minutes and minutes from Earth?
Full disclosure here, I have no involvement in the Dragonfly mission. I'm just a super fan. But my understanding is what it's going to do is when it kind of goes up the one time. When it flies up one time, it's going to survey, it's going to look around. Then it'll come back down recharge its batteries. And during that time, when the data gets back to Earth, people are going to look around and say, let's go here, right that place over there looks kind of interesting.
So it'll be a combination. Some of the in time flight.
Stuff is going to have to be done by the spacecraft by itself, but when it comes to making decisions about where to go next in terms of big steps, that's going to be done by the people back.
Your on Earth.
I really liked your comment about needing to sample several places. It seems obvious that if you only land on Earth in one place, you might conclude, oh, this whole place is grantede or oh look it's all beautiful marble or something. Obviously you need to look around to get a better sample. And so when we only land on one place on the Moon, like the Apollo astronauts, you know, only looked or near where they landed, we may have gotten to bia a sample of what's going on up there. So that's really cool that they're going to explore it. So other than landing on the surface. In your book, you were talking about seeing what's inside a planet by basically how it wobbles. Can you walk us through the physics of that, the moment of inertia, and how it gives us a picture of what's inside.
Yeah.
Absolutely, So all of the planets spin to some amount, right, That's why we have a day on the Earth. And when it spins, a planet doesn't just stay a perfect sphere. It kind of gets fatter at the equator than it does at the poles. Now, it turns out that how fat it gets at the equator versus the poles is directly related to what the material properties of the object are. So, for example, if you had a perfect water planet, right, imagine the small planet made of water and you spun it, there's a specific like ellipsoidal shape you would get for a liquid planet, Whereas if you had a dense core inside the planet and with a solid layer and then a water ocean on the outside, you're going to get a different amount of flattening or a different amount of kind of bulging at the equator. From that, so we can actually use the amount of bulging of these planets when they're spinning to get information about what's inside.
So, for example, you spin a basketball, it stays a sphere, But if you spin a blob of pizza dough, it becomes a disc, right, and so it tells you pizza dough softer than basketballs. I guess we already knew that, But you're saying, we can apply the same thing to planets. By the deformation of the sphere, we can tell basically how rigid it is.
Yes, absolutely, and also where the dense, how dense it is essentially in different parts. So for example, Saturn, Saturn is the bulgiest of all the planets in our solar systems. Even if you look at through a telescope, it doesn't look like a sphere. It actually looks like more of an oblate spheroid. So it's really interesting to look at Saturn through a telescope.
You mean Saturn looks like squished, like somebody sat on it.
Yes, Saturn looks like someone satur on it.
In the best possible way. I mean Saturn's beautiful, yes.
Yes, absolutely, But because of that we know that Saturn isn't just a ball of hydrogen helium. We know that there have to be some rocks inside kind of condensed at the center, and then the gas sphere is kind of more on the outside of it. So we've actually been able to figure that out from the size of its equatorial bulge.
So that's the first way we can use rotation. There are other ways.
So for example, as planets orbit and rotate, they can actually as they're rotating, they don't always point their north pole to exactly the same location, so they can actually process, so their rotational axis can move around in a circle about their orbit axis. And if you've ever played with like a top, like a toy top, and you've spun it and you've seen it make this little wobbly circlar pattern, planets do the same thing. So planetary rotation axis wobble. They process, and they also do this thing called nutating where they kind of dip down a little bit, And the period of those procession motions and the kind of how cyclical they are really tells us about the interior properties as well.
But why does it happen in the first place, I mean, does an angular momentum tell us that it should we spin along the same axis. Is this the effect of like other things pulling on it?
Yes, exactly.
So if Earth were alone, if it was just the Earth, then nothing else was around, we would not have any procession or nutation. But we've got the Sun, we've got the moon nearby, and both of those things causecessional motions and wobbling nutational motions that affect our orbit in our day.
So Jupiter and the other things are pulling on the Earth and changing the direction of its spin axis basically like where the north pole is pointing in the galaxy, And you're saying that tells us something about what's inside the Earth. People I think are used to thinking about the gravitational model of like, well, you can treat the whole planet as a point mass at its center of mass, you can't learn anything else about it. So how is it possible to know something about the distribution of mass inside the planet from how it's spin wobbles.
So the wobbling and the spin can tell you things, for example, like if you have a liquid layer inside the planet. So I don't know if you've ever played this game, but if you take a beach ball and you put like a little pocket of water in it, and you try to throw it to someone, it moves completely differently than if you don't, or even easier, take an egg, Take a raw egg and take a cooked egg, both still in their shells, and put them on your counter and spin them, and you will see that they spin very differently because one of them has liquids inside of it and the other one is fully solid. So we can use the way that the spin axis wobbles to figure out where are there liquid layers in this planet? Is it fully solid that sort of thing I see?
Is this planet more like a soft or hard boiled egg. That's incredible, And can't you also measure the moment of inertia of the planet and tell like where the mass is distributed, and like you can tell the difference between like all the mass being at the core versus all the mass being at the surface.
Yeah, so it's a bit complicated in the math, but it turns out that the procession rate, so how fast the axis of the rotation processes about the orbit normal.
I'll give you any example.
So on the Earth, right now, our north pole points to the North Star. It was named that way for a very specific reason. But it's moving around and in about twenty It takes about twenty six thousand years for that pole to.
Get back to the North Star.
Right, So the period of our orbit is twenty six thousand years, and that period can be used to actually determine the moment of inertia of the Earth through some fancy math formulas. And so if we can measure the precession rate or the period for other planetary bodies, we can also figure out their moment of inertia.
Wow, twenty six thousand years? How long have we been making these measurements? Couldn't be more than a thousand years at maximum?
Yeah, yeah, it's definitely less than that. But you know, you can you can trace out a little arc of a circle, then you can pretty much draw out the rest of the circle.
Yeah, I guess we have a model and we can fit to that little arc. That's amazing, incredible. We can learn so much about what's inside these objects without even ever going inside. All right, I can't wait to talk about this the more, but first we have to take another break.
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Okay, we're back and we're talking to Professor Sabina Stanley, the author of the new book What's Hidden Inside Planets about what's inside the Earth and other planets. Now, I want to talk about sort of how the inside affects the outside, because obviously, if you're just curious about how the Solar system is formed, you want to know what's inside the Earth. But even if you're not, like it has an effect on living on the surface, right, tell us about, like how the magnetic field of these things is generated and how it relates to bubbling soup.
Yeah, so magnetic fields are my favorite topic. Not going to lie. So here's this amazing thing.
Right, We're on the surface of the Earth, and one of the reasons it's such a nice place to live at the moment is because we have this beautiful magnetic field that completely envelops the Earth. And that magnetic field, what it does for us is it shields the surface from high energy particles that come from the solar wind which come from the Sun, and from cosmic rays that come from deep space, and those very high energy particles. If we didn't have our magnetic field, they would kind of blast the surface of the Earth and they would do some terrible things. First of all, they would cause high radiation environments, so we'd likely have high rates of cancer, for example. But also they cause lots of electrical disturbances, and if you think about our power grid, our power grid does not like there to be large fluctuations in electromagnetic fields. That's another thing that is not so good. It also tends to these solar winds that bombard planets. They can actually erode the atmosphere of a planet, so they can take they can basically, you know, it's like pointing a hair dryer at the Earth. You're going to be able to blow off all the gas from it. So there are all these things that the magnetic field actually shields us from. But this magnetic field that surrounds us is actually created deep inside the Earth in the iron core. So iron is great electrical conductor. When you have a great electrical conductor, if you can get it moving around in the right way, then you can actually generate magnetic fields. And the best kind of analogy I can think of for this is if anyone has a home generator or if they have a bike light that they can pedal to get going, you're basically converting the kinetic energy of that motion into electromagnetic energy. So you either you know you're pedaling, causes your bike light, causes currents to flow that causes your bike light to shine right, or similar in your generator. So in the core of the earth, convection which occurs because the center is hotter than the outer parts of the core, so you kind of have bubbling up like like you would if you put a pot of soup on the stove, right, you get the bottom of the pot is hot, the top of the pot is cold, So you get these overturning motions in the soup. Same thing happens in the core, and so that overturning motions they create magnetic fields and you get what called a dynamo. So the dynamo in the center of the Earth generates this magnetic field that protects us on the surface.
Amazing, and so you're saying that it's the convection cells that generate the magnetic field, not, for example, the spinning of the planet.
Right, So there is a somewhat common misunderstanding out there that the reason that Earth has a magnetic field, for example, is due to its spinning, And this has been used sometimes, for example, to explain why Vna, which is spinning very slowly, doesn't have a magnetic field. And it turns out that you don't need spinning at all to generate a magnetic field. So magnetic fields can be generated through dynamo processes without spinning. Now, spinning sometimes helps in organizing motions and stuff like that, but it's not actually a requirement. So it's the convective motions, not the spinning, right.
And so in your analogy, you're talking about like peddling your bicycle to generate electricity, And we haven't seen any magnetic monopoles in our universe, so we know that to generate magnetic fields you have to take a charge and put it in motion, which is how electrical generators work. But what is the charge here? Like we have flows of iron. Iron is obviously metallic and it conducts, but don't you need some ion in motion in order to get a current going? What generates the actual current? If you just have neutral iron. How does that generate a magnetic field?
Yeah, it's actually an induction process.
So what it is is you've got a good electrical conductor and imagine you have a magnetic field and it's frozen into a good electrical conductor. So magnetic fields tend to stick inside good conductors.
They don't like to change.
But imagine then that you start moving that conductor around relative to itself, so you shear it, you pull it apart a little bit. That magnetic field has to go with it, so you stretch and twist the magnetic fields through the motion itself to create new magnetic fields.
Wow. Fascinating. And the Earth's magnetic field, though it's pretty reliable, is not actually constant. Isn't it gradually changing?
Yes? Absolutely so.
We have records from the rocks in our crust. They can be magnetized at the time that they form, and those records tell us that Earth's magnetic field has changed over time. We at least have data that shows it's been around for about three billion years, if not longer. But it hasn't always been the same, So there are times in the past where the field has gotten weaker. There are times in the past where the field has flipped polarity, so the north magnetic pole became the south magnetic pole and vice versa. And even today, on like weekly time scales, we can measure the small changes in the earth magnetic field that are happening from a variety of things. Some things are external, but sometimes we can also see on a yearly scale we can see the changes due to different flows happening in the core of the Earth.
Can we use these changes in the magnetic field to sort of image those flows the same way we can see changes in the gravitational field to give us a picture of what's inside the Earth.
Yeah, it gets a little more challenging the deeper you go. And with magnetic fields, what we can see, for example, is because we know it's a good electrical conductor, if we see a magnetic field pattern drifting in one direction. So for example, there's this kind of famous thing we talk about in geomagnetism called the westward drift. So if you follow certain features of the magnetic field, you see they all kind of drift westward, and we interpret that to being there's flow generally in the westward direction.
There's like a jet stream in the core of.
The Earth that's flowing westward, that's taking the magnetic field with us.
Wow, and so how well do we understand this process? Are there still open questions about like why it's flipping and why it's changing or is it something that we understand pretty well?
So many open questions. So the amazing thing about the process, so fluid dynamics. If you've had any experience with climate modeling or trying to study flows that happen in pipes and so forth, fluids are really complicated. They can they can display turbulence, for example, or laminar flows depending on what types of you know, what the situation is like.
Now, if you add to.
That, add to fluid dynamics magnetic fields and all the things that happen with magnetic fields, you almost get an added complication. And so when we try to think about, well, how do we study the dynamo process, right, we can't really wait thousands of years to watch the real system over time. We want to study it faster. So you can either do experiments or you can try to write a computer model that can mimic what's going on in a core when it's generating a magnetic field. Experiments are really hard. Turns out that dynamos they like three things. They like really good electrical conductors, they like really fast motions, and they like really large length scales. And then you start saying, Okay, I'm going to build my giant sphere of a really good electrical conductor and then spin it really fast, and you just you end up with a huge challenging problem. The biggest dynamo experiment out there is the three meter dynamosphere in Maryland, and it has yet to generate an active dynamo, so that's a challenging problem. We use computer simulations to study dynamos inside planets. The problem there is that planets, the motions, the scales, and the motions are so tiny and so fast that there isn't enough computer power on the planet to run a simulation accurately. So we have to make a lot of assumptions and simplifying type conditions, so we aren't able to fully study the system.
The way we want to. We have to be very nuanced in how we study it. Well.
Do we understand why the Earth's flipping of the magnetic field seems so irregular compared to, for example, the Sun, which has this rock solid solar cycle of eleven years.
Yeah, we don't fully understand why at all. We can't even kind of predict what we would expect for other planets as well. We have what I would call a hand way the understanding, and that we would describe the core fluid as being a very nonlinear system that can have different attractors or different stable systems. And sometimes it's in one stable position, sometimes it's another. And so if you have something near a stable position, imagine you have a ball sitting and you have like a nice valley and two hills on the side, and you stick the ball on one of the tops of the hills, right, it'll pretty much stay there. But maybe if you shake it a little bit too much, give it a bit too many perturbations, it'll sink down and go to the other stable position. So we think that some perturbations in the fluid can sometimes cause the field of flip, but we don't have a good way to, for example, predict when the next flip is going to happen, what's the key factor that causes such a flip for example?
And these are all areas of current research.
Wow. And then as we discover planets in other solar systems. How do we begin to do geology of those planets? And first, I guess it's a trivial question, is would you call it geology? Geology is to study the Earth, So is this like exo planetology? What do you call it?
This is a great question.
I think the norm has been to refer to geology as looking at rocks, and it doesn't matter where those rocks are.
So rocks.
There are Mars geologists, so I'll just get that out there instead of marsologists or whatever you would call them instead. Yeah, with exoplanets, the challenge there is the type of information you can get can be quite limited compared to what we can get when we're in our own Solar system or here on the Earth. But even with the standard techniques that can discover exoplanets, right, if you think about the methods involving radial velocity detection, so where you measure fluctuations in the stars light curve caused by the motion of a planet around it, you get information about the period of the orbit, and that can also give you measurements about the mass of the planet. Then if you use transit where a planet passes in front of or behind a star, you can get information about the size of the planet.
So as soon as you have.
The size and the mass, you already have kind of an average density, a bulk density of the planet. So we have sense of whether when we discover these exoplanets, is it a gas giant, is it an Earth like planet? Is it an ice world like Uranus and Neptune. So we can do some very broad geology, let's say, from that type of information. But what I'm most excited about is the possibilities that are going to come forward with JWST because this new telescope is going to be able to measure the atmospheres of exoplanets and tell us what they're made of. That's going to be crucial information to figure out what's actually going on deeper inside the planet. Right our atmosphere on Earth is the way it is because of interactions with the interior of the Earth, and so we're going to be able to use information about the atmospheres of these exoplanets to also tell us something about the interior.
What do you mean by that? I know our atmosphere is different because we have a magnetic field and because of the surface gravity. What else does our atmosphere tell us about what's inside the Earth.
Right, So if there was an alien flying by our solar system, and all it could measure is kind of the spectrum of our atmosphere, it would be able to tell that there was life here most likely.
Right.
We've done things to our environment to make it very obvious that there is industrial action happening on the surface.
Right.
But also, for example, a lot of the processes that regulate some of the key species in our atmospheres, like carbon dioxide. On Earth, there's the carbon cycle. The carbon cycle not only involves the atmosphere, it involves the ocean, the surface, and the deep interior. So carbon gets recycled inside the Earth, and so we can actually learn about how exchanges of materials happen with the interior and the atmosphere by looking at how much carbon there is around, for example, right. And so the same is true for other element cycles, and so the same could be true for exoplanets.
We had Professor Shields on the podcast recently, and she does exoplanet climate simulations. We're basically building models of these planets and then trying to make them consistent with what we might understand from JWST. It sounds like you're talking about doing something similar, but you're building models of the internals of these planets to explain then the climate in the atmosphere, which then tells us about the light we're seeing from these planets. So it seems like quite a few steps there from the photons we're getting in Jast to our model of what's happening inside those planets. Incredible that we could learn.
Anything, Yes, absolutely agreed.
And what about future missions? I know there are space telescopes that are going to be looking specifically for planets. Are those going to have the capacity to tell us more about these planets or do we need to wait until we can send the landers to listen for exoplanet quakes.
What I'm most excited about for future exoplanet data has to do with magnetic fields again, right, So if we think Earth having a magnetic field is so important for shielding life on the surface, then it might be nice if we knew that exoplanets had magnetic fields.
It maybe it's something we should.
Add to the conditions for a habitable planet out there. And there have been some signs, some evidence that we might actually be able to measure magnetic fields of exoplanets so there's hope that with even more measurements and so forth, we might actually be able to tell in the future if an exoplanet has a magnetic field.
Today, how would that be possible? Are you looking for like the Northern La It's equivalent on the planet seeing the effect of the magnetic field on the atmosphere.
So that's one way kind of so it's not the Northern Light itself. But actually the way we found out Jupiter had a magnetic field. We knew that Jupiter had a magnet field in the nineteen sixties even though we'd never been there, because electrons that spiral along the magnetic field lines of Jupiter get really close to the poles into the atmosphere there, right, and that causes aurora on Jupiter as well. But it also causes a type of radio emissions to come off of Jupiter, and those radio emissions get beamed out into space and we could actually measure them here on the surface of the Earth. So we knew about Jupiter's magnetic field in the nineteen sixties before we'd ever gone there, because we received these radio emissions. Same is true for any other planet. Now, it turns out that the intensity of those radio emissions is really important, so you need really strong magnetic fields in order to be able to measure them. On the surface of the Earth, we have this horrible atmosphere on Earth and it blocks a lot of radio emissions, which is very frustrating, although kind of good for breathing. So I guess, you know, let's say we put a radio telescope on the far side of the Moon. That would be great for helping to detect radio emissions from exoplanets. So there's that method, but they're also what I would call sneakier methods.
Right.
For example, if you look at the transit spectrum, so if you look at a planet that's going in front of a sun or a star and you see kind of how wide the planet is. People can already kind of tell if a planet has an atmosphere by the fact that it could have different thicknesses or different radius in different wavelengths, and that you know, sometimes the atmosphere will let light through, whereas the planet itself won't, right, And so that's how we can tell whether something has an atmosphere, is what particular wavelengths of light get through at different distances. The same can be true about a magnetic field. Sometimes a magnetic field can cause certain light spectra light frequencies to not get through, so we might be actually able to measure a magnetosphere surrounding a planet by looking at transit spectra. You can also maybe see if a planet has like a tail, right, and so sometimes if atmosphere is be blown off a planet, you might be able to see that in a transit spectrum or through other types of light detection. So there might be some sneaky ways to look for magnet fields of extle plants as well.
Wonderful well, I expect that the next generation of scientists will be even more creative about coming up with ways to extract amazing information from these tiny little blips in our telescopes.
Yes, hopefully so.
Wonderful well, thank you very much for coming on the podcast and telling us so much about the mysteries that are under our feet and the mysteries that are out there in the universe.
Thanks so much.
This was fun, all right. That was my chat with Professor Sabina Stanley again. She's the author of the book What's Hidden Inside Plants, which you can get now at all reputable booksellers. Thanks very much for listening. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeart Radio. For more podcasts from iHeartRadio, visit the iHeartRadio app Apple Podcasts wherever you listen to your favorite shows.
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