What’s Hidden Inside Planets, with Dr. Sabine Stanley

and far. A quick bit of author bio before we get started here. Doctor Sabine Stanley is a distinguished planetary scientist and a key contributor to NASA's Mars Insight mission. Holding a PhD in Earth and Planetary Sciences from Harvard u University, she focuses on the complexities of planetary interiors. Currently a faculty member at a top research university, she

leads innovative studies in her specialized field. Beyond academia, Doctor Stanley is a regular speaker at international scientific conferences and serves as a consultant for various space missions. Her research has been published in leading scientific journals, earning her multiple awards for her contributions to planetary science. With a career that blends rigorous research and public engagement, Doctor Stanley remains a pivotal voice in the scientific community committed to enhancing

our cosmic understanding. So I guess now let's jump right into our conversation, Doctor Sabine Stanley, Welcome to the podcast.

a planet with breathable atmosphere breathable to us. And yet you say in your preface that quote, arguably a planet's interior is more important than the surface in determining a world's fitness for life or ability to withstand the pressures put on it by its home star. I think this might be really surprising to people. Could you explain this?

wind particles from hitting us. Now, when you have these high energy particles coming from the Sun, if they actually blast it into the planet without the magnetic field there, they would work to strip off our atmosphere. They would bring high radiation levels to the surface. It would essentially not be a really great place to live if we

didn't have our magnetic field. So one really important thing when thinking about is that planet going to be really good for say, life to form, is does it have a magnetic field and that's really created in deep inside the planet. The other aspect of this is that most of the reasons that we think the surface of Earth is so nice and habitable, right the liquid water, that breathable air, all of that is related to a kind of recycling process that happens inside Earth. Earth's water a

lot of it came from outgassing a volcano. So there's water deep inside the Earth. When you have volcanic flows bringing up magma to the surface, there's lots of gas, particles and water in that that eventually makes it out into the atmosphere. Carbon dioxide is recycled this way. Water is recycled this way. So you can't really just focus on the surface. You have to ask how does that surface interact with what's going on deep inside planet.

going on. So what does generate the Earth's magnetic field?

more complex than just spinning around. So it's not just that the Earth is spinning and that's causing it. It's actually these like helical type flows that occur because of the fact that the Earth is trying to cool down, so space is cold. The inside of Earth is hot. And so just like if you put a pot of soup on the stove, the bottom of the pot is hot, the top of the pot is cold. You get that

boiling action that rolling around. Same thing in the core of the Earth, you get these churning motions as the hot material at the center of the Earth tries to make it up and out to cool down the core and then the planet. So it's convection, that's what we call it. It's convection. That's the motion that's actually creating the magnetic field that Earth has.

in the past. So about four billion years ago, the rocks on the surface of Mars were magnetized from a dynamo that was active at that time. And then there's Venus. For Venus, we have no way to tell if it ever had a dynamo in the past. It does not have one today.

going on inside the human body. Right, We do scans of things, we use gravity, we use magnetic fields, we use seismology. So then you try and use any of these techniques on Venus, and they don't work for a variety of different reasons. So first of all, let's say we wanted to use seismology, right, So the waves that pass through a planet when you have an earthquake, for example, on Earth, the speed of those waves is completely determined

by the material properties they are passing through. So we actually learn a lot about what the materials are inside the Earth by looking at how fast these seismic waves travel through it. You want to do this on Venus, Sorry, the surface is a horrible temperature, and the atmosphere is made of sulphuric acid, and the pressure is ninety two atmospheric pressures, so it's just the materials are just going to melt and dissolve basically, so no chance to do that.

Then you try and use a magnetic field, right, Having a magnetic field is a great way to learn about the interior of a planet. As soon as you know a planet has a global magnetic field, you know, it's got a liquid molten electrical conductor in its interior, and it's got those churning motions. Well, Venus doesn't have that,

so we can't use that. And you try and say, okay, well, why don't we just observe the surface and look at the rotation of the surface, for example, while Venus has this horribly opaque atmosphere that you can't actually look through and try to do that. And then one of my favorite things about Venus is that when you look at all the planets, all the planets, you know we talk about planets being spheres, they're not spheres. They're actually kind

of bulgy oblate spheroids, we call them. So they're fatter across the equator than they are at the poles, and that bulging of the equator is because of the fact that the planet's spin. Now we can use that information how bulgey a planet is to actually tell what material is inside it, how dense it is inside it. And you go and try and do this at Venus, and Venus is rotating so slowly that it basically has no bulge.

So we can't use that either. Basically, Venus is just very frustrating doesn't want us to know anything about its interior.

on Earth. So, for example, the elemental composition of that vast cloud determines the elements that make our Solar System, but also more interesting and subtle things like how the slight initial rotation of that cloud governs so much about our world. Could you talk about some of these connections between the properties of the cloud and the way the Solar System is now?

And so then you put something out in the middle of nowhere, a molecular cloud, right, and you don't have anything really torquing it or anything, and you ask, well, how much spin is it going to have? And it's going to have some random amount, right, like nothing's perfectly isolated and still, so you take all those particles in the molecular cloud and you add up all their spins, and a lot of them will cancel out. Someone will be spinning in one direction, other particles will be spinning

in the other direction. And you add it all up and it almost all cancels out except for a little bit. And that little bit in a molecular cloud. Here's the amazing thing. Once gravity gets hold of that molecular cloud, it starts gravitationally collapsing to eventually form our solar system. That amount of spinning just increases and increases and increases as the cloud gets smaller and smaller and smaller. And you have a total understanding. People have a total understanding

of this effect. If you've ever watched, for example, figure skaters who are about to do a jump with a spin it and they pull their arms in. As soon as you make something more compact, it spins faster, even though you don't do anything to it. And the same thing happened to the molecular cloud. As it became smaller and smaller and smaller, it spin faster and faster and faster. And that's what led to all the spinning we have in the Solar System now. And this is true not

just of our Solar System. We can look out and see other Solar systems forming. We can see other planets around other stars. They're all spinning too. It's all the kind of we share that among all the planets.

processes on these bodies. Where moons can be a little bit different than planets, there's a lot bigger possibility of where they came from compared to where they ended up. So for example, the rocky planets in the Inner Solar System, Mercury, Venus, Earth, Mars, they all pretty much formed close to where they are now. Some moons actually come from very far away and then get trapped in the gravitational field of a planet and then become a moon there. And that happens a lot,

for example in the Outer Solar System. So if you look at Jupiter or Neptune or any of these planets, some of their moons are on these really weird orbits. They're like orbiting in the opposite direction as the planet is spinning. They're not around the equator at all. And those moons we think are actually captured basically comets. They're captured comets or asteroids that were doing their own thing got gravitationally kicked into the Solar System a little bit

close in and then got trapped around a planet. So you can actually find some bodies orbiting these planets that were probably formed much further away, and so in that sense, that's a little bit different than what you see with planets.

put into orbit. And then you ask the question, Okay, so now you have this disk of material surrounding the Earth. How long did it take for that disk of material to become the Moon a single object? Now, that disk of material followed the same laws of physics as the planet's forming out of the disc. Initially on gravity caused collisions. Some of those collisions caused things to clump together. Eventually, the Moon grew and grew and grew, and estimates suggest

it took forty years for this to happen. Now, when you're talking about things in astronomy and in Earth science, you're working on millions of years, billions of years. Those are the types of lengths of time we're used to. So talking about a process that takes forty years is

just shocking. And so I always have this image in my head, and this is obviously impossible because it was four point five billion years ago, But I have this image in my head of like some parents sitting down with their kids and the parents going, you know, when I was your age, there was no moon in the sky kind of thing, right like that. It's on a human generational timescale that this changed.

and the Earth. And it's the same reason, for example, that the Moon only shows us one face, so tidal interactions, the fact that you know, the Earth's not a perfect sphere, the Moon's not a perfect sphere, and they tug on each other when are not kind of facing the right way. That has caused the Moon to start moving further away. It's also caused the Earth to start slowing down its rotation a little bit, and so over time, the Moon's moving further away and it's going to make it smaller

and smaller and smaller. And one reason we know all of this is that the moon, if you look at how bulgy it is, Like I talked about how spinning objects have this bulge on them. The Moon is too bulgy for how fast it's spinning right now, and the only way to explain that is it must have been spinning faster in the past. And the only way for it to have been spinning faster in the past is if it was much closer to the Earth, because we

know it's locked. It's day is locked to the Earth's you know, it's always facing the Earth the same side, so it had to have a much faster spin in order to get that bulge that it had.

especially hospitable now, and Venus is really inhospitable. So what does that tell us about looking for exoplanets that could sustain life elsewhere?

we're using now. But as mentioned, planets are complicated, and you could have a planet that comple letly changes its surface temperature through a greenhouse, a runaway greenhouse effect. That's what happened on Venus. Right, Venus is getting not that much more heat from the Sun as we are, but it happened to go through this climate process, this runaway greenhouse that made the temperture on the surface incredibly hot and not able to sustain any water. The water all

evaporated off of Venus. So we have to be a little bit more careful. We need to understand the dynamics of what's going on inside the bodies and outside, because that's what creates the atmospheres. Another great example of this, I think we also need to kind of broaden the search for habitable worlds, let's say, because if you look in our Solar system, aside from Earth, the next best place to possibly look for where life might form are actually in the water oceans of some of the moons

in the outer Solar System. Now, these oceans are buried like miles beneath the surface, but they're liquid water, they have energy sources, they have complex carbon molecules, all the kind of ingredients that we think might be important for life. So I think we need to think more carefully about what the conditions are for life on exoplanets out there, and we might end up finding life in places we didn't expect.

So this is just not possible, luckily for us. Sometimes the Earth brings samples from the deep up to the surface, and one thing it does is it brings up diamonds. Lots of people know about diamonds because they're important in say the jewelry industry, things like that, And when you're someone who's a jeweler, what you really care about are these pure diamonds, the things that are just pure carbon

and diamond form. When you're a geologist or someone who's studying the in tier of the earth, you want the impure diamonds. You want the diamonds where some little bit of the interior of the Earth got stuck in the middle of the diamond as it was forming. And because diamonds are so hard, they actually maintain their pressure, and so when they come up to the surface, that little bit of material from deep inside the Earth actually stays

preserved in the inside of that diamond. And so we can actually study materials how they are in the deep interior of the Earth by looking at these inclusions in the diamonds when they come to the surface. So that's one thing. Now, meteorites are like the best Christmas gift anyone could ever get from other planets. Right. Basically, you have some sort of collision or something that happened far away that caused a piece of a planet or an

asteroid to get knocked off. Eventually that piece of the asteroid came near Earth and some of it landed on Earth and we can go and collect it. That's rarely where you have samples of the insides of other planets, and we even have meteorites that are very iron rich that come from the cores of previous planetary asteroid like bodies that got broken up. So one of the best pieces of evidence we have for what's in our core is actually looking at the cores of other bodies. They're

all very similar. They all have this iron rich cores that eventually come to Earth and we get to study them.

World's Europa Enceladus moons like that. Sixteen. Psyche is a metal world. So this is a body that is mostly made of iron. It's mostly metal, and we just haven't seen that before. And I'm really excited about what it's going to be like to go look at this thing and you know, answer questions like, hmm, what does a crater look like on a body that's mostly made of metal? Is? What does a volcano look like on this thing? Does it have a magnetic field? What's really going on on

the surface. So it's very exciting. It's a new type of planetary body that we've never gotten to see up close and personal.

like to fall into Jupiter or something like that. It's clearly a captivating question because we see these outer cloud layers and there's just this you question, like, obviously, you imagine you're denser than some outer part of that cloud layer and you could sync down into it, and you've got to wonder what's inside. So what do we know about what is inside the gas giants and the ice giants?

completely different beast. Same thing is true for hydrogen. So let's start there. Hydrogen the most kind of the simplest element we have, thing that we're used to being in gaseous form. Now in Jupiter and in Saturn. As you keep descending into the planet, temperatures are rising, you get to millions of degrees, you can get to millions of atmospheres of pressure. Hydrogen is a very different material under that pressure, and in fact, is you squeeze a hydrogen molecule,

you basically allow So imagine hydrogen molecule. You got a proton at the center and the nucleus, and you got this electron floating around. You squeeze enough of those close together the electrons essentially get freed from the protons and you create what's called what we call a metal. So you can actually have metallic hydrogen going on inside Jupiter and Saturn, and it's a great electrical Conductor's actually where Jupiter creates its magnetic field, same with Saturn. So that's

kind of a material we're not expected. Interestingly, helium take another one, the second most simple element we have, helium. You do the same thing. The helium. Turns out that in the outer layers of Jupiter and Saturn in the atmosphere, helium and hydrogen are nicely mixed. Right. It's kind of like if you put salt in water or sugar in water warm water and you stir it. They're nicely mixed. You can't kind of separate them, but you put them

under high enough pressure. When this hydrogen becomes a nice metal, the helium no longer wants to stay mixed in it, and so helium will actually kind of exolt out of the hydrogen and become these droplets, and then helium's heavier than hydrogen, so they drop out. It rains helium inside

Jupiter and Saturn. So I think that's really cool. Then you get to the ice giants, where you have a lot more complicated molecules methane, ammonia, and you say, okay, what happens to those things under high pressure, and you can get things, for example, like super ionic water that's actually formed in where all the oxygen atoms form a nice lattice and all the hydrogen the protons that would make up water in the H two O flow freely between it, something we've never seen on the surface of

the Earth. And you can even make a diamond ocean deep inside Neptune and Uranus. It's been hypothesized that the carbon in things like carbon dioxide and methane ends up in the diamond phase deep inside, but then it melts, so you actually get a liquid diamond ocean. Diamond actually has this really cool property that water on the surface of the Earth also has, and that's right near that

freezing point. The solid phase is slightly less dense than the liquid phase, which is why we have, for example, icebergs that can float on water. Here. The same is true deep inside Uranus and Neptune. Diamond bergs would actually float on a diamond seed deep inside Uranus and Neptune. So these are just material behavior that we just have no experience with here on the surface of the Earth, and I love thinking about it.

intuitions about how materials work. For example, I think there's a part in the book where you talk about how it's hard for people to understand sometimes that parts of the mantle migrate up and down even though the mantle is solid not liquid. Is that right?

That doesn't mean it can't flow. So we do see that rock move around, it moves on really slow timescales, so it can take hundreds of millions of years for a rock to make it from say the core mantle boundary, up to the surface of the Earth. But it does flow, it does move around, and the only reason we see it in its liquid state at the surface is because it was under a lot of pressure deep inside the Earth. Pressures increase incredibly as you go down, and so that

it was basically pressure frozen. It was basically made a solid because of the high pressure, and then you bring that up to the surface and you release the pressure and everything kind of expands out and becomes the magma the liquid that you see.

When the Voyager two mission, which was the only mission that we have that has gone out to Uranus and Neptune and basically just flew by for a little while, when it got to first Urinus, which is closer, it didn't see a dipolar field. It saw this multipolar fields. There were a bunch of North poles and a bunch of South poles all over the planet. I remember reading about some of this history of when this happened in

the eighties, and people, you know, weren't expecting that. So the first question you have is, all, well, maybe maybe something broke, maybe the magnetometer is not working properly or something like that, and they did lots of tests and

they make sure that wasn't the case. Then you get out to Neptune and it's also this multipolar field, and so you got to say, hmm, well, were we just wrong about the fact that magnetic fields are supposed to be dipolar, and the fact that Uranus and Neptune happen to be the only water rich planets in the Solar System, these ice giants, and they happen to be the only ones with multipolar fields, then you got to start saying, maybe there's a causal relationship with there, right, So I've

spent a lot of time thinking about that and trying to think about how you create multipolar fields in an ice giant, And it turns out that there are some features in an ice giant that might make multipolar fields more likely to occur. Turns out that the dynamo region can be really thin in these bodies, so you just have this really thin shell where the conditions are just right for convection to occur in a good electrical conductor and create a magnetic field. When you have this really

thin shell, you can't make big global dipolar fields. Nothing's communicating the right way. All the length skills are too small. So maybe you get more multipolar fields that way. People are still studying this that we're really looking forward to new mission, hopefully to Uranus sometime in the next decade, so that we can study the magnetic field up close and the interior of the planet, so that we can understand the connection.

so strange. What is the deal with this?

all the materials we have. Now, if you go to some other solar systems out there, they might have slightly different ratios. And if nebula out there that eventually forms a star with planets around it happened to have a little bit more carbon, then the types of rocks that you form, the types of minerals that you form, can be very different, and you can actually create planets that are mostly made of carbon, that have a much higher

carbon content than what we have here on Earth. It's a geologist kind of like dreamscape to think about, what if the chemistry was just slightly different because there's just a little bit more of some tiny sub element. Right, Remember, our solar system was mostly hydrogen and helium, and it was just little bits of these rocks that eventually became the Earth. And now you just slightly tweak the ratio of those elements in extrasolar planets and you could create completely different worlds.

think of as necessary for life. So the plan is to go there and see try to get a sense of what that ocean is made of. Are there complex molecules in there that are kind of what we would consider the building blocks of life, things like that. So that's one really exciting. The other one is actually another moon. There's a mission called Drag and Fly, which is scheduled. Yeah, you know it's going to be a good mission when it has a cool name, right, So Dragonfly is scheduled

to go to Saturn's moon Titan. Now Titan, in my opinion, is probably one of the coolest places in the Solar System to think about. It's the only other planetary body out there to have nitrogen as its main based thick atmosphere. So the Earth is the other planet, right, so it has some similarities to it. It's the only other body out there that we have seen liquids running on the surface. So just like on Earth we have rivers and seas and oceans, Titan has rivers and seas and oceans on

the surface. Now there's a catch, they're not water. Those rivers and season oceans are actually made of things like methane and ethane, so not you know, fun places. But Titan has this thick atmosphere and it's also incredibly small, so it has a really low gravity. So my favorite fact about Titan is that it's really easy to fly there. So you could strap some cardboard on your arms and flap them on the surface of Titan could fly right. So this is amazing. So the Dragonfly mission has decided

to take advantage of it. So it's sending essentially an octacopper, so it's two quad copters, so think of a helicopter, but with eight different blade things. It's sending this thing out there. It's going to be able to land on the surface, do a bunch of science and then take off again, look around, figure out where it wants to go next, travel quite long distances, and then land again

and do more science. So this is going to be the first time we're able to study this world very locally, right, like touch it, and also fly around to very different parts of it. And Titans exciting because it has all the building blocks of life. We know there's a liquid water ocean underneath. We know it has complex chemicals, those hydrocarbons like methane and ethane. We know it has energy sources.

So we really want to understand a lot of the processes that we think are important in the creation of life are going to be happening on Titan, and we're excited to study them up close.

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