Planetary Radio • Jan 17, 2024

What’s hidden inside planets?

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Sabine Stanley

Professor of Planetary Physics at Johns Hopkins University

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

Venture into the hearts of worlds and uncover how we study planetary interiors this week on Planetary Radio. Sabine Stanley, professor of planetary physics at Johns Hopkins University and author of the new book "What's Hidden Inside Planets?" discusses some of the amazing things that lie under the surfaces of the worlds in our Solar System. But first, Mat Kaplan, senior communications advisor at The Planetary Society, gives an update on the first Commercial Lunar Payload Services mission and the timeline for NASA's Artemis program. We close out this show with Bruce Betts, our chief scientist, as he shares information on the new book, "Casting Shadows: Solar and Lunar Eclipses with The Planetary Society."

What’s Hidden Inside Planets book cover
What’s Hidden Inside Planets book cover This is the cover of What’s Hidden Inside Planets by Sabine Stanley. The author takes the reader on a journey inside the interiors of worlds and explains the methods scientists use to peer below the surfaces of planets in our Solar System.Image: Johns Hopkins Wavelengths
Interior structures of Earth, Mars and the Moon, to scale
Interior structures of Earth, Mars and the Moon, to scale All of the terrestrial planets have a three-part layered structure. At the center is a metallic, iron-rich core, part of which may be molten. Above the core is a thick middle layer called the mantle, made of silicate rock (composed mostly of silicon, oxygen, iron, and magnesium), making up most of the bulk of the planet. Above the mantle is a relatively thin crust of less-dense rocky material. The crust has more lighter elements (aluminum, sodium, calcium, sulfur) than the mantle.Image: NASA / JPL


Sarah Al-Ahmed: What's hidden inside planets? We'll find out this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. Today we're venturing into the hearts of worlds and uncovering how we study planetary interiors. Sabine Stanley, Professor of Planetary Physics at Johns Hopkins University and the author of the new book What's Hidden Inside Planets joins us to talk about the amazing things that lie under the surfaces of worlds in our solar system. But before we dive into that subject, we have some major lunar exploration updates. Mat Kaplan, our senior communications adviser at the Planetary Society will share the fate of the first commercial lunar payload services mission. We also have some updates to the timeline for the Artemis missions. We'll close out the show with Bruce Betts, our chief scientist, he's going to share some information about our new book Casting Shadows: Solar and Lunar Eclipses with The Planetary Society by Bruce Betts. If you love planetary radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing, you'll never miss an episode filled with new and inspiring ways to know the cosmos and our place within it. This episode is chock full so let's get this space show on the road to the moon. You've heard me say it before And I'll say it again, humanity is on the cusp of a new age of lunar exploration. Not just one that's more internationally collaborative, but it's also offering private commercial entities more opportunities to get involved. NASA's Artemis program is in full swing that's designed to return humans back to the surface of the moon for the first time in over half a century. And the commercial lunar payload services program completed its first launch. Unfortunately, and we all know this, space is hard. And the uncrewed commercial mission carrying Astrobotics, Peregrine Lunar Lander and its payloads did not go according to plan. Here's Mat Kaplan, our senior communication adviser with the details. Hey, Mat, Happy 2024.

Mat Kaplan: Happy 2024. Very happy new year to you, Sarah, and glad to be back with you on the show.

Sarah Al-Ahmed: Yeah. You were just with us a couple of weeks ago, we were doing a recap of all the amazing space exploration that went down in 2023. And at the time, I remember you mentioning that you were looking forward to something that was coming up in the new year, the first launch of NASA's commercial lunar payload services Program. And that just happened, that was on Monday, January 8th. So we have some good news and some bad news, right?

Mat Kaplan: Yeah. Mixed results for the first eclipse mission. But it was a first shot eclipse and a first shot for the company Astrobotic with their Peregrine lander, they're putting a good spin on it. But where do you want to start?

Sarah Al-Ahmed: Well, right out the door, what is NASA's eclipse program? And why are you so excited about it?

Mat Kaplan: Well, you said it, a commercial lunar payload services. It's NASA's attempt to take the model that worked so well so far with SpaceX and the Falcon 9, and apply that to get into the moon, getting robotic missions to the moon. And the idea is that instead of NASA saying, Okay, it's our rocket, and here's exactly how we want you to build it. And if you go over your budget, well, okay, we'll just have to make up the difference. No, this is a fixed price project or contract. So NASA is basically just contracting out with, in this case, Astrobotic, there's several other companies that they are working with. And if Astrobotic goes over budget, well, they have to eat it. That's their problem. But so far, so good. And everybody had high hopes and this thing had, what? 20 or so payloads on it and then they ran into trouble. Although the rocket, which was also a first did a great job, apparently. We've been waiting a long time for this one.

Sarah Al-Ahmed: Yeah, this was the first flight of the United Launch Alliance's Vulcan Centaur Rocket, cool name. But I wanted to ask you, how does this rocket compare to other commercial rockets that are on the market right now?

Mat Kaplan: First of all, this is out of ULA, which is that partnership between Lockheed Martin and Boeing that has been putting out Atlas and Delta rockets for a long, long time. And this is their sort of this is their new generation of rockets. We should have seen it fly a long time ago, but they've been waiting for the engines, which are built by, of all companies Blue Origin, a place you visited once, the BE for engine. It's the same engine that Blue Origin is going to use in their new Glen rocket which is even further behind. But finally they were able able to deliver these engines to ULA, they put them on the rocket and it worked really well. And eventually, this rocket will essentially kind of come in between the Falcon 9 and the Falcon Heavy in terms of what it can get up into space. In some ways, it's even a little bit superior at weight one way anyway, and how big of a payload it can take up in its fairing. It's even better than the Falcon Heavy for that. But the big difference is, this is not a reusable system. The Falcon 9, at least the first stage we've seen, I don't know, lots of those come home now. And this is a much more traditional booster. So once you use it, that's how it's gone. It burns up. But the military especially wanted to launch companies to be able to rely on and for years now, they've only had SpaceX, because ULA was retiring the Atlas and the Delta, they didn't want to be caught with just one company. So now it looks like the Vulcan Centaur from ULA is ready to take on whatever job they give it.

Sarah Al-Ahmed: That's some good healthy competition, we need to have more than one company to rely on if we're going to back to the moon.

Mat Kaplan: Yep. And eventually Blue Origin, if they ever do make the New Glenn, get up there as well on those engines, we might have three.

Sarah Al-Ahmed: Looking forward to that. The models that I got to see of New Glenn and the actual pieces on the floor when I was at their facility in Florida, were just so cool. But fingers crossed for them, we just need to give them some time, I'm hoping. But one of the things that was on board this launch that I was really looking forward to and unfortunately hit a snag was Astrobotic's Peregrine Lander. So what actually happened with that lander once it gotten to space?

Mat Kaplan: They're still figuring it out. What they do know is that there was a big loss of propellant. Apparently oxidizer, the liquid oxygen that so so many most liquid fueled rockets use, and without enough fuel to ease it down to lunar surface, that just wasn't going to happen. And so something happened, a tank burst, they suspect now in their latest update, because they've been sending these out that a helium valve didn't close completely. It over pressurize the liquid oxygen tank and bluee. And the interesting thing is they figured, okay, now we only have about 40 hours of propellant left. They keep extending that, they keep adding hours to that. And it's a real shame, because in every other way, the spacecraft seems to be working so well. All of those payloads they've been able to send power to them, they've been able to get data from them. And there's some really terrific payloads and of course, our hearts go out not only to Astrobotic, but to all of the groups, including NASA, and others who are behind these various payloads. But Astrobotic is not done. They knew that they're in it for the long haul. We heard as much when we actually had on Planetary Radio, John Thornton, one of the founders and the CEO of Astrobotic, about almost five years ago now. So I suspect that we're going to see another attempt in the Eclipse program from Astrobotic very soon.

Sarah Al-Ahmed: What do we think is actually going to happen with this lander in the next coming days?

Mat Kaplan: Well, it is going to get out as far as the moon and beyond, it's just the moon won't be there yet. I haven't seen yet what kind of final resting place or resting orbit they have in mind for Peregrine. But it's in deep space right now and pretty soon it will be reaching apogee of that orbit and swing back past Earth. I suspect that that's almost a sure thing, because they don't really have the propellant to do much else. And I don't think, this is just a guess on my part, I don't think we're going to see a lunar impact, because that would just be kind of uncool. But we'll see, I mean, may not have a choice, depending on their trajectory.

Sarah Al-Ahmed: Yeah. I mean, as sad as this whole thing is, I'm still really proud of everyone involved. This is arguably the first commercial lunar lander that we've attempted. You could say that Israel's Beresheet mission was an example. But that as well, didn't manage to make it to the moon safely. But the moon is hard and we're just beginning a new age of commercial lunar exploration. So as sad as this is, I'm not too surprised and not too devastated either.

Mat Kaplan: They're gonna get this right. Not just Astrobotic, but other companies are going to be getting this right, because it's just too important a goal. And we did this before people ever went to the moon with the surveyor spacecraft. So Gosh, darn it, we can do it again.

Sarah Al-Ahmed: Yeah, we are. And of course, all of this is part of our new push toward lunar exploration. We're trying to put humans back on the moon with the Artemis program. But we also have another update from the Artemis program that also isn't the happiest thing in the world. So what's going on there?

Mat Kaplan: So disappointing. I mean, I was looking forward to greeting those astronauts when they come back, five miles from where I live here in San Diego in 2024. Well now as you know, we're looking at NASA says at the soonest September of 2025. And that's fine. They've got good reasons for waiting. Several of them, I mean, you want your life support to work really well if you're going to send people out there. We know that the rocket can do it, the SLS. We know the Orion capsule can get out there just fine. But of course, on Artemis 1, the launch that you and I had hoped to see and missed, there were no people. So life support. But then I think you've also seen where they had some degradation of the heat shield on that Artemis 1 Orion capsule, that was kind of unexpected. So they want to figure that out. And I didn't know but you sent me something that said that they've also got a few electrical things to figure out. So yeah, it takes a while to get this stuff right. And when you've got people on board, you really want to make sure it's absolutely right. And, unfortunately, if you're going to push back Artemis 2, you got to push back Artemis 3. That first landing of a woman and a person of color on the moon and the return of men to the moon as well. And looks like that's now they're shooting for roughly a year later, September of 2026.

Sarah Al-Ahmed: Yeah, it's unfortunate, but honestly, I'm very grateful that NASA is prioritizing the safety of our astronauts. We've already proven that humanity can get to the surface of the moon and return, we want to make sure that we do it again safely. So I think this is the right decision even if both of us want to see this happen as soon as possible.

Mat Kaplan: You bet. But I'm going to hazard one more wild guess, which is that Artemis 3 probably wouldn't have happened in 2026 anyway. We need that big starship working perfectly if it's going to land the humans, those folks that are returned to the moon safely and get them back off the moon. And we're a little ways from that. I mean, the latest is maybe the next starship to lift off in February, so stay tuned for that. But that's a big jump from that, to getting people down to the surface of the moon and back up into space and home.

Sarah Al-Ahmed: We need all these pieces to work together in order to get us back there safely. But I know that we can do it. And we know the moon is hard. I mean, as the former US President, John F. Kennedy said in his famous pre Apollo speeches, "We choose to go to the moon, not because it is easy, but because it is hard." One of my favorite speeches of all time. So I'm very hopeful about the future of going back to the moon. Despite all of this, it's just really exciting to be in the midst of this moment in history. People are going to look back on these things and think, wow, I'm really glad they took the time. And look how cool it is that we've finally returned humans to the moon as an international collaboration.

Mat Kaplan: Absolutely right, that Artemis accord collaboration. That is as exciting as anything else about this return to the moon for me, and I'm right there with you, Sarah.

Sarah Al-Ahmed: Well, as sad as all this is, I think it's going to be great. And I'm really grateful to have you wanted to give us all an update.

Mat Kaplan: Thank you so much. My great pleasure, Sarah, as always.

Sarah Al-Ahmed: We actually got a little bit more news about the fate of Astrobotic's Peregrine over the weekend. According to their social media, the spacecraft is on a highly elliptical orbit that extends beyond the orbit of the moon and soon it's going to be swinging back on its way toward Earth. It's hard to fully anticipate what's going to happen next because of that propellant lake. But the company now predicts that the spacecraft is likely to enter Earth's atmosphere when it comes back our way. That means that it's probably going to burn up upon reentry. Our hearts go out to everyone who worked on this mission and all of its payloads. We know how much effort and love everyone put into this mission. Well, it's sad that Peregrine won't be making it soft landing on the moon. This is also part of the human journey. We're just going to keep trying until we get it right. Keep looking up. Now for our main subject of today, planetary interiors. I know what some of you Lord of the Rings fans are thinking right now. Don't dig too deep. That's how you get ball rocks. But you don't even know that half of it. Right now, you and I are on a ball of rock hurtling through space. We're shielded by this thin atmosphere enveloped in a global magnetic field that protects us from the sun and the theory of other rays from space. And none of that would be possible without the symphony of weird physics going on beneath our feet. Every world in our solar system from the terrestrial planet that we live on to the distant ice giants like Uranus and Neptune, are shaped by the conditions inside of their interiors. But some of them are way stranger than others. The real question is, how do you even probe what's going on inside of planets? Today we're joined by Dr. Sabine Stanley. She's a Bloomberg distinguished professor of Planetary Physics at Johns Hopkins University, and a pivotal contributor to NASA's Mars InSight Mission, which was designed to study the interior of the red planet. Her new book is called What's Hidden Inside Planets? Thanks for joining me, Sabine.

Sabine Stanley: Happy to be here.

Sarah Al-Ahmed: We talk a lot on the show about the surfaces of worlds and even their atmospheres. But almost all of that is actually dictated by what's going on underneath the surface so it's really wonderful to have an expert to talk to us about it.

Sabine Stanley: Thanks, I agree with you completely. It's so true.

Sarah Al-Ahmed: Your new book is called What's Hidden Inside Planets? Why did you feel compelled to write this book about the inner workings of worlds?

Sabine Stanley: I found that while I've been teaching and interacting with my students, when you talk to people about what's going on inside planets, I don't think they know first of all, how cool it is, how interesting some of the stuff happening down there is and how much it actually affects their daily lives. So I thought, you know what, having a book out there that's really written for the general public would be a good idea.

Sarah Al-Ahmed: And I think you did a really great job of this because it's understandable to such a broad range of people. People are just getting started, even me who spent my whole life studying these topics, learn some things that I really didn't know from this book.

Sabine Stanley: Thank you.

Sarah Al-Ahmed: How did you first get interested in studying the internal workings of planets?

Sabine Stanley: I like to say I have an origin story, in that and I talked about this in the book, I actually grew up in a town in northern Canada called Sudbury in Ontario, Canada. And it's actually a giant impact crater. So about one point a billion years ago, a meteor smashed into the surface of the earth and created a giant hole. And that created all sorts of resources that we'd like to use in order to build things and so forth to come up to the surface and made it a city for where mining became like sort of the main production of the town. So I like to think that I've been surrounded by planetary stuff my whole life, and maybe that somehow subconsciously affected my choice. But in reality, while I was kind of in high school, and an undergrad, and so forth, I just really liked science, I liked math and physics in particular. And I happened to run into some really great mentors that helped show me my path where I wanted to go and how I got into this field.

Sarah Al-Ahmed: Isn't that so funny? It's like I more broadly, just wanted to study the universe. But as I encountered people and mentors, I honed in on the things that I loved most. But I think I got most interested in the internal workings of planets, because I lived right on a fault line in California.

Sabine Stanley: There you go, yeah.

Sarah Al-Ahmed: So learning about how the earth works definitely helps me contextualize and maybe be a little less terrified of what was happening underneath my feet.

Sabine Stanley: Yeah, exactly.

Sarah Al-Ahmed: And your book takes us on a step by step from the formation of the solar system, all the way to the death of planets, but in a very human way. You put these little anecdotes throughout, and I just wanted to say I loved that little bit about you connecting with your sourdough starter during our collective COVID bread making era, I felt like that was very relatable.

Sabine Stanley: Thank you. Yeah, it was a little bit devastating what happened to the sourdough starter, but you learn and you move on.

Sarah Al-Ahmed: Clearly, learning more about the internal workings of planets is a challenge, because it's not like we can just dig underneath the surface all the way down to the core and count the layers along the way. And before I read your book, I didn't really know that after the space race came this mantle race. What was that all about?

Sabine Stanley: Yeah, absolutely. Also, I was a little bit familiar with it, but in writing the book, I got to kind of read a lot more about it. So you can imagine now we're at a time where there was a lot of competing technology innovation happening between the US and the Soviet Union at the time, this was kind of in the, you're thinking about '70s, that sort of timeframe. And recognizing that we people really wanted to know what's going on deep inside the earth, and that it's really hard to drill inside. So lots of the superpowers of the time decided to try to actually reach the next layer below the crust of the earth, which is the mantle. And it turned out to be much harder than even going to the moon ironically, going even that 10 to 20 miles depth to reach the next layer of the mantle is a real challenge. And so in the book, I talk a little bit about what groups tried to do this, what challenges were faced and why it never worked out. And we actually have not drilled down to the mantle of the Earth yet.

Sarah Al-Ahmed: The movie, The Core, lied to me.

Sabine Stanley: It's my favorite movie, by the way.

Sarah Al-Ahmed: Really? Actually it's kind of spectacular. But in chapter one, you say that your favorite layer of the Earth is actually the core. Why is that your favorite?

Sabine Stanley: Yeah, well, so my love the thing that I study is planetary magnetic fields. So how planets generate the magnetic fields and that all happens inside the iron cores inside, for example, the earth in some of the other planets, the cores are made of different things. But that's why I love that layer. It's also the farthest layer from us so in some ways, it's the hardest to study. So I love that challenge of figuring out what's going on. It's about 2000 miles below our feet here on Earth, and other distances on other planets. So yeah, so it's a little bit about the challenge and it's about all the really cool things that the machinations of the core actually create, that are so important for life, like our magnetic field.

Sarah Al-Ahmed: People commonly say we know more about space than we do about the bottom of the ocean, but like at least the bottom of the ocean, we can get to.

Sabine Stanley: Yes, fair point. Fair point.

Sarah Al-Ahmed: In a chapter two, you talk about how the solar system is like a family. It formed from the same cloud of gas and dust. But there's such a huge variation between all the different objects in our solar system. And this is an absolutely huge question so people who are curious will actually have to read the book to get the full answer. But can you tell us a little bit about how our solar system formed, and how that created the differences between the worlds orbiting the sun?

Sabine Stanley: Absolutely. So our solar system formed in a lot of the same way that other stellar systems out there form. You had a giant molecular cloud filled with sort of hydrogen gas and helium and a little bit of dust and it sort of got close enough together through some mechanisms. Some people think a shockwave from a supernova was the answer for our solar system, that the material started condensing in on itself gravitationally. So everything was attracted to everything, and just start getting closer and closer together. As long as you have a little bit of rotation in that process, which you're pretty much guaranteed will always happen, then you end up instead of forming just a star, you form a star or the sun at the center, and you form a disk of material around it that's rotating around the central forming star. And so that's how our solar system formed. But then that disk of gas and dust surrounding the sun, the proto sun early on, there was material in there, that also was attracted to itself gravitationally. And so things started to clump together. And as clumps got bigger and bigger, they were able to gravitationally attract other clumps, lots of collisions happened. And eventually, you end up in this situation where you've got these eight planets, a bunch of moons and a bunch of small stuff like asteroids and comets.

Sarah Al-Ahmed: And this is a question that was posed, actually by one of our Planetary Society members in our member community app. So all of us I'm sure familiar of those diagrams inside textbooks that show the way that these worlds differentiate over time into layers. That kind of look like job breakers in the textbooks. But this Planetary Society member, Phillip Shane from New York, wanted to know how accurate those are and what it would actually look like if you could like, say, cut the earth in half.

Sabine Stanley: So fundamentally, like if you squint and look from a far distance on a sort of first order, answer, yeah, those are accurate. We know pretty much what the radius of the core is, what the radius of the mantle is, and the cross. So when you see those cut out diagrams, they're pretty accurate. But when we start getting to some of the outer planets, for example, the giant planets like Jupiter and Saturn and Uranus and Neptune, we actually know a lot less. In fact, the Juno mission and the Cassini mission, So Juno went to Jupiter, still, they're getting lots of great data, the Cassini mission that orbited the Saturn system from about 2005 to 2017, that really gave us a lot of information about their interiors. And what we've learned is they're more complex, it's not as simple as three layer models. Instead, the cores what you would call sort of the rocky part inside Jupiter and Saturn probably mixes in a little bit with the outer hydrogen gassy layers, and so they're not as clean in the separation of different layers. So right now, people are spending a lot of time trying to understand how do you do that? How do you keep up a mixture of rocks and gas together for the age of the solar system?

Sarah Al-Ahmed: Yeah, we'll get into Jupiter in a little bit, because that was fuzzy core is very strange and interesting. But I was reading recently about these things that they're calling mantle blobs inside of Earth. I'm just personally curious, what is going on there?

Sabine Stanley: Right. So this is something we've known for a while. So from seismic data, we can figure out what the density of material is as a function of depth and location inside Earth's mantle. And what we know is that there are certain places where the mantle is a little bit denser, and other places where the mantle is a little bit less dense. There seem to be these blobs, you can call them, near the core mantle boundary, just kind of above where the core is. And people have been hypothesizing, testing theories about what they could be. And there are some ideas out there. For example, one thing we know is that the surface of the earth because we have plate tectonics, the outer layer of the Earth, the lithosphere, actually descends back into the Earth at subduction zones. So for example, along the Pacific Rim all the way around, we have subduction zones where one plate is descending back into the earth, and those plates actually descend all the way down to the core mantle boundary and they're little bit colder than the rest of the mantel, because they were at the surface for a while where it's colder. And if something's hold there, it's denser. And so we think that some of the blobs are due to that. But there also seem to be other blobs. And we don't know if they're denser, because they're a little bit different in composition, maybe they have a bit more iron, or something else that's a bit heavier, we don't really know. Maybe they're colder for some other reason. If something's colder again, it's a little bit denser. So people are trying to understand what those blobs are. A recent hypothesis came out in a paper a couple of months ago, actually, is that some of those blobs are actually the leftover of the body Theia, which impacted the earth to eventually create the moon. So we might actually have part of that original planetesimal that crashed into the earth to create the moon deep in the mantle of Earth.

Sarah Al-Ahmed: That's such a cool idea and a great validation of what we already thought might have been the creation story for the moon but I cannot wait to learn more about that.

Sabine Stanley: Absolutely.

Sarah Al-Ahmed: So in order to study the internal workings of planets, we have to use more indirect methods. And this is a huge question. So you don't have to go into super detail on all these, we'll get into it. But what are some of the ways that we actually study the internal workings of planets without having to drill into them?

Sabine Stanley: So I'll talk about maybe four of them quickly. So on Earth, the one that gave us a lot of information was seismology. So whenever there's an earthquake, waves travel through the entire planet, and the speed of those waves is completely determined by the material properties that have that they're going through. Just like sound waves depend on if they're traveling through air or water, same with seismic waves. So we were able to measure the times that seismic waves arrive on different parts of the earth and from that, figure out the speeds of the waves, and basically the structure of the interior of the earth. So that's how we learned we have a core, we have a solid inner core, and we learned about all that structure in the mantle like those extra dense spots. Then you can use gravity. So when we're used to doing problems in our, say, intro physics classes, everyone says gravity on Earth is 9.8 meters per second squared, let's say, right? The reality is, gravity varies. If you're walking on the surface of the earth, and you actually had a really good gravimeter or something to measure gravity, you had measured different values for G, for the gravitational acceleration. And that's because it really depends on what the mass is right below your feet. And so with very precise gravimeters, people can actually determine where there are density differences inside the earth. And again, that gives us information on all the different layers that we know of in the earth and what's going on. Then you have my favorite method, the magnetic fields, so in the cores of planets, if you have a really good electrically conducting region, and you have motions happening in that region, then motions in that electrical conductor can create a dynamo. So just like if you're on a bike, and you have a bike light, and you're pedaling can generate currents that then light your bike light, same thing happens in the core of the Earth, except its motions creating magnetic fields. So the magnetic fields generated in the core, and we can actually measure it at the surface of the earth or an orbit, and watch how it changes. And if we watch how the magnetic field changes, we learn a lot about what's going on inside the core of the Earth, and other planets as well. So those are three methods. And I'll say the fourth one is actually one that we don't think of a lot, but is really important, and that's samples. So there are two sources of samples for the insides of planets. The first is actually meteorites. Whenever meteorites fall to Earth, what we're seeing is part of the interior of another planetary body. It was a body that may have been big enough to have a core, for example. So we have, for example, iron meteorites, we have iron meteorites. We believe those come from the cores of broken up meteors that broke apart, so we can learn about the interiors of some of the bodies that used to be in the solar system. And just like we mentioned earlier that all the planets are essentially a family grew from the same stuff, as soon as you learn about one of the other ones, you learn about Earth as well. But we also have some samples from inside the earth. And that actually comes from diamonds. So a lot of people when they think of diamonds, they think you want a diamond to be really pure and not have anything in it. But geologists really want diamonds that have what are called inclusions in them. So when diamonds form in the mantle, they actually can sometimes trap material inside them. And because diamonds are so strong, they keep that material at the pressure it was and it can't change. So when the diamond comes to the surface in these sort of volcanic activities that bring diamonds to the surface, we can actually maintain a sample from deep inside the earth. And that's how we've learned for example, that there's water deep inside the earth is from water inclusions inside diamonds.

Sarah Al-Ahmed: That's so cool. It's like the the mosquito and Amber of-

Sabine Stanley: Exactly, yes.

Sarah Al-Ahmed: I've wanted to go back to meteorites for a moment because we know that they come from all over the solar system. A lot of them come from Vesta, some of them come from Mars, but how can we tell from their composition, where they come from, if we haven't been to that place to necessarily sample it?

Sabine Stanley: So if we have been to the place, then there are ways to do it. So for example, we know meteorites are from Mars, because we can ground truth it with measurements we have from Mars. If we haven't been to a place, then we have to use some more indirect information. So for example, we can sometimes use a technique called spectroscopy to learn about what the composition of the surface of an asteroid is, so maybe there's a telescope that can really get this information or a flyby mission that can learn what the material is made of. And then you can kind of compare that to what we see in a particular meteorite on the surface of Earth and compare that but it is quite challenging if you haven't visited the place already. Another way you can kind of get some information is if you happen to see the meteorite fall. So if you remember, recently, the Chelyabinsk meteorite that flew over Russia, and if you can actually see the streak of the meteor as it enters Earth's atmosphere, you can trace back the orbit of the thing, and you can figure out where it came from. So sometimes we get information about things that way.

Sarah Al-Ahmed: One of the missions that I'm most looking forward to arriving at its target is NASA's Psyche mission, which we think might be a metallic asteroid, but could potentially be the leftovers of a forming planet that never fully came together. How do you feel about that mission?

Sabine Stanley: I am, psyched about Psyche, let's put it that way. It's one of my favorite missions as well. I like to think of it as, we have some understanding now of rocky worlds, some understanding of gas worlds like Jupiter and Saturn, icy worlds. But now this is like a metal world. This is a world that's rich in metal. And I'd love to understand, first of all, what does the surface look like? What do craters look like if the body's made of mostly of metal? Could it have had a magnetic field in the past that could be maintained on the surface in the rock somehow? So I'm very excited for this mission to get to Psyche and tell us all about it.

Sarah Al-Ahmed: we clearly have similar interests because a couple months ago I got to speak to someone who literally takes iron meteorites and blasts them with projectiles to see how they produce craters because it's so cool.

Sabine Stanley: Nice. Yeah.

Sarah Al-Ahmed: We have this idea that the solar system is like a family and that by studying one world we can learn more about others. But there are these interesting inconsistencies. For example, you talk about the connection between magnetic fields and atmospheres, but if we use the earth as an example, we have a magnetic dynamo. We've got a strong global magnetic field and that allows us to preserve this atmosphere. But Mercury has a magnetic field, no atmosphere, and Venus has no magnetic field and literally an atmosphere is so thick it could crush and melt us. So, that's so fascinating and weird.

Sabine Stanley: Yeah, absolutely. And I think it's an important lesson that we have to be careful in planetary science about generalities. It is definitely the case that if you have a magnetic field, it influences the atmosphere like the ability of the planet to keep the atmosphere. But we don't know how much it influences it. We don't know if, well, our own solar system shows us that just having a magnetic field doesn't mean you guarantee to have an atmosphere. And having an atmosphere doesn't guarantee that you have a magnetic field. So we already know that it's complicated. And so I think we have to look at it and study it more carefully. And in fact, the Maven mission at Mars has been thinking about this a lot because we know Mars is a planet that had an atmosphere in the past and that that atmosphere got blown away and it seems to be related somehow, or at least temporarily around the same time as when Mars lost its magnetic field. So I think Mars is really the greatest kind of test planet for this whole hypothesis or trying to understand how does a magnetic field influence a planet's ability to keep its atmosphere?

Sarah Al-Ahmed: And that's an interesting question too. How does a magnetic dynamo die? Why did Mars' magnetic field disappear?

Sabine Stanley: Yeah. So it turns out for the rocky planets, we should be asking the question, why do any of them have magnetic fields rather than how do they die? How have they not all died, is probably a question we end up asking ourselves more often. So the key point is, in order to keep a dynamo going, you need to have convective motions happening in the core of the body. And if you look at a planet like Earth and you ask the question, how much heat is escaping Earth today and how much of that heat is coming from the core? Because it's the escape of heat that causes the convective motions in the core. But if you just do some simple math with it, it turns out that the amount of heat coming out of the core on Earth today is really close to how much heat it could actually just remove with what's called conduction. With just the transfer of heat through molecular vibrations, essentially. So you wouldn't have to have the fluid motions in the core. So we do know we have a magnetic field, so we know there are motions, but we're really close. We're almost on like the edge of where we could sustain a magnetic field. And you look at this for other planets and the same thing holds, it turns out to be easy to remove the heat from a planet just from that conduction process. So the fact that we have convection is in some ways telling us that there are some very careful details in the whole thing that are really important to think about. Because we do know that Mercury has a dynamo today, Earth does, we know Mars had one in the past. The moon had one in the past. So we have a lot of kind of test cases to look at this, but it's all about how these planets remove heat.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Sabine Stanley after the short break.

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Sarah Al-Ahmed: Another weird thing that existed potentially on Mars that no longer is there is plate tectonics. And I get this question from people a lot. Why is it that earth can have this plate tectonics that allows us to have earthquakes and things like that, but we haven't really detected that at this time anywhere else in our solar system?

Sabine Stanley: Yeah, it's a great question. So it is absolutely the case. I would argue there's very little to no evidence that there was plate tectonics ever on Mars. And we don't fully understand what the requirements are for a planet to have plate tectonics. So we can't say, and this is really important too, when we think about exoplanets, we can't yet say that if you're of a particular mass, a particular size or composition, you will have plate tectonics or you won't have plate tectonics. What we know helps us here on earth have plate tectonics is the following things. So our lithosphere, this outer rigid layer of the planet that makes up the plates is very thin compared to the size of the earth. And because it's thin, it's a little bit easier for it to move around on the surface and descend back in at subduction zones, for example, part of the cycle that is plate tectonics. Now, if you get to a smaller planet like Mars, turns out that smaller planets can cool faster and if something cools faster, the lithosphere gets much thicker. So because it's basically colder rock, that's what makes it the lithosphere. And so the lithosphere on Mars is much thicker compared to the size of Mars than say the lithosphere on Earth is compared to the size of Earth. So if you have a much thicker lithosphere, turns out to be very hard to get it to go back in into Mars. So we think that plate tectonics is easier on bigger planets, which might be one of the reasons Mars and Mercury don't have plate tectonics. And even Venus might, Venus is a bit smaller than Earth. Is Earth kind of just big enough to have plate tectonics. Is it going to be a lot easier on much bigger planets like super earth around other stars? That's a question I think we're looking at. The other really important thing is you have to have some sort of lubrication of the whole process. On earth, we think that water and other volatiles that we have in the mantle can help lower what we call the viscosity of the rock or basically make it easier for it to flow. And on other bodies, so in Venus for example, there's very little water left in the mantle. Most of the water escaped when the planet went through the greenhouse gas phase and it is in its current state. So maybe this is because Mars has less water in its mantle as well. So it's not as lubricated, it's not easy for those motions to happen.

Sarah Al-Ahmed: These are even more reasons to think that Earth is just such a special place in the universe. I'm so gratefully that this is the way it is now because we all get to be here and talk about it.

Sabine Stanley: Absolutely.

Sarah Al-Ahmed: And you got to learn a lot about the internal workings of Mars because you worked on the Insight mission. But what's interesting about what you wrote in the book about understanding the internal workings of planets using the seismic activity was that it doesn't just apply to terrestrial worlds. You can actually use this same kind of science to understand bodies that are made of fluid and gas, like even the sun, Jupiter or Uranus.

Sabine Stanley: Yeah, absolutely. And my favorite application of seismology to like a non rocky body is the fact that turns out that the rings of Saturn have waves in them that are triggered by density variations in the inside of Saturn. So that's kind of a seismology. We can use the waves that travel through the rings to study the composition of the interior of Saturn. So that's my favorite example of that.

Sarah Al-Ahmed: Even more reasons why the Cassini mission and its, look at those rings, it's just so pivotal. It's gotta be one of my favorite missions in the history of history.

Sabine Stanley: I agree completely.

Sarah Al-Ahmed: Well, let's actually talk about some of the internal workings of these worlds. Let's start with Mercury. Why does Mercury have such a giant core relative to its size?

Sabine Stanley: I think all the evidence that we have points to the following type of scenario, but we don't know for sure if this is what happened. It's possible that Mercury was much bigger when it started, so it kind of had the core that it now has, but maybe a much bigger mantle layer. So it was more like maybe a Mars sized planet or who knows, even an earth sized planet. And then it suffered a really large collision, a glancing collision with another body early in its formation that kind of blew off all of the rocky layer, the mantle layer. And so the reason there's so little mantle left is because it all got removed from the planet early on. And the reason we think that, well, first of all, it would explain why you have the the core as you do, but there are lots of simulations, computer simulations that happen of how planets form and we know that these types of collisions happen early on and that they can do this kind of thing. So that's sort of the current leading hypothesis I would say about why Mercury is such a iron rich planet.

Sarah Al-Ahmed: The solar system was a really wacky collisional place early on. Totally terrifying. I think it it impacted, if you'll forgive it, almost every world. But then in chapter six, you said something that actually kind of tickled me. You said that at some point you've called almost every world your favorite except for Venus. So my question is, what is your vendetta against Venus apart from the fact that it's just like Earth's evil twin?

Sabine Stanley: Venus is the worst planet out there. I will say this right now, it is absolutely the worst. Okay? So here's the issue. No, obviously I love Venus too, but as someone who studies the insides of planets and thinking about how challenging it is to study the insides of planets, Venus has basically said, "Nope, I don't want you to use any of the techniques you've been able to use for other planets. Nope, none of them are going to work here." So it's very non-cooperative basically. So I'll give you some examples. Seismology, we've now been able to do seismology on the earth, on Mars, on the moon. But you try to do seismology on Venus, you've gotta put a seismic station on the surface of Venus, which is an incredibly inhospitable place. It just won't work. So that's one challenge, so we can't do seismology. Okay, then you think, well maybe magnetic fields. Well, Venus doesn't have a dynamo today, so we can't use magnetic fields. Then you think, well, one other way we actually can learn about the interior structure of a plant is by looking at its shape from its rotation. So the fact that planets rotate, that they spin means that they're bulgy at the equator. So the distance around the equator is larger than the distance around, say the poles. And you can use how bulgy a planet is based on its spin rate to actually determine what the interior structure is like. So for example, Saturn is the bulges of all the planets in our solar system, and it tells us something about how the mass is concentrated inside. You try to do that for Venus, unfortunately, it spins so slowly that it basically has no bulge. So we can't use information from that either. So Venus is just, it thwarts us a lot, I will say that. It just does not want us to use any of our really ingenious techniques that people have developed to study inside the planets. Venus just won't let us do it.

Sarah Al-Ahmed: And another case where the rotation of a planet might've been impacted by an impactor, Venus doesn't even rotate the right direction. So everything it does is just thwarting us.

Sabine Stanley: Yep.

Sarah Al-Ahmed: But all the more reason why we need to send more missions there because there's so much we don't understand and it's literally right there just waiting for us.

Sabine Stanley: Yeah, absolutely.

Sarah Al-Ahmed: We've talked a lot about the similarities and differences between Mars and Earth, but as someone that worked on the Insight mission, what are some of your favorite insights from the inside of that planet, from this mission?

Sabine Stanley: So first of all, the fact that Mars is still tectonically active, I think was a really important insight. We had seen features on the surface that suggested that there was sort of fracturing might occur, stretching of the surface that would cause Mars quakes, but we actually detected them. So I think that was a major finding. But because of the Mars break we were able to detect, we were actually able to determine the structure of the interior. So one big finding that came from the insight mission was we actually now know how big the iron core inside Mars was. We had some estimates before, you can get an estimate if you think about what the mass of Mars is and some of those other techniques like gravity and other things. But seismology really nailed down exactly the size of the Martian core because we could see seismic waves bounce off the horal boundary in Mars. And the core of Mars is a little bit bigger than we thought it was. And in fact, because it's a little bit bigger and we know the mass of the planet, it means that the core has to be a little bit less dense, lighter than we thought it was. So how do you do that? You're like, okay, well we know the cores of planets are mostly iron mixed in with some nickel, but we also know there have to be some lighter elements. Even in earth's core, we know there's about 10% of things like oxygen, sulfur or silicon. But if you do cosmos chemistry stuff, you ask the question, when I'm building a planet, when planets are forming early in the solar system, what lighter elements would actually go into the core when the planet is differentiating, when the iron is sinking to the center? And it seems to be that Mars' core is so light that we can't explain it with our current geochemistry models, cosmos chemistry models for what light elements can go into the core. So something very strange is happening or we're going to have to refine some of our theories of Cosmos chemistry to understand how the core of Mars got so light.

Sarah Al-Ahmed: Well, it sounds like we need Insight 2.0. I mean, we still need to know more about what's going on with the heat transport within that planet because they tried, they attempted to nail that mole down underneath the surface and it didn't necessarily work. So clearly we need another one of these missions to learn more.

Sabine Stanley: Absolutely. And I think the whole situation with the mole really shows how hard it is to do these missions. When you're saying you're going to do something for the first time on another planet, you don't fully understand the material you're digging into, the properties of the material, nothing. It's such a challenging thing and I'm so impressed with the work that was done by the engineers and the scientists to try to get information to improve the moles descent into Mars and so forth. And it was just a really amazing experience to watch those people come up with these genius ideas on how to do this. And the good news is we still got information from the mole. It just wasn't the information we were looking for. We now know sort of the temperature structure in the upper part of just under the surface as opposed to say 10 meters down. But it was really amazing to see the amount of creativity that had to go into problem solving that whole issue.

Sarah Al-Ahmed: Mars is constantly throwing those challenges at us and watching teams over the decades find cool new ways to go around the fact that that rover wheel doesn't work anymore. I can't hammer this down to the soil. It's absolutely inspirational. But let's move on to Jupiter because I still have this burning question in my brain about its fuzzy core. We've been orbiting that world with the Juno mission taking gravitational ratings. But I always anticipated there would be some kind of like solid boundary, solid core, and then the rest of it would be fluid or gas on top. But it doesn't appear that way.

Sabine Stanley: Yeah. And there are two ways you could think that this might have happened. So when we think about how planets form, we usually think our sort of standard model for how Jupiter formed is that it kind of started like earth. It was this rocky body but it got big enough about 10 times earth's mass that it could actually gravitationally attract all that hydrogen gas from the solar nebula. And with that model, you would expect the separation, you'd have this rocky core in the center and then this gas layer on top. But what people have to think about and what people have thought about is the fact that, well that's true, but as soon as you put all that gas on top, the pressures and the temperatures inside Jupiter increased massively. So you're talking about probably millions of bars of pressure, millions of sort of surface atmospheres of pressure, tens of thousands of degrees temperature, and what happens to rocks when they're in contact with gases at those temperatures and pressures. And even before the Juno got to Jupiter, people had done experiments and theoretical calculations and shown that, you know what, rocks and gases kind of mix and they make their own kind of fluid together when you put them under those conditions. So maybe it's not surprising that over time it's almost like the rocky layer has dissolved into the gassy layer in the interior. Another possibility that people talk about is that just as with all the other planets, we know that impacts have occurred on Jupiter. So imagine you have a rocky body impacting into this gas giant ball over time. Some of that rock ends up in the gassy layers and maybe what we're seeing there is kind of the accumulation of that.

Sarah Al-Ahmed: So weird. And deep down inside there, most of Jupiter is made of hydrogen and helium, which are the most abundant chemicals in the universe, but you're putting them under these amazing pressures and we'll get into what happens with helium when we reach Saturn. But it's possible that within Jupiter there's this layer of what they call liquid metallic hydrogen. What is that and why is that so special?

Sabine Stanley: So it actually, most of Jupiter, I would argue, is probably liquid metallic hydrogen. So it's not even just a plausible layer. It's probably most of the planet. So here's what you want to think about. Something's a gas because the molecules in the gas are far enough apart and they have enough kind of kinetic energy that they can maintain in that state. But when you put something under pressure, you're basically squeezing it. And the more you squeeze it closer and closer together, eventually those molecules get so close together that they can form bonds. And so that's what happens as you go deep inside a giant planet like Jupiter. Eventually the hydrogen gets closer and closer together. And so the hydrogen, the protons, basically the nuclei of the hydrogen atoms essentially join the way they would in a metal. And so most of the interior of Jupiter is, I would say a fluid hydrogen planet that allows the electrons to flow around the hydrogen atoms. And so they have vitalic properties, and that means they're really good at conducting electricity and heat. And that's why, for example, that's the region where the magnetic field in Jupiter is created is in the metallic hydrogen region.

Sarah Al-Ahmed: And that would explain why when Juno got there, the magnetic readings were just off the charts. I mean, I recall thinking that is just way higher than I thought it should be.

Sabine Stanley: Yeah. Jupiter has the strongest magnetic field of any planet in our solar system.

Sarah Al-Ahmed: But then going out to Saturn, we have this idea that potentially inside Saturn it is raining helium.

Sabine Stanley: So take that exact same story I just told you about Jupiter and remember that all the gas in the giant planets, it's not just hydrogen. There's about 20 to 25% helium mixed in there too. And at the atmospheric type pressures and temperatures, so in the outer layers, hydrogen and helium mix very nicely. But you start putting hydrogen and helium under that high pressure when hydrogen becomes metallic, when it goes into that phase we just talked about, helium no longer likes to stay mixed in with the hydrogen anymore. And so the helium separates out. And if the helium separates out, just like if you had a mixture of let's say oil and vinegar that you had shaken or mixed enough to make a salad dressing, but you then you left it over time and they separated again, the helium weighs more than the hydrogen, so it's going to sink. And so that sinking process, which essentially happens is that droplets of helium form and they sink through the hydrogen to greater depths. The same thing might actually be happening in Jupiter too, but the layer where this happens might be much thinner than in Saturn.

Sarah Al-Ahmed: I love thinking about the weird ways that it rains on other worlds especially on Titan, that moon. But as we move out to the ice giants, it gets even weirder. And before we talk about that, what is it that differentiates these ice giants internally from gas giants?

Sabine Stanley: The ice giants are gaseous in their outer layers, just like the gas giants are, but they have much less hydrogen in helium than Jupiter and Saturn do, for example. So we think only about like, say 10 to 20%. The outer layer is actually a hydrogen-rich gas layer. There's a lot more of what we call icy materials, things like water, but also materials, other volatile materials like methane, ammonia, those types of chemicals that ended up being able to condense out into liquids and solids in the outer solar system while planets were forming because temperatures were so much colder. So those planets have more of those things. Now, we don't really know exactly how much water, ammonia, methane type stuff the ice giants have compared to say Jupiter and Saturn, but we think that they're the majority of those planets. So then not only do you have to think about what happens when hydrogen and helium get under high pressure and high temperature, but you also have to think about what happens to water when it gets under those high pressure and temperatures. What about ammonia, methane, mixtures of those things? Add some rock in there. Then what happens? So it gets much more complicated for the ice giants.

Sarah Al-Ahmed: And we have so much less information because we've only ever sent one space mission in the history of history past those two worlds. And it was just cruising, just cruising out into interstellar space. So we know very little,.

Sabine Stanley: Yes, we absolutely need more missions to the ice giants in our solar system.

Sarah Al-Ahmed: Yeah. And I know you're very passionate about this because your PhD thesis was about what was going on within these ice giants. So first I'll ask because I know that people have asked this a lot and are very interested in it. What is going on with the idea of diamond rain inside of these worlds?

Sabine Stanley: Yeah. When people think about what happens to these materials, this water, this ammonia, this methane. When you put them under high pressures and temperatures and what people who study these materials under high pressure and temperatures using computer simulations, what they've found is that the carbon that you might find in methane, for example, can actually separate out from the other materials when you're under high pressure and temperature. And so the pressures they would form you would actually get diamonds and those diamonds would then rain out of whatever the mixture is in. So that's how diamond rain forms. And it even, there are some papers out there that suggest it gets even weirder that there might be a layer near sort of the deeper parts of Uranus and Neptune where that diamond layer collects and is liquid. So there might be liquid diamond seas. And diamond actually shares one of the same properties that water on earth surface shares is that the solid phase is slightly less dense than the liquid phase. So there might even be diamond icebergs floating on the Diamond Sea inside Uranus in Neptune.

Sarah Al-Ahmed: I want to play that level in a video game. But I think the thing that's actually really interesting, I mean we love the idea of diamond icebergs, but what's really cool is what's going on with the water because You end up with this layer of ionic water. What is that?

Sabine Stanley: So in the outer atmosphere, water will be in like a vapor phase. It'll be in molecular like you would see in water vapor here on earth. But again, you ask the question, start taking that water, squeeze it together, what happens? The first thing that happens is that the H2O, so you're thinking you got two hydrogens and an oxygen they break apart, those molecules can break apart when they're under high pressure and temperature. So you might get some OH is flowing around and some H is flowing around. And those are ions, so they have a charge. So that actually makes a layer inside Uranus and Neptune that can conduct electricity, the ionic water layer. But then if you go even deeper, a different phase appears. So when you actually break up the hydrogen and oxygens entirely, the oxygens can actually form lattice themselves. So they almost, all the oxygen oxygens connect to each other and then the hydrogens from the water actually flow through the oxygen lattice. And so it's kind of like what a metal does, except a metal does like nuclei stuck together with electrons flowing through. Here you have hydrogen atoms or protons flowing through the lattice of oxygen. So we call this super ionic water and it's a new layer. It's been hypothesized to exist inside the ice giants. It's been theorized through computer simulations to exist in our computer labs. And recently it was actually created inside an experimental lab. So we know that supersonic water is a real phase of water

Sarah Al-Ahmed: That is so nutty ridiculous. What's also really interesting to me is, as you said just a little bit ago, water on earth when it freezes, expands, but there are these other forms of water ice deep out there in the outer parts of our solar system that actually don't operate that way.

Sabine Stanley: Absolutely. When something is frozen, it's made into a solid, it has a crystal structure. So it has some way that the atoms are bound to each other and water just is able to have all these different kinds of ways of binding to itself when under different temperature and pressure conditions. So that's why we have all these different phases of water you might've heard of, like ice three, ice five, ice seven, ice 11, ice nine, all these things are different crystal structures for frozen water. And so it's really interesting to think about what the properties of those are inside planets. So, and where we see this happen a lot, for example, is on the icy moons in the outer solar system. So moons like Europa, Enceladus, even Ganamide et cetera, they all have ice layers in their outer regions. And because it's so cold out there, some of these other different phases of water, solid water can actually occur there. So it's really interesting to start thinking about what happens when you have an ocean in between two ice layers of different phases.

Sarah Al-Ahmed: And that's another really cool thing that despite their distance from the sun, we can still have liquid water inside of these worlds even potentially all the way out to Pluto. And that is just bonkers.

Sabine Stanley: Absolutely. It makes me think that we really have to reconsider what we call the habitable zone when we start talking about exoplanets.

Sarah Al-Ahmed: I've been thinking about that a lot recently. Because it's quite possible that most of the habitable territory in our universe is inside of these water worlds and not on rocky planets like Earth, which are clearly pretty rare as far as we can tell.

Sabine Stanley: Absolutely.

Sarah Al-Ahmed: One more thing I wanted to talk about is one of my newest favorite moons in the solar system, Triton, which is orbiting Neptune. That one is really wacky because it didn't actually form around Neptune as far as we can tell. It's probably a captured Kuiper Belt Object. What do we think might be going on in there? We don't have a lot of information clearly because we haven't sent a mission to it.

Sabine Stanley: Right. So we caught some great pictures of Triton from the Voyager 2 mission when as it was going by Neptune. So we have some information there. And you're absolutely right that one way we think we understand how moons form is if you have a moon that's sort of orbiting in a direction opposite to the rotation of the planet or in a different plane from the sort of equatorial plane of the planet, then it was probably captured. Because it was doing its own thing too close to Neptune, then got trapped in the gravity field and now it's doing its own thing. But Triton, it's on this elliptical orbit, it's going around the planet in the wrong direction. And so we think that it probably experiences a lot of tidal heating. So when you're different distances away from the planet and the interior, you experience different gravitational forces with different distance from the body. So Triton is probably tidally flexing a lot, and that might be creating some heat in its interior and that might be activating some of the stuff we see on the surface. So we think, we see, for example, cryo volcanoes on the surface of Triton. Triton is a very important moon for us to go study in the future because there's a lot of activity happening there.

Sarah Al-Ahmed: And we could get into a whole tangent about all the things that vulcanism in our solar system can tell us. But what's really interesting to me is that we're just beginning on this journey of understanding the internal workings of the worlds in our solar system and out beyond our solar system are just thousands and thousands and thousands of worlds that we'll probably never be able to delve into their interiors. But by comparing the worlds in our solar system, we might actually get some understanding of how they work. And that is so cool that we're connected across these cosmic distances that way.

Sabine Stanley: Absolutely. And I would also add to that that just like we learned in our own solar system, there might be observables at the surfaces of some of these exoplanets that tell us about the interior. We could, in theory, measure magnetic fields of an exoplanet that would tell us something about the cores of those exoplanets and the fact that they're convecting and generating magnetic fields. We can see out gassing of the atmosphere on these worlds. Those atmospheres are created in the interiors of these bodies. So we have some information that we can use to help us understand what's going on inside exoplanets as well.

Sarah Al-Ahmed: There's so much we could discuss. Exoplanets the death of worlds, maybe what we have to look forward to in the future as we're exploring our solar system. But for that, our dear listeners are going to have to actually pick up your book. But I wanted to thank you for joining me, Sabine, this is such a fascinating topic and clearly we're just at the beginning of learning so much about how these worlds operate. So it's wonderful knowing that there are people like you that dedicate their lives to this.

Sabine Stanley: Thanks so much. This has been a lot of fun.

Sarah Al-Ahmed: Thanks. Wow, there are so many questions I wish I had time to ask her. We didn't even get into the magnetic multi poles on Uranus and Neptune, but thankfully Bruce Betts, our chief scientist is right around the corner. I'll ask him in What's Up? Hey Bruce.

Bruce Betts: Hey Sarah. Glorious festive day to you.

Sarah Al-Ahmed: Hey, glorious festive day to you as well. I had a really fun time talking with Sabine about planetary interiors. So I gotta put this question to you. What is your favorite wacky thing going inside one of the planets in our solar system?

Bruce Betts: That is a wacky question and I think it would have to be metallic hydrogen. It's just so weird the fact that it exists inside at least Jupiter and Saturn and just weird. I mean, I guess it's not that weird a concept, but it's so far from our reality of having hydrogen just and under high pressure and all the electrons just running free and just chaos. Chaos ensuing.

Sarah Al-Ahmed: I had no idea that liquid metallic hydrogen even existed until after I graduated with my degree. I don't know how that got past me, but it might be one of the coolest substances I've ever heard of.

Bruce Betts: Yeah, no, it's awesome. Would now be an appropriate time for a bone I have to pick with the naming of the world's and interiors of giant planets?

Sarah Al-Ahmed: Yes, please.

Bruce Betts: That was my forum.

Sarah Al-Ahmed: Give the tea Bruce.

Bruce Betts: No one's going to change what they do, but it makes me crazy, particularly is someone trying to like write, say children's books and explaining that now the Uranus and Neptune now for like the last 20 years or more are called ice giants. The ice isn't there right now. I mean they were like thin clouds in the atmosphere, but it sounds like it's big fall of ice. But the ice was there way back when it was forming. There were things like water and methane and things that you didn't have as much of it. Jupiter and Saturn that came together as ice, but then they turned into this hot smooshy ball. So the whole mantle, so called mantle of these planets is actually super hot, high pressure liquids, mostly water with other good, you know, methane and the like in there. But anyway, that's why it's confusing. They should be mushy, wet, moist, maybe moist planets.

Sarah Al-Ahmed: People would hate that though. I hate the word moist.

Bruce Betts: I know that's why I said it because it amuses me that people find that word just weird. I don't know. That's a whole other subject for a linguistic program.

Sarah Al-Ahmed: I always think of them as like slushy planets almost.

Bruce Betts: But they're not even slushy. I mean, my impression.

Sarah Al-Ahmed: You have top layers, but yeah, you're right. That's weird.

Bruce Betts: Definitely the top layers and the atmosphere, no question. We see ice clouds, methane ice for example, like in pictures from Neptune, from Voyager, you can see those. But anyway, you can take that or leave it or cut it outta the show, whatever you want.

Sarah Al-Ahmed: Would you have the same bone to pick with gas giants though? Because the entirety of gas giants isn't just gas.

Bruce Betts: Yes, but most of an ice giant, it has the same kind of atmosphere as well. So yes, that is a problem, but it's not as much of a problem for me because you do at least have the outer bunch of atmosphere that is gas. But that's a good point. Maybe we should figure out what to call them.

Sarah Al-Ahmed: But going back to Uranus and Neptune, I think what was really cool that I learned while reading Sabine's book is that inside of Uranus and Neptune, there's this layer of ionized water underneath that a layer of super ionized water, which is really weird, but this actually is part of what contributes to the magnetic field of these ice giants. I'm just going to keep calling them ice giants.

Bruce Betts: No, that's the official term. I purge my issue with it but that's what we call them. So you have the giant planets are the four, the gas giants are Jupiter and Saturn and the ice giants are Uranus and Neptune. So go for it, stick with convention.

Sarah Al-Ahmed: We'll stick with convention. But what I didn't realize, and I feel like I should've known this ages ago, is that for most worlds that have these global magnetic fields, they're a dipole. You got your north and your south pole, but on these worlds it looks like they might be multipolar magnetic fields. What does that mean and what would the consequences be?

Bruce Betts: End of all life on that planet, certainly. Now the dipole is your basic bar magnet concept. You've got a north pole and a south pole like the earth's magnetic field that usually are lined up roughly with the rotation axis, but not necessarily. And they're generated by a conductive fluid of some kind deep down moving around. And so the dipole is the simplest form of a magnetic field where everything comes from can be modeled as those two poles. And there's usually some type of complexity. But Uranus and Neptune are just, I mean they surprised a lot of people, if not everyone, I'm not sure it was everyone. But when Voyagers went by and it's like wow, they have weird magnetic fields. They're way off the rotational axis and they're shifted. So instead of going through roughly the center of the planet, depending on which planet we're talking about, they're shifted out by like 1/3 of a diameter roughly in terms of the pole. And then you got this multipolar thing, which is basically, we have a complex magnetic field. So it's not generating just something that can be modeled as a dipole, but you have to like have a north pole over there and another north pole over there in a south pole here in a south pole there to fit your model of what's going on. And it probably, as I understand it means you're generating the field from like significantly different locations in the planet and you've got these weird obs, they've got the fluidized ionized liquids in their melty goo in the mantle and that may be generating something. And then now there are these recent studies of most recently ice 19, ice XIX, which so water ice anyway. Water ice can be in a bunch of different forms. So they're just weird and messy and that's what makes them interesting.

Sarah Al-Ahmed: Man, that is so strange. We need more dedicated missions out there to go check out these ice giants because that is so, so wacky. And I'm glad that we have JWST because now we can see things like we could potentially study their Aurora and other things that are impacted by these magnetic fields, but without getting a closer, closer, closer look, there's a lot that we won't be able to learn.

Bruce Betts: This is true. Magnetic fields are limited by where you are as opposed to you can't just build a bigger telescope to detect your magnetic field. But although as you say, you can do secondary things like looking at Aurora.

Sarah Al-Ahmed: Super cool.

Bruce Betts: Aurora.

Sarah Al-Ahmed: You mentioned just a hot second ago that the naming of ice giants has created an issue for you when you're trying to write kids' books. And I wanted to let everybody know that you actually just released a new kids' book called Casting Shadows.

Bruce Betts: The Planetary Society and Learner Books release this. Yes, it is Casting Shadows: Solar and Lunar Eclipses with the Planetary Society. So it's our first book in a series that we're working together with Lerner Publications and it's targeted at roughly second to fourth graders. Lots of big pictures and descriptions, but that doesn't mean it won't be of interest to other grades and adults. But yeah, that's available and you can get it on Amazon or you can get it on the Lerner L-E-R-N-E-R, their website directly from them.

Sarah Al-Ahmed: I'll put a link to the book page on our website for this episode of Planetary Radio because we've got a major total solar eclipse coming up in the United States on April 8th. So now's a good time to like educate the kids about what that's going to be like because this is going to be so cool. All right, I think it's time

Bruce Betts: It's time for [inaudible 01:10:11].

Sarah Al-Ahmed: Wow. That was a long one. You flying around while?

Bruce Betts: Yeah, I was actually opening my notes so I had to make it longer. Although I'm going to talk about things that flew in the atmosphere, which is the names of the space shuttles. Take your pick. But the last one's always interesting because it's endeavor and it doesn't have the American spelling, it has the British spelling because as many people know, named after James Cook's first voyage far into the world where they went to observe a transit of Venus, so it's all relevant. But do you know what the others are named after? Bruce is named after me.

Sarah Al-Ahmed: Space Shuttle Bruce

Bruce Betts: Columbia, a lot of ships named Columbia, including the frigate in 1836 that they named it after Challenger, a Navy ship and discovery two ships because you can't get enough, including Henry Hudson's trying to look for Northwest Passage, which exists now, thanks global warming. Atlantis was named after a Woods Hole Oceanographic Institute ship, which I thought was interesting, kind of rather different than the others. 1930 to 1966, a sailboat that traveled more than half a million miles in ocean research. There you go. Oh, and then of enterprise, of course, the drop test only in atmosphere named oddly after the Star Trek enterprise.

Sarah Al-Ahmed: I guess it counts.

Bruce Betts: That of course was named after a long history of navy ships, US Navy ships named enterprise including carriers.

Sarah Al-Ahmed: That explains it though. I have always wondered why Endeavour wasn't spelled, I commonly misspelled space shuttle Endeavour. So that totally explains why I have that issue.

Bruce Betts: Yep. No, that's why because it was British Explorer James Cook on its maiden voyage in 1788. All right, everybody go out there, look up the night sky and think about vertices of your favorite regular polygon. Thank you and good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with some really exciting announcements from our space advocacy team. You can help others discover the passion, beauty, and joy of space science and exploration by leaving a rating or a review on platforms like Apple Podcasts. Your feedback not only brightens our day, but also helps other curious minds find their place and space through Planetary Radio. You can also send us your space slots, questions and poetry at our email, at [email protected]. Or if you're a Planetary Society member, leave a comment in the planetary radio space in our member community app. Planetary Radio is produced by The Planetary Society in Pasadena, California, and it's made possible by our members who aren't afraid to ask the deep questions. You can join us as we dream of diamond rain on distant worlds at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor, Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.