We’re learning more and more about the outer planets of the Solar System. Uranus and Neptune are ice giants, filled with water and other volatiles that we’d consider ice if it was here on Earth. What’s inside these worlds, and what could we expect to find across the Milky Way?
Gas giant (NASA)
The Realm of the Ice Giants (The Planetary Society)
The Skies of Mini-Neptunes (The Planetary Society)
Moment of Inertia (Hyperphysics)
VIDEO: Conservation of angular momentum (Khan Academy)
Escape Velocity (Hyperphysics)
Kuiper Belt (NASA)
As Planet Discoveries Pile Up, a Gap Appears in the Pattern (Quanta Magazine)
Mushballs explain missing ammonia on ice giants (EarthSky.org)
Scientists find strange black ‘superionic ice’ that could exist inside other planets (University of Chicago)
Studies of ‘amorphous ice’ reveal hidden order in glass (Princeton University)
Dynamo Effect (University of Oregon)
Observational biases for transiting planets (arxiv.org)
Hot Jupiter (NASA)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast. Episode 618: Ice Giants. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how you doing?
Dr. Gay: I am doing well. This is my favorite weekend of the year. It is Halloween and this year, I am the walrus, goo goo g’joob.
Fraser: Nice. Yeah, but you have a million costumes that you can just select from any point.
Dr. Gay: I do. I do. But, it’s a cold blustery winter’s day and dressing up as a walrus seemed the right way to stay warm.
Fraser: Yeah. I just wear a whole bunch of layers to stay warm here in the trailer, the Universe Today headquarters trailer. All right. So, let’s get into it. So, we’re learning more and more about the outer planets of the solar system. Uranus and Neptune are ice giants filled with water and other volatiles that we consider ice if it was here on Earth. What’s inside these worlds and what can we expect to find across the Milky Way as we find more? All right. So, I’m having trouble sort of explaining this. But, in my mind, Jupiter, Saturn, Uranus, Neptune, they’re all the same. They’re all just gas giants.
Dr. Gay: Nope.
Fraser: Or, at least that’s what we used to think.
Dr. Gay: Yes.
Fraser: But, that’s not true.
Dr. Gay: We were wrong.
Fraser: They are dramatically different. Yeah.
Dr. Gay: And so, now, we have gas giants, we have ice giants, we have rocky worlds, we no longer say “terrestrial worlds” because that’s a little solar system centric, not that that generally stops us. But, Saturn and Jupiter are these giant worlds with atmospheres dominated by hydrogen and helium that have densities that allow craziness to exist in their cores that we will go into in another episode. But, Neptune and Uranus, we’re now finding out, might be these really cool transition objects between super Earths and tiny Jupiters.
Fraser: So, if I took a sample of the sun with primordial hydrogen and helium, and I took a sample of Jupiter, and I took a sample of Saturn, and I put them all in front of a planetary scientist and let them all cool down to the same temperature, they would have a tough time telling which one was which. Right?
Dr. Gay: So, they would have – if you grab a big enough sample, they would have no real trouble telling them apart as long as they had a really good mass spectrograph –
Fraser: No, all they get is they get to smell it.
Dr. Gay: No, then they’re stuck. Well, maybe not. So, you have both hydrogen and helium making up the majority of all four worlds’ atmospheres. You have water vapor in all four atmospheres. But, you see it most in Jupiter’s atmosphere as whitish clouds. You have ammonia in all four atmospheres, but it happens to be at just the right temperature to make Neptune and Uranus blue, and it’s in slightly larger amounts proportionately, not because they necessarily started out with different materials, but because they held on to different materials.
Fraser: And so, I guess where I’m driving with this is that if I took a beaker of Uranus and put it beside a beaker filled with Jupiter, it would look very different.
Dr. Gay: Yes, if you looked closely enough. We benefit from being able to see such a large amount of the atmosphere.
Fraser: But, I’m not talking necessarily about the atmosphere specifically. I’m just talking about the composition of what they’re made of internally.
Dr. Gay: Oh, yeah. Yeah.
Fraser: Not their surface. Like, just inside, it’s water, and ammonia, and methane, and stuff as opposed to Jupiter, which is hydrogen and helium.
Dr. Gay: Helium. Yeah. And, this is where I have to say I have a whole lot of bias because I really enjoy painting, and these are four worlds that I have spent a lot of time trying to figure out how to paint. Saturn’s easy. It’s like big, beige circle with stripiness.
Fraser: But, you’d want to breathe Jupiter and you’d want to drink Neptune.
Dr. Gay: Fairly accurate. Fairly accurate. I’m not sure I’d want to smell any of the four of them.
Fraser: No. I don’t think you’d want to do either of those things that I just mentioned. But, if you saw them there, that would be instant. The point just being how dramatically – like, I just feel like I need to deprogram childhood Fraser from the the “there are four gas giants in the solar system” education.
Dr. Gay: So, let’s take it back a different way. Atmospheres, you have less hydrogen and helium in the smaller ones because they’ve lost it. You have more cool molecules, but their cores are radically different. Neptune and Uranus probably have just regular rocky cores that if left alone would be called a super Earth. We’ll get to that later. Whereas Jupiter is just shy of nuclear reactions going on. We’re talking seriously dense hydrogen and helium in its core, and Saturn, well, a lot further away. No one would ever accuse it of wanting to have nuclear reactions. It’s still not exactly something that’s rocky in its center the way we think of rocks.
Fraser: Yeah. Yeah. So, I guess how did we learn what’s inside the ice giants?
Dr. Gay: A whole lot of computer modeling combined with information on moments of inertia. One of the worst things to try and understand in first year physics is moment of inertia. It’s that characteristic in a rotating body that says how much energy it takes to rotate. With an ice skater, you have conservation of angular momentum. And so, when they throw their arms out, they increase their moment of inertia and they slow down. And when they wrap their arms around themselves, they reduce that moment of inertia and are able to spin all that much faster.
And so, when we fly a spacecraft past one of these bodies, with a whole lot of super complicated mathematics that I look at with dread and foreboding, we are able to take how these spacecrafts’ motion is impacted by its passage near our world and reverse engineer what the moment of inertia is for that world. This is also how we figured out that the moon’s core is actually off-center. We figured out Jupiter has a fluffy inside. It all comes down to what is the moment of inertia of a planet and what physics allows you to get to the density distribution that explains the total mass of the object, the total volume of the object, and the angular momentum.
Fraser: So, why? Why? Not the moment of inertia, but why are the ice giants so different from the gas giants?
Dr. Gay: It all comes down to mass. One of the really cool things we’re starting to understand is as you grow the core of a future world bigger and bigger, it gains increased ability to hold onto an atmosphere. And whether it keeps it or not has to do with what kind of magnetic field does it have, what kind of attack does its sun institute upon that atmosphere, and as we transition from worlds like Earth that are dominated by rocks with a thin skin of atmosphere, to worlds that have a rocky core and a massive atmosphere, what we’re seeing is if you go too small, atmosphere gets blasted away. If you just go slightly bigger, it gets to hold on to its atmosphere but only to a point.
Those hydrogen and helium and other super lightweight gases are still going to be able to ricochet off bigger particles, ammonia, things like that, and in the process, they’re gonna get sent off at escape velocities. So, it’s they have a big enough core to hold onto the atmosphere, but not so big that they get to hold on to all their lighter weight elements.
Fraser: And, they’re out almost to the Kuiper belt, this region that is largely dominated by watery objects. So, it does make sense. Now, you’re clearly fascinated, borderline obsessed I would say with the atmosphere, with the outer layer, the cool swirling storms. So, just let me know what you’re thinking. You clearly have a lot that you’re pretty excited about. So, what is so cool about their atmospheres?
Dr. Gay: Well, their atmospheres, other than being my favorite color, being able to show seasonal storms that are both light and dark in color, depending on what their composition is. These atmospheres allow them to basically be on just the other side of the mass gap in planets from super Earth, and there are now theories for planet formation saying the reason we have this valley of not that many planets between these super Earths and the ice giants is because there is a range of masses that if you form within this range of mass with your big beautiful atmosphere, your sun’s gonna go, “No. You may not keep that,” and is going to blast that atmosphere away.
And, this starts to imply that worlds like Mercury might have formed with significant atmospheres and just had them blasted off. And, I love this idea that rocky worlds are the failed cores of ice giants where had they just gotten bigger, they could’ve been an ice giant, whereas Jupiter-like objects are failed stars. So, it feels like we’re a solar system of failure orbiting a star.
Fraser: Well, except for the ice giants which are perfection.
Dr. Gay: Exactly. Exactly.
Fraser: Yeah. That’s awesome. So then, if you could float in the cloud tops of Uranus or Neptune, what would it look like?
Dr. Gay: So, the cloud tops, you’re looking down on this thick blue atmosphere, and the cloud tops are just gonna be, like, flying an airplane above the clouds here except a bit bluer, by which I mean a whole lot bluer. But, if you descend through the clouds, that’s when cool stuff starts happening. I live in the Midwest. We have a lot of hailstorms. I was once giving a talk, and during my talk, a tornado and hailstorm hit the building I was in, but luckily the auditorium was the tornado shelter. So, while trying to give this talk, there were apple-sized chunks of hail hitting the thing and making the sound like massive snowballs falling apart on the roof. That sound is something I will never forget because I’ve never experienced hail that big before.
Fraser: That’s madness.
Dr. Gay: And, there are ammonia versions of this in the atmospheres of Neptune and Uranus, we think.
Fraser: I wonder how big. Do we know how big these mushballs are?
Dr. Gay: So, it’s the early days of the modeling. I’m afraid to jump at that number. But, the way it’s described, think of the size range you imagine for snowballs, and you’re looking at ammonia mushballs.
Fraser: Right. And, what are they made of? Is it pure ammonia? Ammonia, and ice, and other materials?
Dr. Gay: It’s ammonia and other ices, and it’s this combination of different things that freeze at different temperatures that give them their mushball characteristics.
Fraser: And so, if you were flying in that airplane through this atmosphere above the cloud tops, there would be mushball hailstorm –
Dr. Gay: Beneath you. Do not dive through those clouds.
Fraser: – beneath you forming.
Dr. Gay: Do not dive through those clouds.
Fraser: Yeah. If you go down deep enough, you’re gonna be getting hammered by gloopy ammonia methane water mush. It’s crazy. So, we’ve talked about sort of the outer atmosphere, the beautiful blues and greens that you love to paint, the cloud tops and so on, but, there’s some pretty interesting research that just came out fairly recently. If you go deeper, what’s down there? So, what happens if you did decide you were gonna brave through the mushballs and try to make it down towards deeper into the planet, what would you see?
Dr. Gay: So, I have to admit I saw this paper come out, and we just finished our 36-hour hangout-a-thon. And so, it’s been a harried week. And, it talked about super ionic ices, and those were a whole lot of words were I decided I would figure that story out this weekend. But, it sounds like you’re a step ahead of me.
Fraser: Not too far. But, the gist is that under immense temperatures and pressures, what would be water behaves in really strange ways. It’s like the pressure could be so high that what would be liquid water, because it’s very warm, is crunched into a kind of ice. You get a lattice structure the way you get with ice. And so, the theory is that as you descend down through the inner material of Uranus and Neptune, you go through different unusual phases. And the new discovery is that researchers were able to create some of this in the lab. So, yeah, they were able to essentially use a diamond vice to make a droplet of water behave in this super ionic way that is thought to be in the centers of Uranus and Neptune. And in fact, not only is it probably in these, but it’s probably the most likely form of water in the universe.
Dr. Gay: Oh, wow. So, this gives us three different phases of ice. There’s the anomalous ice which it flash-froze before crystals could form. There’s the formed more slowly and is at higher temperatures normal crystal in ice, and then under sufficiently dense conditions, you get super hot, super ionic ice. What can’t water do?
Fraser: And then, right. Well, I’m about to tell you. So, I guess I’m about to add a thing that it can do. And so, the other piece of research that I’m sure you’ve seen is how this relates to their magnetospheres.
Dr. Gay: I have not.
Fraser: So, the other big piece of research – it’s been like Uranus and Neptune week. But essentially, this ice, moving, shifting behaves essentially like metallic hydrogen inside Jupiter the way the dynamo effect works inside the Earth with a rotating iron and nickel core. And so, you end up with its ice that is generating the magnetospheres around Uranus and Neptune while it is metallic hydrogen at Jupiter and metal on Earth.
Dr. Gay: What that means is the electrons are able to flow freely between the different molecules of water just like they flow freely through wire. And, that is truly bizarre because while ice on Earth isn’t exactly an insulator, it certainly isn’t something I’d use to wire my house.
Fraser: No. You know, you don’t want to be in the water when a lightning storm is happening because you can be electrocuted. So then, we’ve got Uranus and Neptune in the solar system. But, are they outliers?
Dr. Gay: No.
Fraser: Are they what’s normal across the Milky Way?
Dr. Gay: So, this is why they’re so exciting to me. Forget all that chemistry. The universe is hydrogen, and helium, and metals I maintain as an astronomer. Now, what is so cool about Uranus and Neptune is ice giants like them seem to be about the most common kind of planet that we are finding. And, part of this is observational bias. We are not yet able to, as consistently find smaller rocky things. And, ice giants are what you get before you get the smaller rocky things, and there is a gap that we have mentioned in the mass between these two. So currently, as we look out there, there’s plenty of hot Jupiters. There’s plenty of cold Jupiters. There’s plenty of Jupiters. They exist. They’re out there.
Saturns are out there, although, we haven’t yet clearly defined anything with ring structures. But, we have found things of that mass. But, the mass of Uranus and Neptune, that mass range seems to be the sweet spot for creating planets that we have been able to discover. It’s highly possible that as we come out the other side of this mass gap where objects have their original atmosphere blasted off of them that we’re going to find there’s an enormous number of super Earths. And, I just love this idea that it goes from ice giants to super Earth, and there’s this whole range of objects in between that wanted to be ice giants and instead just lost that atmosphere.
Fraser: So, they call these things mini Neptune’s, right?
Dr. Gay: Yeah.
Fraser: You got super Earths, you’ve got mini Neptunes. And so, we can expect to find an object that is four times the mass of the Earth and in some cases has all that similar ice, not necessarily the same pressure. But then, you’re gonna have the ones that were too close to the sun and then had all that blasted away. So, if you took Uranus and brought it into the inner solar system, say, put it around the orbit of Venus, would it turn into a super Earth?
Dr. Gay: I’d have to do the mass.
Fraser: Right, how much material’s in the core.
Dr. Gay: Yeah. And, not only that, but just at what distance does the sun become dangerous. You couldn’t bring it in as close as Mercury, I don’t think. But, exactly where does it decide it can no longer hold on to that atmosphere? That is the word for a computational person who does planets and studies the stuff I do.
Fraser: But, it’s thought. I guess what you’re driving at is it’s considered that that’s the process, that you get a super Earth by blasting away the material from a mini Neptune or a Neptune.
Dr. Gay: Yes. Yes.
Fraser: Amazing. Yeah.
Dr. Gay: It’s awesome.
Fraser: And so, to think that we could have a different composition of planets if they were shifted to different locations in the solar system.
Dr. Gay: And, as we start finding objects further and further from their stars and as we start finding objects that are smaller and smaller at greater distances from their star, it’s gonna be interesting to see just how the distribution of planet sizes varies with star size without the observational bias we’re currently dealing with.
Fraser: Yeah. I mean, it seems like the universe is filled with hot Jupiters.
Dr. Gay: Yes.
Fraser: Which is not – we can’t assume that that’s what’s really out there.
Dr. Gay: No.
Fraser: Just like the night sky is filled with super hot, very bright stars that we see with our eyes.
Dr. Gay: Right. Not reality.
Fraser: That’s not actually the way – that’s not reality. Yeah. Awesome. All right. Well, thank you so much, Pamela. Oh, one last question. Since the ice giants are such absolutely fascinating places, I’m assuming there’s all kinds of missions planned to return and explore them?
Dr. Gay: No. No. But, yeah. No.
Fraser: That’s it. That’s it. That’s all we’ve got.
Dr. Gay: We really want to, but no, instead of sending three spacecraft to Venus – and I don’t begrudge Venus its missions, but, three?
Fraser: Something had to go and go with the mission to Triton. Yeah.
Dr. Gay: Oh well.
Fraser: So, currently, officially, right now there are no concrete plans for anybody anywhere to send any mission at all to Uranus or Neptune?
Dr. Gay: That I know of.
Fraser: Or that I know of.
Dr. Gay: Yeah.
Fraser: That sucks.
Dr. Gay: Yeah.
Fraser: It’s so cool.
Dr. Gay: And, part of the problem is it’s slow science and it requires a ton of energy, and the planets aren’t exactly where you want them right now.
Fraser: All right. Well, thank you, Pamela, and we’ll see you next week.
Dr. Gay: Bye-bye, Fraser. And –
Fraser: And you’ve got some names for us.
Dr. Gay: – before I say goodbye to all of you, I want to thank all of you that contributed during our 36-hour hangout-a-thon, and I want to thank all of you who were there month after month supporting us through Patreon allowing us to know this program can plan ahead. And this week, I want to thank Adam Annis-Brown, Frank Tippin, Alexis, Helge Bjørkhaug, Ben Leiberman, William Baker, WandererM101, William Andrews, Jeff Collins, Harald Bardenhagen, Scott Bieber, David, Alex Cohen, Matthew Horstman, David Gates, Marco Iarossi, Phillip Walker, Randa, Nial Bruce, Matthias Heyden, Justin Proctor, Paul L. Hayden, Jeff Willson, The Lonely Sand Person, Gregory Singleton, Tim McMackin and Paul D. Disney.
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Fraser: And you can and you’re not strapped for cash.
Dr. Gay: Yeah, we totally understand a lot of you out there listening do not have the funds. But, if the average donation size was just $5 from all of you out there, we’d have no more funding issues. So, please just consider giving if you can, and if you can’t, we get it. And, if you really can, maybe give a little extra for that person who can’t. We’re gonna be here as long as we can and we’re so grateful for all you’re doing for us.
Fraser: Thanks, everyone.
Dr. Gay: Thank you.
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