What in the world is metallic hydrogen? What does it even mean? Where does it exist in nature, and can we make it in the lab? I discuss these questions and more in today’s Ask a Spaceman!
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Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know what time it is. It's time for Ask a Spaceman. I'm your host, Paul Sutter. You've got questions and I've got answers. You go online to Twitter or Facebook, use the hashtag AskASpaceman, and I will take those questions and I will answer them eventually. You can also email me at AskASpaceman at gmail.com or visit the website AskASpaceman.com, kind of convenient, or visit my YouTube channel, youtube.com slash PaulMSutter.com. You can ask questions all over the place. We have one simple goal with this show, complete knowledge of time and space. And on the road to complete knowledge of time and space, we have today's questions. We have Tom S. and at Abguntha via email and Twitter, respectively, asking about metallic hydrogen with several question marks after that. We have Andres Del C via YouTube. Could you please talk more about liquid metallic hydrogen? And we have Colin E via email. What's inside Jupiter and how did it get there? Metallic hydrogen. Okay, I need to take a few steps back.
Let's talk about phases of matter. Good old phases of matter. We're used to thinking of things very simply. Solid, liquid, gas. And transforming from one phase to another is very easy. Like take water. Take some solid water, aka ice, add some heat to it, add some temperature, increase its temperature, and it becomes a liquid. Then add some more heat and it becomes a gas. Okay, so that's pretty easy. If I want to take a material and change its phase from one thing to another, I just need to add heat or subtract heat. But pressure also gets to play in the phase changing game. So let's take room temperature water, which is a liquid, and stick it in a vacuum. Put it up in space. Put it in a vacuum jar and suck out all the air. What happens? It boils. It becomes a gas. So same exact temperature. Same exact room temperature water. Water that would be neither pleasant nor disgusting to drink. Just kind of meh. In a vacuum, it becomes a gas. And so we need to play both the heat game and the pressure game in order to change phases.
And we map this out using what we call phase diagrams. These phase diagrams say, okay, if I'm at this temperature and that pressure, I will have this state of matter. And every element or molecule or material gets its own unique phase diagram. So I'm Hydrogen will have a different one than hydrogen combined with oxygen to make water. And that will be completely different than oxygen by itself. And from carbon and lithium, everyone has its own phase diagram. Everyone has its own response. to temperature and pressure. And the phases in a diagram are like the countries, are like countries on a map, where there's boundary lines. And if you have this temperature and pressure combination, then you're within this boundary, you're within fluid land, and you obey the laws of the physics of fluids. But if you have another combination of temperature and pressure, you're in a different section of the map. You're in Gazistan. And you have to obey the ideal gas laws. And usually they're very strict boundaries where you can travel along this diagram just like you travel in a country and then you hit a border and you go through border control and then boom, you're in a new country with a new set of laws.
And that's what we see as changing faces. You're a liquid, you're a liquid, you're a liquid. Boom, you hit a border in temperature or in pressure and you keep adding heat or you keep adding pressure or keep removing heat, keep lowering the pressure or whatever. Then boom, you cross that border and now you're a gas. But there are some interesting places in these maps, in these phase diagrams. There are no man's lands. There are places that don't follow the usual rules of solid, of liquid, of gas. And it's usually when you go outside our normal everyday experience that these strange things happen to these materials. I think these strange things are properties of the materials themselves, but we only get to see these properties under extreme conditions. We don't normally see how water might behave or hydrogen might behave in this weird way until we really crank up the temperature, really lower the pressure. Then interesting things get to happen. It's like international waters. Like if you really crank up the temperature and pressure, these definitions of solid, of liquid, of gas get really, really fuzzy.
And if you're out in international waters, well, what rules apply to you? You're not really in any country. So who do you obey? How do you govern yourself? An example of this is what's called the critical point when it comes to water. There's a certain temperature and pressure that if you increase the temperature and increase the pressure beyond a certain point, water is no longer a liquid and it's no longer a gas and it's definitely not a solid. It's its own thing. It's its weird mix of liquid and gas properties. It kind of has some properties of gas and some properties of liquid, but But then some properties are missing, so it's its own thing. And if you only get there by cranking up the pressure and temperature past a certain point, what's called the critical point. Hydrogen has its own phase diagram. We usually don't get to experience almost all of that phase diagram. Under normal conditions here on the surface of the earth, with everyday temperatures and everyday pressures, it's almost always a gas.
If you find some hydrogen and it's by itself, it's a gas. It's just going to be a gas. You can heat it down to zero Celsius or heat it up to 100 Celsius. Nope, still a gas. Very low pressure, very high pressure, still a gas. If you cool it way down to 33 Kelvin, that's 33 degrees above absolute zero, it does become a liquid. And if you cool it down below 14 Kelvin, so anything less than 14 degrees above absolute zero, it becomes a solid. So you can make liquid heat hydrogen and you can make solid hydrogen. It just has to be really, really cold. But that's at typical normal everyday pressure. So if you keep the everyday earthly pressure, one atmosphere, say, and you lower the temperature, then you get to liquid and a solid. If you crank the pressure up, you can raise the temperature and you can actually get back a hydrogen solid. So there is solid hydrogen at high temperature, but that's only at very, very high pressure. And hydrogen has these particular properties because hydrogen doesn't like to be by itself.
Hydrogen is just a single proton with a single electron in a shell around it. That's it. That's all it takes to make a hydrogen atom. And a single proton with a single electron is like the most eligible bachelor or bachelorette in the elemental world. It'll just hook up with anybody. Doesn't care. Usually, it just pairs up with itself. So we usually get hydrogen as twins. Hydrogen, in our normal experience, is diatomic. It merges with itself. It combines with itself to make molecular hydrogen that is much more stable, much less reactive, just likes to hang out by itself or with itself because it's a twin. All right. And so when you encounter hydrogen gas, you're actually seeing diatomic hydrogen gas. Hydrogen molecules are paired up together. When you cool it down to make a liquid, that's diatomic hydrogen as a liquid. Freeze it so it's solid when you're below 14 Kelvin, and that's diatomic hydrogen frozen solid. But if we're devious enough, if we're crazy enough... We can split those bonds.
We can break diatomic hydrogen apart. And we can get monatomic hydrogen. All right, it sounded, it looked cooler when I wrote it down in my notes, and it didn't sound that great, but we're just going to roll with it. We can play some games where we can split that hydrogen apart. What does it take? It takes Patreon. Go to patreon.com slash pmsutter to learn how you can support the show and all the education outreach activities I do. There's way more details on the Patreon page itself. I can't thank you enough for all of your generous contributions for this show, but I'm going to try again. Thank you so much. That's patreon.com slash pmsutter. What does it really take to split hydrogen apart so we can see the bare elemental hydrogen? It takes incredible temperatures and or incredible pressures. And if you just take a hydrogen gas and you really heat it up and you keep the pressure low, you're going to get a plasma where the protons and the electrons separate from each other and you just get a thin, hot, soupy mix of protons and electrons.
Like the sun, the interior of the sun is this hot, soupy mix of protons and electrons. It's a plasma of hydrogen. What if... You increase the temperature and you increase the pressure. What if you did both? So you get a plasma, a fourth state of matter, if you increase the temperature but you let the pressure go. What if you have both high pressure and high temperature? Well, past a certain combination of temperature and pressure, the bonds between the paired hydrogen are forced apart and and you get individual hydrogen atoms, aka protons, and they float around. What do individual protons floating around look like? It looks like a liquid. But this is a very different kind of liquid than the cold temperature liquid hydrogen. That one at 33 Kelvin was still diatomic. It was still two hydrogens glued together, paired up, fluiding around, doing whatever a fluid does. This is a very different beast because the bonds have been broken. Because the hydrogen has been ripped apart, now it's just single protons and single electrons floating around.
That's going to behave differently than the low temperature version, we think. I'm saying we think because this is largely theoretical. I'll get to the experiments in a bit. You need, in order to make this happen, this weird state of hydrogen, you need at least one million times sea level air pressure and preferably, you know, three or four million times sea level air pressure. And you need at least around a thousand Kelvin to do the trick as well. We think hydrogen will behave this way because we understand the chemistry, because we understand electrostatic forces, because we understand quantum mechanics. We can predict that this is how hydrogen ought to behave at high temperature and high pressure. Let me talk about this some more before I get to the experiments. The best word I can use to describe the state of hydrogen at extreme pressures and extreme temperatures is weird. Not only are the protons separated from each other... because of the extreme pressures, but the electrons are forced out of their shells and they also float around.
So you get a soup of protons and electrons like the more familiar plasma, but with insane pressures. And it's so different from a plasma that we give it a new name. We call it metallic hydrogen. Why metallic? Well, think of a metal. How would you define a metal? Is it hard, shiny, a good electrical conductor, probably dense, doesn't have to be dense, but typically dense? Metals, and most elements, by the way, are metals, have these properties because the atoms or the molecules that make up the metal, they link up, they link together to form a lattice. So their electron shells overlap, right? They link together, and each atom will have a few electrons left over. So they'll use some electrons to form the bonds, the linkages between them, and then there'll be some electrons left over that just float around and just hang out. So you get a lattice, a structure of ions, that's the atoms that are linking up together, and then these are embedded in a sea of free-floating electrons. And it's these free-floating electrons that give the metal most of its properties.
That's what makes it shiny, that's what makes them good electrical conductors. And if you melt down a metal, you get a liquid metal. You can take gold and melt it down and it's still gold. It's just the liquid phase of gold. And it still has all the essential properties, except for being hard. But it has a lot of the essential properties. There's still these linkups of the gold ions. There's still free-floating electrons. So you can have liquid metals. The concept of a liquid metal isn't that crazy. And it's the properties of those free-floating electrons that make a metal a metal. So here we have hydrogen at high temperature and high pressure. And hey, look at that. The electrons are floating around. That's a metal. It's probably shiny. It's probably a good electrical conductor. It's probably pretty dense. That's not so bad. All right. Metallic hydrogen. It turns out if you take hydrogen and subject it to high temperatures and pressures, it forces apart the atomic bonds. It forces apart the electric bonds and you get a new soupy thing that kind of sort of looks like a metal.
It's not the only element that does this. There are things like carbon that when you subject it to high temperatures and high pressures may not normally be a metal will start acting like a metal. And so that's not really the crazy part. Here's the crazy part. A normal medal, like a bar of gold, that you can donate to patreon.com slash bmsutter. When you pick it up and say you squeeze that bar of gold, you pick up that bar of gold and you want to squeeze it. It resists you. All right. It's pretty tough. It doesn't like being squeezed. What is resisting you? Well, it's those it's those ions. It's the atoms and molecules that are all linked together that have formed this lattice. They don't like being squeezed any more than they already are. So they share electrons so they can link up. But that's as close as they're going to get. And so you can try squeezing on it, but you're pressing, you're trying to overcome the electrostatic bonds, the electrostatic repulsion between these ions. They just don't want to get any closer to you.
And it's just the electromagnetic force that's supporting that pressure where you try to squeeze, but it's going to say, nope. I've got electrostatic. I am literally repulsed by my neighbor, and I'm kind of linked up to them against my will, but here I am, and there's no way you're going to get me to squeeze any tighter. Metallic hydrogen does not do that. It's a liquid. It's a soup. It is also not going to want to be pressed together, but... There's no latticework. There's the free-floating electrons, which make it behave like a metal, but there's no latticework of ions because the ion of hydrogen is just a proton. It's just a single proton. It's got nothing left to give. It's not going to link up. It's not going to share electron shells in orbitals and blah, blah, blah to form a lattice. It's just a proton, folks. It's not going to make that latticework. So it's not electrostatic. It's not the electromagnetic force that's preventing you from squeezing metallic hydrogen tighter together.
Instead, it's degeneracy pressure. It's quantum mechanics, folks. It's the good stuff. degeneracy pressure, normally you only encounter it in discussions of, say, white dwarves or neutron stars. You can only put so many electrons in a box. Say you fill up a box with electrons. They have all the same charge, so they're going to repel. They're not going to want to get closer together. That's electrostatic repulsion. You can squeeze on them and make them get closer together. And you can do a pretty good job. So if you take a bar of gold, you can put it in anvil or whatever. You can squeeze it. You can make them get closer together whether they like it or not. You can overcome electrostatic repulsion. But then you'll hit a limit. You'll hit a limit because no two electrons can share what's called the same quantum state. They cannot be in the same place at the same time. They just can't. So even if you're able to overwhelm the electrostatic force, you're going to hit a wall, and that wall is degeneracy pressure.
Here's another way to think about it and why it's a quantum mechanical thing. And this way to think about it is from the Heisenberg uncertainty principle. If you take, say, a couple electrons and you squeeze them really close together, what are you doing? You're pinning down their position. You're saying, you're going to be right here, not over there. You're going to be right here, right between my fingers that are squeezing really, really, really, really tight. So that is reducing the uncertainty in the position. They can't escape. They're being squeezed down. But if in the Heisenberg uncertainty principle, if you reduce the uncertainty in position, you increase, you enhance the uncertainty in momentum. Momentum is velocity. So it's the more you try to squeeze two electrons together, the more they're going to vibrate. the more they're going to buzz. And that buzzing, that vibration, that momentum is like a pressure. You know, they're like, imagine trying to take two bees in a small metal box and you're making that box smaller and smaller and smaller.
You're saying, no bees, you're going to be right here in this tiny little box. But they're going to go nuts. They're going to go buzz, buzz, buzz, and they're going to bounce against the walls of that little container, especially as you're trying to make it smaller and smaller. And that will resist them. you trying to push it together that will resist you trying to make it smaller. That is a pressure. And that is degeneracy pressure. And that is a pure quantum mechanical thing. This isn't the electromagnetic force supporting liquid metallic hydrogen against further collapse. It's quantum mechanics. That's what makes metallic hydrogen weird. It's a liquid, it's a metal, and it's dominated by quantum mechanical forces. That's the secret sauce. That's the specialness in liquid metallic hydrogen. To make liquid metallic hydrogen, you need pressure and heat. You need them both. Where can you get? The sun is pretty hot and there's a lot of pressure, but there's also a raging nuclear fire in the core, so it's a little bit crazy there.
So it turns out the material in the sun, the hydrogen in the sun, just turns into a plasma. Earth's core is very hot. And it's under a lot of pressure, but there ain't a lot of hydrogen there. So good luck with that. What about the gas giants? Yeah, yeah, yeah, yeah. Jupiter and Saturn are gas giant planets full of hydrogen, also helium. So you have all the right ingredients in Jupiter and Saturn. You have hydrogen, which if you're going to make liquid metallic hydrogen, good to have some hydrogen on hand. You have hydrogen, you have high pressure because, you know, just, you know, go down under the cloud tops and the deeper you go, the more stuff is going to be on top of you. So that's naturally going to provide a lot of pressure. What's really going on inside of Jupiter? Well, we honestly don't fully know. I mean, we haven't stuck a lot of probes deep inside Jupiter, especially thousands of kilometers where we think things might start getting really interesting. We know the cloud tops.
We know surface activity. We know about the atmosphere just underneath it. That's from observations with different wavelengths of light that can penetrate the very topmost layers. But also by looking at the formation of storms and cloud patterns that put certain requirements on what's happening just underneath it. We can use the strength of the magnetic field around Jupiter to understand what's happening mixing up deep in its core. We can use variations in the gravity as we orbit spacecraft around it. And we can use simulations and computer modeling and compare with observations and figure out what's going on inside. Mostly... Jupiter is hydrogen with helium and a few other things. And getting that higher pressure, again, we don't know for sure what's happening inside Jupiter, but slowly over time, we're getting a better picture, especially with instruments like the Juno spacecraft, which is in orbit around Jupiter right now, unless you're listening to the show deep into the future, in which case Juno was a spacecraft that orbited Jupiter.
Higher pressure is easy. Just go deeper. There's more crushing weight of atmosphere on top of you. Eventually, actually, the hydrogen gas gives way to a diatomic hydrogen fluid that passes critical point. So it's not quite a fluid or a gas, just like water passes critical point, blah, blah, blah. So there's like a thin transparent layer of hydrogen fluid slash gas. And then you go even deeper. But you need the temperature too. And how do you get hot inside a gas giant? It turns out it's this really interesting mechanism called the Kelvin-Helmholtz mechanism. Not to be confused with the Kelvin-Helmholtz instability, which is another show. Feel free to ask about that. This is the Kelvin-Helmholtz mechanism. Check it out. You have a ball of gas. Could be a planet, whatever. It's a ball of gas. Its surface is exposed to space. So it's going to radiate heat because that's what things exposed to space do. It'll glow a little bit. Radiate heat. What does a gas that radiate heat does? It cools off.
What does a gas that cools off does? It compresses. It gets smaller. So... You have the outer layer of the planet cooling off, which is going to make it compress. That squeezes the core. What does the gas that gets squeezed do? Doesn't happen? Whatever. It heats up. The core heats up. That makes the surface hotter. What does the hot surface do? It radiates. What does that radiation do? It cools off the surface. What does the cooling surface do? It squeezes the core. What does the squeezing core do? It heats up. What does the heat do? It's a cycle. It's a mechanism. It keeps the interiors of giant planets warm. This is what prevented them. Otherwise, they would have cooled off a long time ago. But this act of radiating heat, compressing, can make the cores very, very hot. There's other possible mechanisms of heating the interior of Jupiter. You can have helium rain form and falls through the metallic hydrogen. Spoiler alert, there's metallic hydrogen in Jupiter. There's friction here and that can generate heat too.
We don't know what is the most important mechanism, but there's a couple. How did these elements get inside of Jupiter? They were born with it. Our solar system was three-quarters hydrogen, one-quarter helium, and a small percentage of other stuff. The pre-solar disk, that's what it was made of because that's what basically everything in the universe is made of. And the gas giants got to, and the sun, got to retain a memory. Here on Earth, we lost our hydrogen, we lost our helium, we just have the heavier stuff. But the outer planets past where it's too cold to obliterate by radiation any ices that might form, you get to retain a memory. You get to keep that image, kind of that window into the early solar system. And that's why studying Jupiter is so important. It is a window into the very early solar system. It's a picture of what our pre-solar disk looked like before we got here. But that's another show. So there, in the core, or in not quite the core, we think the core of Jupiter might be solid, but there is a very thick layer.
Most of the volume of Jupiter actually has the right conditions, the right temperatures, and the right pressures to make metallic hydrogen. Liquid metallic hydrogen. So most of Jupiter, we call it gas giant. Most of it is actually liquid. Most of it is actually liquid with an atmosphere, a gaseous atmosphere on top of it. You dive down deep enough, you will encounter this strange state of matter. That is a liquid. That is a metal. That is supported by quantum mechanical forces. How crazy is that? Usually you think of these weird quantum mechanical effects and you have to go to a white dwarf or a neutron star. No, you can just go in our backyard. Right there. Go in your backyard, look up, find Jupiter, find Saturn. You're looking at a giant ball of liquid metallic hydrogen. How awesome is that? That is significantly awesome. Before I go, I do want to talk about the experiments that have been done. We've been trying to make liquid metallic hydrogen in the lab, or at least some form of metallic hydrogen.
You can also have solid metallic hydrogen. Every few years, there's claims of making it in the laboratory, most recently at the end of 2016, using a diamond anvil, which is a great band name, by the way. Feel free to use that if you're trying to cook up a band name. Diamond anvil creates an enormous amount of pressure, which is what you need to make metallic hydrogen. They thought the latest experiment, the end of 2016, may have been a lock. In fact, in the abstract of the paper where they announced it, they said, yes, we totally made metallic hydrogen. It was disputed almost immediately. And the sample of metallic hydrogen survives. It turns out metallic hydrogen, for various reasons, is what's called metastable, which means if you make it and you don't bother it, it will actually hang around for a while. But apparently they bothered it and then it disappeared and they haven't been able to make it since. And so that is still up in the air of whether metallic hydrogen has been created in the laboratory.
Right now, the only place where we're pretty sure metallic hydrogen exists is in Jupiter and in Saturn. But we're not 100% sure. Remember, this is a theoretical state of matter based on our understanding of the laws of physics and our understanding of chemistry. Maybe wrong, probably not. There's probably liquid metallic hydrogen in Jupiter and Saturn. More measurements by the Juno probe and... What's left of the Cassini probe after it plunged into Saturn. The data we're still analyzing, trying to understand that, you know, what is the character of this very exotic, very strange and yet kind of normal. Hydrogen had this superpower in it the whole time. You just needed to subject it to extreme stress in order to bring it out like normal superpowers. Before I go, two quick announcements. Again, Space Radio is live where you can talk to me on the radio and ask me questions and I'll answer right away. Spaceradioshow.com. We record every Thursday at 4 p.m. Eastern. Call 888-581-0708 to talk to me.
And also astrotouring.com. Fraser Cain and I are at it again. We are going on a Caribbean cruise to experience some beautiful dark skies, to experience the Kennedy Space Center, to experience some mind ruins together. And we want to do it with you. Not you, you. Yes, you. astrotouring.com. Sign up, because it'll be fun. Thank you so much to my Patreon contributors, especially Justin G., Matthew K., Kevin O., Justin R., Chris C., and Helga B. It is your contributions that keep this show going and growing, and all my education and outreach activities. I'm eternally grateful. Thanks to Tom S. at Abguntha, Andres Del C, and Colin E. for the questions for today's episode. You can ask more questions by following me on Twitter and Facebook at Paul Matt Sutter using the hashtag Ask a Spaceman. Go into AskASpaceman.com. Go into YouTube.com slash Paul M. Sutter. Whatever you do, just get me questions, and I will see you next time for more Complete Knowledge of Time and Space.