 Okay, so we're going to just do this lecture via video. It's not quite as good as being in the classroom, but if you have any questions, just feel free to email me or post something in Blackboard, figure some way out of doing Q&A. But since we had our class canceled last night on us due to that situation on campus, I thought we'd just go ahead and knock this out via video. So this talk tonight is about the worlds of gas and liquid. We're talking about the giant planets, and there are four giant planets in our solar system. They are, of course, Jupiter, Saturn, Uranus, and Neptune. Now you often will hear these four planets referred to as gas giants, but in reality they're really kind of subdivided into two subclasses of giant. The first two, of course, are the gas giants. Jupiter and Saturn. The second two are the ice giants, and each of these planets are essentially two of a kind. Uranus and Neptune look very similar to one another, just on appearance, even in this illustration. Jupiter and Saturn, if you ignore the rings around Saturn, which we'll cover in a future lecture, you'll see that the two planets are largely very much the same. So both of these sets of worlds have characteristics that are very similar to one another and very different from each of the other pair. And let me just kind of illustrate these characteristics at a high level, and then we'll delve into them at a slightly more deeper level. So the first characteristic is obviously the size. And in astronomy, we measure sizes by radii, or the radius. So going from the centers of the planets out to their very edges of the cloud tops, we find that Jupiter and Saturn are similar in radii and radius rather to one another, whereas Uranus and Neptune have similar radii to each other. But both, of course, just by looking at it, are very different from one type of giant to the next. The second major characteristic is their composition. Jupiter and Saturn largely consist of mostly hydrogen and helium, whereas the ice giants themselves, they do have hydrogen and helium, but it's a much lower percentage. And instead what you find in these giants and these ice giants are more water and other volatiles such as ammonia and methane and so forth. And in fact, as we're going to learn, these two worlds, these ice giants, are so far away from the sun, it's actually cold enough that these molecules, things like ammonia, methane, and so forth, condense and form ices, and that's why they're called ice giants. They really do have basically icy chunks floating around in the atmosphere, you might think of it that way. So Jupiter and Saturn are much closer to the sun at 5.2 and 9.6 astronomical units respectively, whereas Uranus and Neptune are much farther from the sun, especially Neptune. It's way out there all the way at 30 AU. So the fact that they're farther from the sun and they have a different chemical composition is no coincidence, and we're going to explore that in a little bit more detail. Finally, at the centers of these planets, they both have kind of rocky cores. It's just a question of how large are they relative to the planets as whole? In the case of Jupiter and Saturn, their cores are relatively small compared to the overall size of the planet, whereas in the ice giants, we have relatively larger cores. Both cores, I'm sorry, all four cores, you might say, for both sets of planet are composed of the same kinds of rocky and metallic material mix, and we'll talk a little bit more about composition as we get further in. So there are many characteristics that they have in common with one another, not the least of which is that they don't have any solid surfaces. They all have managed to hold on to their primary atmospheres, which just have a composition of relatively light gases and some complex molecules, but that's about it. And you might remember that we talked about the formation of our solar system and planetary systems in general, but when planets form, they initially have a certain complement of gas surrounding them. Those make up the planet's primary atmosphere. It's just that for one reason or another, actually for several reasons, particularly the planets that are closer to the sun, they lose those primary atmospheres. So living here on Earth, we are living under our secondary atmosphere. We once had a primary atmosphere that was blown away by the solar wind. Earth wasn't massive enough to hang on to it, et cetera, lots of reasons. But out here where the giant planets formed, they were massive enough to retain those primary atmospheres. So they're still exhibiting the initial atmospheres that they formed, and they'll never be a secondary atmosphere to Jupiter, Saturn, Uranus, and Neptune because they're all holding on to their primary atmosphere. The second thing, or the third thing rather that they have in common, is the fact that they're very massive and they're very large and they rotate very rapidly. So let's just dig into these one by one. First of all, no solid surfaces. It's really important to understand that we're only looking at the actual cloud layers in their atmospheres. We're not actually seeing through or waiting for a clear day on Jupiter or Neptune or any other planet to happen and we'll suddenly see down into the surface. There is no surface. It's all clouds all the way down until you get all the way deep into the cores in the mantle regions, which we'll talk about later. Now you might be thinking, well, okay, there's Venus and we can't see its surface, but we know it's there. All we can see are the clouds. Yes, that's true. We do know that there's a solid surface underneath Venus's cloud layer. The thing that I want to keep in mind is that the surface of Venus is not very deep. You don't have to go very deep into the clouds before you strike surface, whereas with these giant planets, you'll go all the way down to their cores before you encounter anything that is remotely surface-like, rocky or metallic or otherwise. So we're not talking about small terrestrial planets surrounded by enormous atmospheres. No, no, we're talking about giant gas balls orbiting the sun. That's really what we're dealing with. So we're not really making a comparison to Venus. So let's talk about their composition. As I said, they're composed mostly of light elements. What do we mean when we say light weight? Well, we're typically talking about hydrogen and helium. You'll find that there's a tremendous abundance in these giants and it's much, much, much more than what we'll find here on Earth or Mars and Venus and Mercury. The terrestrial planets just don't have an abundance of these light elements, namely hydrogen and helium, or as there is great abundance in the giant planets. And why is that? Well, they all formed past the frost line. You might remember, we talked about this before, and we talked about the formation of planetary systems, that the farther away you get from the star, whether it's still forming as a protostar or even after it's formed, it doesn't matter. The farther away you get from the star, the lower the temperature gets. So what that means is that when you're close to the star, say from the distance of Jupiter or closer, and we call this an imaginary line, we call that the frost line, if you are, in this case, to the left of the frost line, anything that constitutes what we call a volatile element, a volatile material such as water, ice, molecular hydrogen, hydrocarbons and so forth, it simply is too hot there. They're receiving way too much radiation from the sun. The temperature is simply too high to permit those molecules to condense, or those elements, I should say, to condense. As a result, they remain in a very rarefied gaseous state. They just simply don't condense anywhere onto any of the planets. And so Venus, Earth, Mars, Mercury and so forth, all of these are relatively devoid of hydrogen. And as we later on would learn, where do we get most of the hydrogen and the volatiles that we do have on Earth? They were carried inward from the outer solar system via comet. So that means that once we are past the frost line, going to the right, if you will, we now have an abundance of these volatiles. And all of these materials, including the refractory materials that we have a great abundance of here on Earth, to the left of the frost line or interior of the frost line, the Jovian planets or the giant planets, rather, have access to just about as many refractory materials as well as all of those volatile materials as well. So it's getting a double whammy, a double helping of materials to get larger with. So the key thing just to remember is that you can only get these volatile ices in the outer disk. Okay, but when you're interior of the frost line, you're stuck with refractory materials, basically rocks. And that's what we live on is one of those rocks. Okay, so we're talking about the composition of these gas giants, I'm sorry, of these giant planets, not just the gas giants, but the ice giants. We have plants that formed past the frost line. And really just about everything that you spot in the atmospheres of these planets are bound in some way to hydrogen. That's largely because hydrogen is the most abundant element there is, followed by helium. So everything that you're going to see, if you're going to find, for example, carbon in the atmospheres of these planets, they're going to be bound to four hydrogen atoms giving us methane. Or if you were to find nitrogen flying around, it would have long since bonded to three hydrogen atoms to give us ammonia. So what you're really are taking a look at, what you're sampling in these atmospheres, are really just manifestations of hydrogen-based compounds or hydrogen-based molecules. Any element that is heavier than helium, by the way, we refer to those in astronomy as a heavy element or a metal. So we have hydrogen and helium, and then everything else, carbon, boron, argon, nickel, et cetera, oxygen. All of those are considered, quote unquote, heavy elements, or even sometimes we refer to them as metals. So just keep that in mind that you're, you know, whereas here on Earth, we have lots of silicates, lots of heavy elements, out here at the giant planet regions, we find mostly light elements. So what are we composed of specifically? Let's just take a look. Well, when we have Jupiter and Saturn, the gas giants, we are looking mostly at hydrogen and helium. We've got some trace amounts of heavy elements. I mean, there is some ammonia and methane in there, but just not that much. When it comes to the ice giants, on the other hand, we've got a much greater percentage of these heavy elements. So less standalone hydrogen or molecular hydrogen, and less standalone molecular helium. Instead, you're going to find it mostly combined with other stuff, and we'll talk about that in a little bit more detail. So the other characteristic is that they're all, of course, very massive. I mean, they just have a lot of stuff. As a matter of fact, Jupiter is more massive than all of the other planets in the solar system combined. In fact, I think you can even add up not just all of the other planets, but all of the asteroids and all of the comets and all of the dwarf planets, and you still come up short compared to Jupiter's overall mass. The only thing more massive in the solar system than Jupiter is the Sun itself. What is also a little bit less obvious is that the planets themselves, these giant planets, all have relatively low densities. So they're fairly large, they're fairly massive, but their densities are low enough that in the case of Saturn in particular, its mean density is less than water. Water, as you might remember, is equal to one gram per cubic centimeter. Saturn is about 0.7 grams per cubic centimeter. In other words, if you could manufacture somehow a bathtub large enough and, I don't know, I guess get like a gravity field slightly greater than Saturn's to pull Saturn into that bathtub, you would find Saturn bobbing in the giant bathtub. I don't know how you're going to build that bathtub or much less fill it up, but good luck. It's just kind of cool to know that you can calculate these things and actually discover that Saturn can float in the giant interplanetary bathtub. Okay, so in addition to being very massive, we often equate large or highly massive objects with very large objects, and that doesn't always have to be the case in nature, but certainly in the case of the giant planets, they're called giant for a reason. They're big, they're really big. Now, we measure their distances, as I said, we measure rather their sizes, excuse me, we measure sizes by their radii, so we're talking about an imaginary line going from the dead center core of the planet all the way out to its outer cloud tops, and we measure that line at the equator, so we're talking about going from the center out to a cloud top that's somewhere on the planet's equator, and we're going to get into why we talk about the equatorial radii in a moment, but this means that combined with their size, well, that their size rather gives us effectively the planet's volume, so the planets are huge. And matter of fact, it's not obvious looking at this, but it turns out that if you take some calculations, you'll discover that Jupiter's got enough volume, it's just physically large enough to accommodate all of the other planets in the solar system combined, so these things are huge. Jupiter is a little more than 11 Earth radii across in size, Neptune is the smallest of the giants, it's just coming in at about a little under four Earth radii, but they're quite large, even the smallest of them is staggeringly huge compared to Earth. So now we're going to talk about why we think about radii as a function of going out to their equators, and that's because they have, actually I'm sorry, let me take it back, I'll get to that in a second, they orbit, let me jump ahead, they all rotate very, very rapidly, and so what this means is that at just under 10 hours for Jupiter, which is the fastest rotating planet of the giants, indeed the fastest rotating planet in the solar system, this causes the equatorial regions of these planets to bulge out, you can almost think of it as like a centrifugal force, I don't like to use that because it's technically not a real force in physics, but okay, we'll just go with it for the time being. You can imagine taking a ball on a string and twirling it above your head, you know that the ball is attempting to go as far away from you as possible, it's extended all the way out, it's a lot like that here, the equatorial regions are rotating the fastest on these planets, and therefore they are trying to effectively fling themselves away from the planets, leaving toward this overall equatorial bulge or an oblate, that is to say non-speroidal shape. You can even just take a look at, these are real images of the planets, no more illustrations, and you can take a look at Jupiter and Saturn particularly, and you'll see that they're clearly not spherical, they are kind of oblate, obloid shapes, they're a little bit squishy at the poles, and that's precisely because of their relatively short, i.e. fast rotation periods. Another thing that they all have, well one of the things that they don't have in common is their obliquities, they are kind of all over the place, well maybe not to a huge extent, unless of course you take a look at Uranus, it's knocked over effectively on its side at about 98 degrees. Now keep in mind that all of these obliquities are just another term for the planets axial tilts, and they're tilted with respect to what? They're tilted with respect to their plane that they share with the Sun, i.e. the ecliptic, effectively. So Jupiter is tilted not very far from perpendicular, just a little over three degrees, Uranus all the way has flopped over at 98 degrees. It's not clear exactly why Uranus ended up this way, but something during its formation we presume must have knocked it over, and it sounds like it would involve a very sudden powerful collision, but it's a little bit more complicated than that, and we're not really sure exactly why, but one of the clues that it might not have been just a sudden catastrophic event is the fact that not only is the planet knocked over on its side, but its ring system, and we're going to talk about rings in another lecture, but its ring system is also in the same knocked over configuration you might say, and even its moons orbit in the same knocked over configuration. So maybe it was like a gradual title shift that brought the rings and the moons into this knocked over position along with the planet. It's kind of interesting. One of the things though about Uranus, it's extreme obliquity, is that this leads to extreme seasonal changes on Uranus. You might remember that the axial tilt is the reason for the season. It's the reason why we have winter, summer, spring, autumn. Uranus and all these planets indeed experience seasons to varying degrees, but Uranus has the most extreme seasons, and one other thing about Uranus is that the seasons are closely associated with what you and I might think of as a day, when we think of a day as the sun rising and the sun setting. Well, in this case, the sun remains more or less over one of the poles for approximately 42 years, and then you get 42 years of daylight at the other pole, which means that you've got 42 years of darkness on either side. So imagine 42 years of darkness or 42 years of daylight. It's kind of crazy. It's just very different than anything we've experienced here on Earth. So this here is a quick wrap-up of all of the physical properties. It's just a convenient table for you to go back and reference. Feel free to take a look at this. You don't have to necessarily memorize every single one of these statistics, but they're really good to go back and take a look at once in a while while making some comparisons and thinking about some of the questions that you might get asked. So take a look at it, get familiar with it, and then when we come back, we'll talk about the cloud structures of the giant planets.