 So, despite receiving so little sunlight, these giants experience some very complex weather, including some very powerful storms and some very high-speed winds, particularly in the case of Saturn and Neptune. So what causes all of this? Well, there's essentially two types of causes or two main factors. The first factor we want to talk about is the rapid rotation. That's what gives these winds these very high speeds. And what we have here overlaid on top of the illustrations of these planets are velocity diagrams, particularly the speeds going toward the east, which are effectively positive values and then westward speeds are negative values. So if you take a look at these planets, you'll see that they all have a different arrangement of different speeds. Unsurprisingly, Uranus has the strangest one because despite the fact that the equator is drawn in the same direction as the other planets, you've got to remember that the planet is knocked over on its side at 98 degrees. So these planets are all rotating very rapidly and as a result, they develop very strong Coriolis forces. We talked about the Coriolis force a little bit in the previous class. And basically, you just got to remember that as the planet is rotating, a parcel of air experiences a slight deflection either to the east or to the west. And that's because the planet is rotating underneath that parcel of air as it makes its way north or south. So depending upon where you are in the planet and the direction that you're moving in and from where along the planet you're moving, i.e. the equator, if you're moving very fast you'll find that your speed is deflected the strongest if you're going from the equator to the north or to the south and if you're coming from the north or toward the south and you're heading toward the equator, you'll find that your apparent deflection speed is a little bit less in any event. The Coriolis force is the reason why hurricanes on earth rotate, and this is the reason why the hurricanes and the storms in these giant planets rotate so rapidly. So Jupiter is rotating just about once every 10 hours. For example, Neptune is once every 16 hours. So the Coriolis forces on Neptune are a little bit less than they are in Jupiter. Nevertheless 16 hours is a lot faster than what we experience here on earth. So we're seeing some pretty strong Coriolis forces. Now the most extreme winds that we know of anywhere in the solar system are to be found in the atmospheres of Neptune and Saturn. I mean they can reach maximum speeds of 2000 kilometers per hour and in fact Neptune I think holds the record for the fastest wind speeds at about 2100 kilometers per hour. As you might expect, the speeds are strongest near the equators of the planets. Remember these planets are all rotating, therefore the parts of the planet that are rotating the fastest are going to be at the equators. That's of course where you'd expect to find the fastest winds. On Jupiter the alternating east-west speeds of these bands, the alternating east-westerly winds, as you can see there in the illustration of Jupiter, the speeds are alternating east to west, east to west, east to west and so forth. This is why we have these strong banded clouds on Jupiter. And a little bit less so of the reason why we have it on Saturn. It's there, it's just that it's a little less pronounced. But this is exactly why we have this. We don't see the same, you know this is why we have the strong bands on Jupiter but Saturn is a slightly different story. As we saw before in the photographs the bands are not as pronounced. It's not clear really why these circulation patterns are different from one plan to the next. So you know it's just something that we don't yet fully understand. This is something that we still need to investigate. So we know that the rotation develops the wind. But where is the energy? So we know that, let me rephrase that, we know that part of the energy is coming from the energy of motion. But thermal energy is also required to develop these storms. Well as we saw the thermal energy received from the sun is negligible. It's a puny amount compared to what we see here on Earth. So it turns out that the heat for the storms is coming from within the planets themselves. At least for Jupiter, Saturn and Neptune. Uranus? Not so much. But Jupiter, Saturn and Neptune all have significant internal heat. And as we know heat flows from warm places to cool places. So there's a constant flow of energy from the interiors of these planets outward. The flow of heat has a significant effect on global circulation patterns, especially when you have strong convection currents resulting from the heat flows. So remember convection is this idea of just imagine a pot of boiling water, a parcel of air is at the bottom of the pot of boiling water, it gets heated by the stove, warm air rises and a pocket of air forms a bubble so that bubble rises, the water surrounding the bubble breaks at the top of the boiling water, the heat is released, the water sinks and it goes and picks up another bubble, carries it up. That is convection in a nutshell. So what's happening is that as this material is or as this thermal energy I should say is being pumped up from the planets interior, as that materials is boiling over, if you will, into the upper atmosphere, those convection, I'm sorry, those Coriolis forces rather take over and they carry that material eastward and westward depending upon where it happens to spill into. So you get these, you know, convective vortices and these zonal winds helping to give shape to both the bands and the storms. So Jupiter has the most internal heat of all the giant planets. It's much hotter than it would be if the sunlight were the only source of energy in Jupiter's upper atmosphere. So here again just a few notes just to just to remind us, you know, Jupiter is packing the most heat, you might say, and when it comes to heat, remember heat is just a form of electromagnetic radiation. It's infrared and we might remember that the luminosity or the flux or the amount of energy being radiated goes as the temperature to the fourth power. In other words, a tremendous amount of energy can be released for a modest increase in temperature. So if you were to compare the actual temperature of Jupiter compared to what you would expect if Jupiter did not have heat, if it was just relying on sunlight, and you take those two values and you take their ratio and you raise them both to the fourth power, you find that Jupiter is putting out more than one and a half times more than 150% of its energy output. It's actually pumping out more energy than it's taking in from the sun. So Jupiter is effectively its own heat source. It almost, well, it almost doesn't need the sun. So where is this heat coming from? I mean, how is it that these planets are actually generating their own heat? Well, the answer is gravitational contraction. That's one of the neat things about these planets being made of liquid and gas. Liquid and gas is very susceptible to compression. If they were solid, they wouldn't compress very much. This compression is caused, of course, by their gravitational pull. So the planets are gradually, very gradually, contracting a little bit. Jupiter contracts at a rate of about two centimeters per year. It's not very much. Jupiter's not going to shrink away anytime soon, but that tiny amount of contraction is enough to raise the temperature just a little bit, giving off a tremendous amount of heat because the energy released goes as the temperature raised to the fourth power. So for a tiny amount of contraction, you get a lot of heat and that's where it's coming from. So let's talk now about the internal structure of these planets. Astronomers have measured the temperature and the pressure only at the outermost layers of these planets. As I said before, we could only look at the upper clouds. Maybe within infrared, we could peer a little bit deeper into some of the cloud layers, but whatever is underneath it has to be inferred from other observations. And so we developed these models, effectively, to calculate temperature and pressure from within the planet, essentially saying, well, if we can come up with a given model, we should be able to see the following surface temperatures and we adjust the model until we get something that matches our measurement, effectively. We fit the model to the measurements. So what we find when we do these calculations is that Jupiter and Saturn, because they have relatively more hydrogen and helium and fewer dense materials than the ice giants, they have these large pressures in the atmosphere. However, the gas is compressed so much under pressure that it actually liquefies at a depth of about a few thousand kilometers. So we've now gone from gaseous hydrogen to liquid hydrogen. And sometimes liquid hydrogen at this very high pressure and this higher temperature, it actually can act like a metal. So this liquid hydrogen is often referred to as metallic hydrogen. I just don't want you to think that we're talking about a solid slab of hydrogen metal. We're talking instead about a liquid mantle, if you will, of liquid metallic hydrogen. It's pretty cool. At even greater depths, the liquid hydrogen can be heated and compressed so much that the electrons are able to move freely. And now we have this liquid metallic hydrogen as illustrated by the brown color. So we've gone from molecular hydrogen to metallic hydrogen. And then you have the rocky core, maybe a liquid mix of water, rock, metals, things of that nature. So the cores of these planets would naturally be very, very hot, very dense liquids, heavy metals. Water, rock, and so on. Uranus and Neptune, however, have more water and volatile materials to begin with. So they're packing things like methane and ammonia, in addition to water and so forth. And so these volatile materials or ices are compounds of heavy element and that evaporate at very low temperatures. That means that you've got to get to very cold temperatures before these will condense. And that's why we're so far away from the sun. We're receiving much less heat from the sun. Now it's cold enough that we can begin to see a strong abundance of these highly volatile materials. So the temperature and pressures inside these ice giants, they're not going to be as high as those of the gas giants. They do not have any kind of a liquid metallic hydrogen, but they do have deep liquid oceans. And in these oceans are where gases and salts could be dissolved, giving rise to an internal structure that is much more ocean-like at relatively modest depths. So where did all this stuff come from? Well, remember, we talked about some likely scenarios for how the giants were formed. We talked about how Jupiter and Saturn were formed from the protoplanetary accretion disk. We had plenty of hydrogen, plenty of helium, still in great abundance back in those days. But then the sun matured and it became a proper star and a massive solar wind blew away all these gases. So Jupiter and Saturn formed first. There was no Uranus and Neptune at this time. After the solar wind blew and cleared everything out, all of these denser, colder, icy materials were left in the outer fringes of the solar system. And it's believed that that is where Neptune and Uranus came from. But to be really honest, many of the details of this process are not yet fully understood. Astronomers are constantly working on more sophisticated models of the early solar system. And we're really taking a lot of what we're learning from extrasolar planets. So it's possible that as we learn more about these other solar systems, we can learn more about our own and get some more clues as to why Jupiter and Saturn are the way they are and why the ice giants are the way they are. So these planets are also characterized by some very strong magnetic fields. And you might remember that whether, well, magnetic fields are generated by the motion of electrically charged particles. So in the gas giants, these materials would either be the liquid metallic hydrogen or those oceans that I talked about in the ice giants with the dissolved gases and salts. That's where you're going to find these charged, these electrically charged materials moving around. And so they give rise to some rather interesting magnetic field lines. In many ways, the magnetic fields act like a large bar magnet, basically just housed somewhere inside the planet. And I want to make sure it's clear we're talking about a bar as in a rod bar type of shape as opposed to a horseshoe magnet. So each magnet has its own set of poles, a north pole, and a south pole. We know on earth the north pole is near the Arctic and the south pole is in the Antarctic. But on earth, if you might remember, the magnetic field is not exactly aligned with the rotation axes. Well, that's true of the giant planets as well. As a matter of fact, not only are they not aligned, but they might not even necessarily be anywhere near the rotation axis. If you look at this illustration, you'll find that here's a bit more detail. Jupiter's magnetic poles are offset. Saturn's are fairly close to its orbital axis. Uranus is flopped off. It looks like the bar magnet has shifted away from the axis of rotation entirely. As a matter of fact, consider that the axis of rotation is running from the left to the right. And the north and south pole is not even perpendicular to it. It's completely off. And look at Neptune. There's this axis of rotation dotted in yellow. And the bar magnet has completely shifted into some other region of the planet entirely. It's just completely different. Why that's the case is not 100% clear. There's a lot of different theories. And again, astronomers are adjusting their models as to how these planets arrived at their magnetic configurations. But for now, it's enough to simply know that they can be very lopsided. They don't have to correlate with their axis of rotation, not unlike what we have here on Earth, but certainly much more extreme. So these planets all feature strong magnetospheres. You might remember that these are the outer magnetic fields that extend very, very far out into space. But the region around the planet in which the magnetic field becomes important to the planet is called the magnetosphere. So the magnetosphere is just a subset or a subregion of the magnetic field that can affect the planet in some way. The key thing about these magnetospheres is that they interact with the solar wind. I mean, even at these distances, they're still charged particles from the sun being blown out. And it carries charged particles. They then get trapped and they spiral down into the magnetic fields, colliding with the outer atmospheres of these planets at their poles. So this creates auroras, like we see here on Earth, and I'll show you this in a moment. But just to get you some idea, Jupiter, perhaps unsurprisingly, has the largest magnetosphere. Remember, magnetic fields are the product of electrically charged particles rotating or moving. Well, Jupiter is moving very fast. It has the most rapid rotation and it also has the greatest abundance of the stuff. So unsurprisingly, its magnetosphere is huge and this is kind of what the magnetosphere might look like from Earth if we could somehow see it. And for scale, there is the full moon. Let's widen up the shot a little bit and if we had a magneto vision, you would see that Jupiter's magnetosphere is a staggering six astronomical units. It's huge and receives a tremendous amount of charged particles from the sun, just based on its sheer size, despite being so far away. So these charged particles interact with the magnetospheres of Jupiter and Saturn in particular and we've been able to image those northern and southern lights of Jupiter and Saturn. And by the way, I should point out that you're seeing a composite of two images each for each planet. The first is a visible light image so you can actually see the planet and then ultraviolet observations made with a Hubble Space Telescope, of course, were then used to, you know, were then overlaid at the poles, but really no kidding, those are the aurorae. We can actually see these aurorae happening as these charged particles spiral down the magnetic field lines and interact with these planets' atmospheres. So with all of that said, I think we are just about done here. Yeah, we are done. That concludes this lecture. I apologize for the format on which we had to do it, but if you have any questions, give me a shout and I will hopefully see you next week. Take care, bye-bye.