 Greetings and welcome to the Introduction to Astronomy. In this video we are going to continue talking about the giant planets, but most specifically their atmospheres. So now we're going to look at the outer layers, the areas that we can actually see of these objects, and try to better understand those. And these are the areas that have been explored and photographed by various spacecraft. So let's get started with some of the general properties of these planets here. And what do we know about them? Well, all of the giant planets are made primarily of hydrogen and helium, and their next highest components are what we call the hydrogen compounds, and sometimes we've been calling these ices. And those are things like water, H2O, methane, and ammonia. So these three make up a large percentage of the remaining part. Hydrogen and helium are the vast majority of the compositions of these planets. But the remaining is these icy compounds of water, methane, and ammonia. Now the Galileo spacecraft, which studied Jupiter, actually sent a probe into its atmosphere. So we know therefore the composition of that atmosphere very well, because it's one of those things we've actually had the chance to sample. So we've been able to study the atmosphere of Jupiter, unlike the atmospheres of the other objects in the outer solar system. Jupiter's atmosphere, at least the portion that we sampled, is very well known. We have a very good idea of what that atmosphere is made up of. And that probe, of course, only survived the very few outer layers. The pressures and temperatures in Jupiter's atmosphere get extremely strong as you get further down. So it's not like it could explore a great deal. But certainly the outer layers that we can see are now well studied. So let's look a little bit about what we see, and we'll start here with Jupiter. Jupiter is, of course, the largest of the Jovian planets. And what we see on Jupiter are the clouds in the atmosphere. The vast majority of the atmosphere is invisible, and that is the hydrogen gas. Hydrogen gas is completely clear, so we can't see through that. So we cannot see through the hydrogen and see the rest of the material, but we're not seeing that. We're seeing things like the Darius Isis forming clouds in the atmosphere of Jupiter. Now when we divide this up, we see that Jupiter is divided into alternating light and dark bands. We call the lighter-colored areas zones, and the darker-colored areas the belts. So there are different sections, and we look at what those are. It's because we're looking at an atmosphere, not the surface of a ball, for example, that the zones, the lighter-colored zones, are actually areas that are higher up in the atmosphere. The belts are lower down. The zones are known to be made of ammonia clouds, although the composition of the belts is still something in question. Now if we look overall at the structure of Jupiter, let's look at what that looks like. And what we see is that as we head in towards the surface of Jupiter, so here we're in the very outer portion of the atmosphere, and here we get down towards the surface layers, and here at about 50 kilometers above the surface of the clouds, and between that and a little below, right around the surface, we get clouds of three different types. We get ammonia clouds, water clouds, and in between them what we call ammonia hydrosulfide. So there's three different cloud layers, and when we look at Jupiter, those are the clouds that we are seeing, so we're not seeing its primary component, hydrogen gas. We're seeing very specifically these three types of clouds, and the reason they're so prominent is where they happen to occur on Jupiter, and that is that they're very close to what we see as the surface. If they were further down, there would be more haze, and they would not be nearly as visible. So this is what we're actually able to see here. Now if we look at the other planets, we can look at Saturn. When we note Saturn, we see that it has less distinct cloud bands. As I mentioned, Saturn is further in the solar system, so it's cooler in terms of temperature, so you have to get deeper down inside and closer to Saturn in order to be able to form those clouds. So it's further from the sun, it has cooler temperatures, and therefore the clouds form deeper. So clouds are deeper below what we see as the surface, and that is why when we look at Saturn, we can get some hints of darker and lighter areas, but they are hidden well below a much hazier layer that blocks them out and makes them less distinct than we saw on Jupiter. Now if we look at Uranus, Uranus is almost completely featureless. So it is a nearly featureless atmosphere. It has a very stable atmosphere because it does not have an internal heat source like the other Jovian planets. So when we look at Uranus, we get essentially no structures at all. When we look at Neptune, so pull up Neptune here, what we see is that it has a little bit more structure, and we know that Neptune does have an internal heat source which may be why it has this structure, that we can see things like Voyager found the great dark spot on Jupiter, as well as other storms and clouds, and we can start to see the structure is a lot more significant than it was on Uranus. Now the other thing that we note about Uranus and Neptune is their distinct blue color, and that is due to methane in the atmosphere. So methane absorbs red light. So that means that the light from the sun that has all the colors for the rainbow strikes things like Uranus and Neptune. The reds and the oranges are then absorbed, leaving only the blues and the greens to be reflected back to us. So that's then what we're able to see for these. Now we can look at overall structures as well, as in a side-by-side comparison. So let's take a look at these. And what we have is that we see the clouds here, as we've talked about. Jupiter's clouds are up very close to what we see as the surface of Jupiter. For Saturn, something to the same scale, the surface would be up here. But note how the clouds are much deeper down, hundreds of 100 miles further down within the surface. And you've got a much hazier area up here that then makes them less distinct. When we look at Uranus and Neptune, profiles are similar. You get things like methane up here, but again, at a much higher level. Again, Uranus has no internal heat source, so maybe nothing to drive weather, which is why it is such a very bland atmosphere. Neptune does have a more significant heat source, and perhaps that's why we see it as a stronger weather system, as more going on inside at that planet. Now we do see some large storms on each of these planets. The best known would be the great red spot of Jupiter. This is a large storm on Jupiter that is larger than the Earth. So the entire Earth would fit within this storm. It is a high pressure region, and that is in contrast to what we see on the Earth. So we talk about it as being a storm, but it is a high pressure rather than a low pressure region. Low pressure regions are storms on Earth. Whereas on Jupiter, this is a high pressure region. It has been present for hundreds of years. Galileo's telescope would not have been powerful enough to be able to resolve this. However, it was not that much longer, a few decades later, that the great red spot was actually seen in the late 1600s, and has been continuously observed for about 200 years now. So continuous monitoring of it. And what we've noticed over that is that it is slowly shrinking. Less than 100 years ago, it was easily twice the size of the Earth. Now it's down to closer to the size of Earth, and seems to be shrinking. If it continues at its current pace, it could be gone within decades. Now we don't know whether it will continue at that pace. Measurements will continue on it. But we do not know whether that will continue at the same rate, whether the rate will accelerate as it gets smaller and smaller, and it will quickly disappear, or whether something else will stabilize it and allow it to continue. But we do know that measurements over the past few decades have shown that it is very significantly changing in size. Now the great red spot on Jupiter is not the only storm in the outer solar system. We see some others, and we can look at those here. There is on Neptune, there was the great dark spot that we see here, which was present in 1989 when Voyager 2 flew by Neptune and took the image that we see here. So we do see the great dark spot and some of the cloud structures around it, but just a few years later in the mid-90s, the Hubble Space Telescope was able to take images of Neptune and saw nothing. So the great dark spot had disappeared sometime between 1989 and the mid-1990s. Now we don't know how long it had been there prior to 1989, whether it had been there for years or decades or whether it had just appeared. And we don't know how often these storms appear. We don't have a long enough baseline of study to be able to get any good statistics where we can say we've seen hundreds of storms and they on average last 10 years, 100 years, or 1,000 years. We don't know that until we are able to see a lot more of them. Now, we also see them on the other planets. So for example, in Saturn, in 2011 we saw a global storm and you can see that up here in the top, we're stretching out across the whole latitude of Saturn. So we see them on Saturn as well. And even on Uranus, again, we don't see a lot of surface features because it is so bland, but enhanced images do give us some very light banding structure. And in fact, if we zoom into certain sections here, we can see that there are also dark spots on Uranus. This scene in 2006, and it's wondered if these are something that is seasonal. Uranus has the most extreme seasons of anything in the solar system because its tilt is the greatest. It is pointing almost directly towards the sun, giving it the most extreme seasons. So for one season or about 21 years, one pole points towards the sun. And after that, you'd have 21 years of summer, then 21 years of fall, 21 years of winter, and then 21 years of spring. So seasons would be very long, but would also be very extreme because during that summer season, the entire pole is pointed directly towards the sun. So let's finish up here with our summary. And what we find is that all of the Jovian planets have complex atmospheres. So none of them have very simple atmospheres. There's a lot going on there with various temperatures and structures deep inside. The structures that we see are determined by the temperatures and the internal heat sources of the planet. So one of the reasons we believe perhaps that Uranus does not have very significant storms on its atmosphere and very significant structure could be that lack of an internal heat source. And we've talked about large storms. We see these on all of the Jovian planets to differing extents from the great red spot on Jupiter observed for hundreds of years to the great dark spot that was present on Neptune to other smaller storms that are known on Saturn and Uranus. So lots of different storms that occur in these complex atmospheres. So that concludes our lecture on the atmospheres of the giant planets. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.