 Okay, so now that we've talked about some of the general characteristics and the similarities and the differences at a higher level, let's dig a little bit deeper. In particular, we'll talk about some of the cloud patterns that we see here, starting with Jupiter. This is an image made by the Galileo spacecraft I visited during the 1990s and gave us the highest resolution images of Jupiter's clouds that we have seen yet to date. Incidentally, there's another spacecraft called Juno that's on its way, so hopefully we'll get some better images from Juno when she gets there. But for the time being, these images from Galileo will do us just fine. One thing about Jupiter is that it has far and away the most complex cloud patterns of all the giant planets. You'll notice these very prominent alternating light and dark cloud bands. In fact, these bands are visible from Earth, even in a good pair of binoculars, and those of you that have accompanied me to the observatory after class a couple times this semester, we've had a chance to take a look at Jupiter, and right there, just taking a look at the first thing you notice about Jupiter is that it looks like a disk. You see a complete planet there, and you've got those prominent dark and light bands alternating, and that tells us something about the characteristics of Jupiter that we're going to explore in greater detail, but for the time being, it tells us a lot about the characteristics of its rotation as well as the chemistry that's taking place in the atmosphere. So there's a lot of really interesting stuff going on, but mostly we're talking about a very complex chemistry that we have. Hydrogen is the most abundant element in Jupiter, so most of the chemistry involved is going to be a manifestation of hydrogen. There'll be some compounds of hydrogen reacting with other compounds of hydrogen, thus giving us these interesting colors. We also see storms in the atmosphere as well. The most famous of which is the great red spot. I never said that astronomers were good at naming things. It's just what it's called, the great red spot, and it's been visible for quite some time. In fact, it wasn't long, as I mentioned in an earlier lecture, Galileo discovered some of the characteristics of Jupiter using his spyglass, and it wasn't long after Galileo's initial observations that astronomers began noticing that Jupiter had a spot in addition to these bands, and that spot turned out to be the largest storm in the solar system. As a matter of fact, it's about twice the size of Earth. I mean, two Earths could fit in there. It's pretty huge. But there are many other storms that come and go throughout, you know, we can even watch them happen on a time scale of as short as a few months to just a few years. The great red spot, however, has endured for about 350. Some astronomers think the spot may have been around for 500 years. So it's a massive cyclone. But one of the things that's interesting about this for its enormous size and its aptly named title, Great Red Spot, it's not as great as it used to be. As a matter of fact, it's been shrinking. These are some Hubble Space Telescope images of Jupiter, and you can clearly see that the great red spot has gotten smaller from 1995 through 2014, just last year. As a matter of fact, we can line these up against some parallel lines, and you can really begin to see that it actually has lost a decent, you know, not insignificant amount of its former self. In fact, it seems to be shrinking at a rate of 580 kilometers per year, and it's really not quite clear as to why that's happening. There could be kind of a reduction in the fuel that's powering it. Maybe there's some thermal differences going on under there. It isn't clear, but one of the things that I forgot to mention, I'll like to mention right now, that makes the great red spot unique, and one of the reasons these storms can last as long as they do on these planets is, again, there is no surface, right? So it's not like we think about a hurricane here on Earth making landfall, and as soon as it comes over the land, it runs out of the moisture to be drawn up. There's no more ocean underneath it to fuel the hurricane. That's not the case here. There's nothing but fuel underneath these storms. There is no surface to break the storm up. So it isn't clear what is causing the great red spot to shrink right now, and it's not clear if it will continue shrinking and just become a spot and eventually die out or if it will grow back again. So it remains to be seen, but it's a very interesting observation. When we take a look at Saturn, we do see some patterns that are similar to Jupiters, but they're much less pronounced. These colors are subdued, first of all, they're much less pronounced. There's few of them. They're fewer cloud patterns. They're not quite as intricate and twisted up as you see in Jupiter. And the colors, of course, are much more subdued. This is a visible, really a natural color image of Saturn taken by the Hubble Telescope. If you were on board a spacecraft looking at Saturn's atmosphere, it would probably look something like this. And yes, you do see some bands there, but it's not as pronounced. The colors are subdued. Well, again, Saturn's atmosphere is composed largely of hydrogen and helium, just like Jupiter, but the colors are different. That tells us that the chemistry going on in the atmosphere must be slightly different. You know, often you'll see some small scattered bright and dark clouds appearing and disappearing, but there's just not a whole lot going on. Although storms are not as frequent on Saturn, but it does in fact seem to have a jet stream. I mean, there is some interesting activity observed in the atmospheres of Saturn, including something that kind of looks like our jet stream here on Earth. I mean, you've got a zone of fast moving wind or some kind of material is blowing through what appear to be alternating high and low pressure regions. Now if you've ever learned anything about fluid dynamics, i.e., the motions of gases and liquids are collectively known as fluids, you'll know that fluids tend to want to flow from high pressure to low pressure. So they're kind of zigzagging, picking up speed as it approaches the high pressure, and accelerating toward the low pressure, and then picking up some speed from the high pressure and just giving us this wavy pattern, not unlike our jet stream here on Earth. But Saturn is also capable of having storms. It's just not as often as you see here on, as we've seen earlier rather on Jupiter. So take a look at this. This is a sequence of images made by the Cassini spacecraft in late 2010 and 2011, and a storm erupted that was carried by Saturn's very high speed winds. It blew the storm debris, if you will, all the way around the planet, wrapping itself around the planet. And here's a detail of it. It was a really spectacular storm. And these storms are probably not exceptionally rare, but they're certainly far less common than we typically will see on Jupiter. So what about the ice giants and what cloud patterns do we see here? Well, these two images of the ice giants were captured by Voyager 2, and there are undoubtedly the highest resolution images, the clearest, sharpest images we'll ever see of these planets, until we build a colossal telescope or send, better yet, send a probe to visit these worlds once again. But when Uranus was passed by in 1986, there wasn't really any weather going on there. Neptune, however, had some interesting weather that we could then do some detailed studies of. But in general, when it comes to these cloud patterns, you know, they're really featureless, really compared to Jupiter and Saturn, particularly Jupiter. And this is because whenever we look at these worlds, there is a tremendous amount of methane in the atmospheres. And I'll talk about that in a minute. But for now, I'm just going to make some observations that, you know, the banding is present. You can see it. This is a slightly enhanced image made by the Hubble Space Telescope. We're kind of stretching things out a little bit in our photo imaging software to kind of bring out some of these details. You can see that there are, in fact, some tiny storms here and tiny relative. If you look just to the right of the equator, kind of on the right half of the image of Uranus here, you'll see a darker smudge that's about the size of North America. So you do see storms, but rather than storms plowing into one another and colliding and forming with one another, as we saw on Jupiter and on Saturn, here the storms are kind of isolated. They kind of come and they go. It's not quite as spectacular. There are some upper atmospheric ice crystals that formed in Neptune's cloud tops as Jupiter sped on by. And it's worth mentioning that everything that we know about the rotation of these gas giants is based on observations, very careful observations of these cloud features. So anytime we can find a feature, whatever it is in the atmosphere, it might be a storm or a low pressure region or whatever, anything that we can visually lock onto, we can then watch it and watch it rotate around the planet. That tells us about the planet's rotation. One of these occasional storms, though, the very famous storm that was spotted by Voyager 2 was on Neptune. It was an immense low pressure region. It was dark in color, so astronomers called it the great dark spot. Again, I never said we were good at naming this stuff. But Neptune is so far away from the sun, remember, we're 30 astronomical units away from the sun where the farthest planet effectively from the sun, everybody assumed that Neptune was going to be completely dead. There wouldn't be anything interesting to see at Neptune. And lo and behold, there was incredible weather, including this huge earth-sized storm, the great dark spot, excuse me. However, unlike the great red spot on Jupiter, the great dark spot had disappeared by the time the Hubble Space Telescope got a look at it in 1994. So in just five years, the great dark spot was gone. But there was a new dark spot that had formed in Neptune's northern hemisphere. So these are storms that come and they go. And sometimes they're pretty spectacular and sometimes not. So it's not 100% clear as to why that's the case, but it is very interesting. And by the way, in order to really explore Neptune and Uranus, you pretty much have to go into the infrared in order to really begin to peer beneath some of that upper atmospheric haze and reveal some of the cloud features or some of the atmospheric structure underneath. It's just a consequence of dealing with gas giants that are dominated, in this case, I should say ice giants, excuse me, that are dominated by methane and so forth. They're really good at absorbing red light and they scatter the blue light. So infrared is kind of a handy way to do this. But let's talk a little bit about getting underneath the cloud layers. And let's talk initially about the two gas giants. So we have Jupiter's atmosphere as depicted in the illustration. And these outer planets have been extensively studied by flybys, orbital missions. In the case of Jupiter and Saturn, they've each had orbiters flying around them. The ice giants had a flyby, one flyby each by Voyager 2. And on each planet, the density, the composition and the circulation patterns are very different at each altitude. So, in all cases, the temperature and the pressure both increase as you work your way downward into each atmosphere. So the temperature and the densities, they become high at least compared to Earth's atmosphere, they become pretty high after only a short distance into the atmosphere, particularly here at Jupiter. As you can see, we're only 100 kilometers down and we're at roughly 10 atmospheres. Okay, so you can get, considering the size of Jupiter, we're talking 11 times, a little more than 11 times the radius of the Earth. You get just 100 kilometers down and you're already at 10 atmospheres. It's the pressures building up pretty quickly and as is the temperature. So that tells us a lot, something interesting about Jupiter. It's a pretty, as gas giants go, it's dense, if you will. One of the things that we could take a look at, in fact, let me bring up some of these bullets here, just to have these. The different cloud layers are formed at different heights. Okay, so you have an appearance of the planets that are rising because of the way their cloud layers are structured. They form at different temperatures and at different pressures. So for example, here on Jupiter, the bright upper layer clouds are made of ammonia crystals. Okay, and they form at a temperature of about 133 Kelvin. But then going deeper, the darker, lower level clouds are formed from ammonium hydrosulfide and that forms at 193 Kelvin. So you gotta get a little bit deeper where the temperatures are warmer to start picking up the ammonium hydrosulfide ice. That's when you can begin to get those orange-like tabby colors in the cloud tops of Jupiter. Just to compare to Saturn, just to give you some idea, again, it's largely the same structure. However, because Saturn doesn't get, the pressure doesn't reach 10 atmospheres until you are nearly twice as deep in the atmosphere. So Saturn is far less dense. Therefore, the atmospheric layers aren't nearly as compressed as they are in Jupiter. So you have a broader region of ammonia ice and a broader region of ammonium hydrosulfides and so forth. Again, all of this greater depth results in a slightly different chemistry, resulting in a slightly different appearance of Saturn. So what about the ice giant cloud layers? Well, here we're looking at Uranus and the temperature and the pressure increase downward as we saw with the gas giants. So you do get to increase temperatures and pressures the deeper you go, but because the ice giants have less mass, they have less gravity and because they have less gravity, the values of these pressures and temperatures just don't increase as high as they do for the gas giants. So therefore, the highest clouds that we see are of a very different composition. They're made mostly of methane ice, which absorbs the redder wavelengths of light and that leaves the blue wavelengths to be scattered in the atmosphere, making the planets really appear, gives them their bluish-green appearance. So the color of the clouds in Jupiter and Saturn, by the way, should be crystal clear. We should see well through and deep into the atmosphere, but because they have impurities in their atmospheres, they, that helps give off their color. That's how Jupiter and Saturn get their colors due to impurities. Uranus and Neptune get their blue colors due to the presence of methane that is very good at absorbing the red light and scattering the blue. So just to compare that again with Neptune and Uranus, again, very similar. Uranus is slightly less massive than Neptune, therefore, its cloud layers are a little bit broader. Now, one of the things that's really interesting about these planets is that they have all of this weather or at least three of them do. Uranus, not so much, but even Uranus has a little bit of weather. They all have these dramatic storms and so forth, and yet they are extremely far from the sun and the farther away you go from the sun, the less sunlight you get. Remember that sunlight decreases due to the inverse square law and that's not true of the sun. That's true of any light source. So by the time you are at Jupiter's distance, the sun is much smaller in the sky and you're receiving considerably less light. Not just, not a linear amount, but going off as one over the distance squared. So it really drops off very rapidly. Jupiter is getting just about 1 27th as much sunlight as Earth. And yet it's got all of that weather. Neptune has all that weather and it's way, way out at 30 astronomical units. So the sky from these planets has a brightness similar to twilight here on Earth. You know, it's just kind of dim. There's not much to see. So where are they getting all of this energy from to have all of this weather? I mean, just take a look at some of this in Saturn. You know, again, at Saturn, we're getting just 1% of the sunlight. And yet the temperatures of the cloud tops is a very chilly 135 Kelvin. But it happens to be at a pressure of one bar, which means that if you were floating around in a spacecraft in the upper atmospheres of Saturn, you could go outside and you wouldn't need to wear a pressure suit. You would definitely need to carry your own air supply with you because there's no air debris that's a bunch of other stuff, but you could just walk around without a pressure suit. You would wanna wear a parka, however, because it's really cold. So where do we get this weather from? Well, that's a question that we're gonna talk about in the next segment.