 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss the large moons of the Solar System and specifically those moons around the outer giant planets. Now there are a number of moons in the inner Solar System, just three of them. So let's look at the moons overall here. We look at, these are not all of the moons in the Solar System, just a few of them. In fact, there are more than 170 moons known in the Solar System and of those three are in the inner Solar System. That would be around Mercury, Venus, Earth and Mars. The inner Solar System ones here would include the Earth's Moon and the two small moons around Mars. The other large moons in the outer Solar System, so in the outer Solar System there are six more large moons, four of them around Jupiter, one around Saturn, Titan, and one around Neptune, Triton. So those are the six large moons in the outer Solar System that we're going to look at. And you can see from the scale diagram here that they are comparable to the Earth's Moon in size. In fact, most of them are actually a little bit larger than our own Moon. There are also a number of other medium sized moons that we see some of here. A Uranus we see does not have any large moons but does have a group of medium sized moons. And there are some other medium sized moons also around Saturn and Jupiter as well. There are also small moons, very tiny things, which are generally captured objects and those do make up a good percentage of these 170 moons that we know of in the Solar System. So these larger planets have had more debris around them, more chances to catch objects and to capture some moons that way. So let's start off looking at these moons with the Galilean moons of Jupiter and these are the moons that were seen by Galileo. He certainly didn't see anything like the image I show you here. He just saw them as little dots of light orbiting around Jupiter. But they were the first time anything was ever seen that was not orbiting either the Sun or the Earth. So these were first seen by Galileo back in the early 1600s. There are four of them. Io, the closest one, is volcanically active and is the one that we see here. And we're actually seeing volcanoes. We do not see any impact craters on it because it is constantly being resurfaced. It is actually the most volcanically active object in the Solar System. Europa here has an ocean below its surface. So we see a surface of ice, in fact water ice. But down below that there is a liquid water ocean. So there is a lot more to Europa that we're going to want to look at. Ganymede, the largest moon in the Solar System. And Callisto, the outermost of Jupiter's four Galilean satellites. They still have some structures to them, but generally have older and more cratered surfaces. We don't see any craters on Io. We see a handful of craters on Europa. But we see a lot more when we get out to Ganymede and Callisto. So let's look at each of them in a little bit more detail. First of all, Io. Io is the most volcanically active object in the Solar System. More volcanically active than the Earth, and this is an object that is about the size of our own moon. In fact, about 25% of the surface is warm lava. So it is just a lava-covered world orbiting around Jupiter. It has no known impact craters. That makes it the youngest surface that we know of. I should say youngest solid surface. That we know of in the Solar System, because it has no impact craters. Even the Earth has nearly 200 impact craters. Other objects have far more. But of the solid surfaces, this one is the only one that is known of to have no impact craters. Now, why is it so hot? Well, let's take a look at this, what is happening with it. And we see that it is in the interior. It does have a metallic core and then a rocky mantle. Unlike many of the others, we do not see any kinds of ices. However, what it has is tidal heating due to Jupiter. And in fact, the moon is being constantly stretched by the tidal forces of the planet. Around the Earth, the moon causes tides on the Earth and causes water to flow. But it doesn't deform the actual Earth itself to any significant amount. However, Jupiter, 300 times more massive, creates much greater tides on Io and actually stretches it. So, because Io has an elliptical orbit here, and gravitationally interacts with Europa and Ganymede, that means that it is constantly being pushed and pulled. That sometimes, in an elliptical orbit, it is a little bit closer to Jupiter. So, sometimes Io is here a little bit closer, and sometimes it's a little bit further away. So, as it comes around closer, it's getting stretched, the tidal forces become greater. And then, as it comes back out, the tidal forces can be relaxed, and they're less, and the moon deforms back into a more spherical shape. And what it does is kind of like needing a lump of clay. If you take a lump of modeling clay and start working it, you find that it's, first of all, it's very hard to move. However, as you continually work with it, you continually need it back and forth, you are constantly warming it up. And Io has had this going on for billions of years. So, any ices that were once present on Io, as we'll see on other moons of the outer solar system, would be long since gone because of these tidal interactions and this tidal heating of Io by Jupiter. So, it is an interesting moon, a very unusual surface, as it is extremely volcanically active, and as I said, the most volcanically active object in the solar system. So, the next moon to look at, Europa, here, is actually a very watery world. So, what we're looking at is an icy surface that this is all composed of ice. So, unlike Io, which was closer to Jupiter and had a rocky surface, this one is all completely ice on the surface, and it is frozen, and you can see a lot of the cracking present here. So, it does have below it an ocean of liquid salt water that is down below this. So, once we dig down through this ice, we would find that there is actually liquid water below the surface. And we can see that here, that if we looked at an interior sketch of the planet, we see the ice, thick icy crust here, and then we have an ocean here of liquid water. So, that there is an ocean there well below the surface. Now, that's not a very thin crust of ice. It is incredibly thick. That can be tens or hundreds of miles. But when we look at this, Europa is actually smaller than the Earth's moon, but has more water than the Earth. So, if we take all of the water here in the ice and in the liquid water below, then we could count that up, compare it to the Earth, and find that that is more water than all the oceans and all the ice caps and everything, lakes and rivers and everything here on the Earth. So, very small, but actually has more water than the Earth. It has very few impact craters. We can see an example of one here. So, there are a few, but because there are few impact craters, that means the surface is very young. You see some possibly tectonic features on the surface. Maybe some of these cracks are tectonic, and there is some kind of plate activity on Europa. That is something that is being studied. And we get icy flows from the interior. So, Io had no craters because of volcanic activity. Europa is getting icy volcanic activity, where icy material will flow up through the cracks. And then fill in any craters that form. So, Io in a way is similar to the Arctic Ocean here on Earth, where you have a thick layer of ice over the liquid salt water. Now, we can look at it in a little bit more detail here and see a closer up image, and you can actually see some signs of various flows that have occurred in areas where the ice has re-crystallized. So, a lot of flowing features. You can see some of the streaks flowing around on the crust where material has welled up from inside and filled in. Filled in all the lower lying areas, much as our moon was filled in with lava. Now, Europa has been filled in with an icy flow from beneath its surface. Now, as we work our way out, we also see, we then see Ganymede, and Ganymede is the largest moon in the solar system. It is significantly more cratered than Io or Europa, so it has an older surface. But when we look at this, again, we are looking at ice. Everything that we see on the surface is ice. Again, there are signs of possible tectonic activity and ice flows due to tidal interactions with Jupiter. And Ganymede, like Europa, has a liquid water interior. So, we know again of a couple more places that there is liquid water present in the solar system. Also, interestingly, Ganymede has a magnetic field, which means it must have something molten in the interior that is generating that magnetic field. That could be on the Earth, that is the liquid metal core, on some other objects, like Uranus and Neptune. It is maybe something in the slushy mantle, so something like that, either something metallic or something icy that is generating a magnetic field, must be present on Ganymede as well. Now, the furthest out of these is the moon Callisto. So, Callisto is the most distant of the four Galilean satellites. It has a very ancient and cratered surface. So, it looks a lot like our own moon, except once again, this is all ice. So, we're not looking at rock, we are looking at ice. But it is an icy surface with a lot of impact craters, similar in many ways to the lunar highlands. So, not seeing any maria here where areas have been significantly flooded. And what we have to remember is that in the outer solar system, ice behaves like rock does in the inner solar system. So, the rocks that we see that melt and flow in the inner solar system in volcanic activity, that job is taken by ice when we get to the outer solar system. So, the material that is impacting in here is ice. And you have to think this is incredibly cold ice. This isn't ice in the verge of melting, as we tend to think of it. This is ice that is, you know, hundreds of degrees below zero. So, it behaves much like rock does in the inner part of the solar system. We also note that callisto never fully differentiated, which means it never melted completely through and allowed separation by density. So, that is quite different than other large objects in the solar system and something else that astronomers, you know, wonder about this giant moon. Now, moving out to Saturn, Saturn has only one large moon for us to look at here, and that is Titan. Titan is the largest moon of Saturn and the only large moon around the planet. It is also the only moon with a significant atmosphere. And in many ways, it's comparable to the Earth. The pressure is about the same. And it's made primarily of nitrogen. So, even though you could not breathe there, if you could actually stand on Titan, the atmospheric pressure would feel a little bit heavier than you, about 50% more, but not unbearable like the pressures we get on Venus. And the composition of it would be very similar in nitrogen. The difference would be that the temperature is much lower. It is extremely cold on Titan. So, you would not be able to stand it. The pressure you could stand, the nitrogen wouldn't bother you, but the extreme temperature would be a problem. Now, this is the only object in the outer solar system on which we have ever landed. So, we actually had a lander, the Huygens lander, that landed on Saturn. That was part of the Cassini mission, where Cassini orbited Saturn and Huygens was the lander that actually landed on Titan and for about an hour or hour and a half, was able to give us some details of the surface. Because like Venus, Titan is constantly shrouded in clouds so we cannot see the surface directly. So, here's an example of an image and we see all of these rocks present on the surface. And these, remember, these are made up of ice. So, these are blobs of icy materials, primarily water. So, the water is just solidly frozen and behaves again like rock does in the inner solar system. We also see, and from some of the orbital, some of the radar imaging, we were able to see that Titan also has a liquid on the surface and that is liquid methane. So, when we look here, we see all of these darker areas. Here are lakes. There are lakes of methane on the surface and it is the only object in the solar system other than the Earth that has this liquid cycle on its solar system where you can get the lakes and the rivers but you can also get methane rain. So, it can rain down methane and then we can find rivers and we see the example of the lakes here as well. So, Titan may have a methane cycle like the Earth has a water cycle where you can evaporate the water and then have it rain back out and have a complete cycle going. Titan does the same kind of thing with methane. Now, the last of the giant moons we want to look at is Triton. Triton, not to be confused with Titan, is the largest moon of Neptune. So, this is very far out in the solar system. It has ice volcanoes. Again, I've mentioned this that the ice behaves like rock. Well, you can have molten ice and the lava is now a water or a water-ammonia mixture that erupts out and behaves not that different than lava does in the inner solar system. We have to remember that the temperatures are extremely cold out here when we get to the depths of the solar system. So, ice is not what we tend to think of as ice, but it is actually frozen completely solid and very hard so it behaves in many ways much like the rocks. When we look at the surface of Triton, it is very icy so much like the other moons we've looked at, things like water, nitrogen, methane, carbon monoxide, and other materials that are frozen and make up its surface. We do know that it has some unusual surface features indicating activity. So, it's not a heavily cratered moon like Callisto, but it actually has some unusual structured materials here and a different type of terrain down here. Some craters, but not completely cratered, so we wonder what is heating it up? What is giving it some source of internal heat to cause that activity? Things like tidal heating from Neptune or possibilities. Are there any radioactive materials that are still heating up the interior? Or is it some kind of icy greenhouse effect where the sunlight penetrates the ice and warms it but then is unable to escape giving it a source of internal heat? But there has to be something to give it this kind of activity and give it these unusual surface features that we see. So, let's finish up with our summary. So, our summary here, what we have gone over in this lecture, we have that there are six large moons in the outer solar system and there are four around Jupiter, one around Saturn, and one around Neptune. Most of these are icy and they do show geological activity based on ice like we have with rock in the inner solar system. So, inner solar system volcanic activity is based on rocky materials. Outer solar system, it is based on ice. And we find that tidal interactions are important between the moons and the planet that can also play a large role in what surface features are visible. So, there is some kind of activity between these that helps to heat up the planet and cause the various surface features that we see. So, that concludes our lecture on the large moons of the outer solar system. 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.