 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the large moons in the Solar System. Now there's not a whole lot of them, but they have been among the best... Now there are not a lot of large moons, but they are among the best studied moons in the Solar System. So we will have a little bit more detail to talk about these than the smaller moons. So let's look at the moons of the outer Solar System. And what we see, these are selected moons. We'll come and talk about some of these here. But there are now more than 200 moons known in the Solar System. Three of those are in the inner Solar System. That is Earth Moon and the two small ones that orbit Mars. There are six large moons in the outer Solar System. So there's seven total, one in the inner Solar System. That's our Moon. And then six in the outer Solar System. Four around Jupiter, one around Saturn and one around Neptune. Now we can also look at some of the medium moons. And I will discuss those separately. And the small moons, which I really won't discuss in any detail here. Many of the small moons are captured objects. So captured objects from the Kuiper Belt or from the Asteroid Belt. And I will not be going into great detail in these. Many of them are very small and very tiny bits of rock and ice that have been captured. So let's start out looking at the four moons around Jupiter. So these were first seen by Galileo and recorded. And if you recall, this is one of the first instances of seeing something where objects did not orbit either the Earth or Sun. So it was different. We were seeing that other objects could be orbited and that there didn't have to be one center of motion in the universe. We will look at Io, the first one here, which is volcanically active and is the most volcanically active object in the Solar System. We will look at Europa, which has a very watery and a great ocean below its surface. And we will look at Ganymede and Callisto here, which have much older, heavily cratered surfaces. So we will look at each of these in turn in a little bit more detail. Now let's start out with Io. Io is, as I've already said, the most volcanically active object in the Solar System. About 25% of its surface, one quarter of its surface is warm lava and there are no impact craters. So everything we see here is volcanic. And there are some volcanoes. The volcanoes do not last a long time. The planet is essentially turning itself inside out and resurfacing itself in a very short time scale. So any craters that do form, now remember that means it does get impacted by craters, but any impact craters simply do not last long enough. The erosional and volcanic processes here are much greater than they are on Earth. We do see the large volcano, known as Pele, which is again comparable, you can see comparable to Olympus Mons and you can see again how they compare to the Hawaiian Islands, not just the volcanoes, but the entire islands themselves. So a very large volcano there, but much of the surface is constantly changed. Now the question comes why? Why is Io, which is about the size of our own moon, so volcanically active? And it all comes down to tidal heating, tidal effects of Jupiter that will heat up the interior. Now, when we look at the interior, as we believe Io looks, let's take a look at that here, and Io has a rocky metallic core and then a very, very hot rocky mantle. It has very little ice, no ice of any kind. So what it is, is it is constantly being pulled by the force of Jupiter. Now remember how a tidal force works. Jupiter pulls stronger on one side than it does on the other because one side is closer to Jupiter and that causes it to be deformed. Now in the case of the moon pulling on Earth, it's not enough to really distort the solid rock that Earth is composed of. However, Jupiter is 300 times more massive than Earth. And Io is about the size of our moon and about the same distance our moon is from Earth. So the tidal forces are much, much larger and it can actually distort that significantly. Let's take a little look at that here in this animation and we can see how as Io moves around Jupiter, it is constantly being stretched and pulled. Now note that it is locked to Jupiter just as our moon is to Earth. One side always keeps facing Jupiter. Now if it just did that, it would just be an elongated object pointing towards Jupiter and there would be no significant tidal changes because it would just be stretched out and that bulge would always point toward Jupiter. However, Io is in an elliptical orbit and there are interactions with Europa and Ganymede, the next two moons out, that will constantly twist its orbit a little bit and that gives it this tidal effect. Now it ends up being something like needing a lump of clay. So if you take a lump of clay, it's cold and hard and doesn't want to move much, but if you sit there and hold it and work it for a little while, it will slowly become warm and pliable. And that's what's happened to Io over billions of years. The interior has warmed up by this constant heating of the tidal forces from Jupiter, making it incredibly volcanically active. Now that's Io. Let's look a little further out as we work our way out from Jupiter and we see Europa, which is a water world. It has a very icy surface, so while the surface of Io was rock, the surface of Europa is ice and it does have an ocean of salt water down below. Now interestingly, this is actually smaller than our moon, but it has more water than the entire Earth. Now we think of ourselves as a water world, but remember the water in the oceans is only, goes down a few miles and it goes thousands of miles down to the core. So the vast majority of the Earth is dry. Here on Europa we do have that metallic interior, that metallic core, and then a rocky mantle. And then above that we have an ocean of water, many tens or a hundred kilometers thick. So it's a very thick deep ocean there and that is where we may think, could life possibly exist there? And then it has a very thick icy crust, so much thicker than Earth's crust, but it still does get cracked by tidal forces from Jupiter. Now Europa is further away, so the tidal forces are far less, but they still exist and that will heat up things interior of Europa as well. Like Io it has an extremely young surface with only a handful of impact craters seen. We see cracks on the surface and icy flows from material. So if the surface cracks a little bit you can have material flowing up like an ice volcano that will flow out and fill in the lower lying areas. In a way Europa is much like the Arctic Ocean here on Earth. The Arctic Ocean is all below a big layer of ice. Now we can look at some of those cracks we see on the surface. Let's take a closer look at those. And here is an example of one of those where we've seen material that has flowed out from the interior. And the various different structured terrains from watery material slushy material that reached the surface and was then frozen. So you can get some flows, but nothing liquid will last because there is no atmosphere. So it's a vacuum there and once that material starts to go it will very quickly solidify once it reaches the surface of Europa. But that constantly resurfaces it again minimizing how many impact craters we can see on the surface. Now let's go a little further out to Ganymede. Ganymede is the largest moon in the solar system. It's actually larger than the planet Mercury. It has more craters than Io and Europa. And it has an older surface. Now when we look at the surface here we have to remember when we get to the outer solar system this surface is made up of ice. So we're not looking at rock here. We are not looking at impacts on rock. We are looking at impacts on ice. Now we have to think of ice a little bit differently than we do here on Earth. Here we have ice usually relatively close to its melting point. In Ganymede it is way below its melting point. It's not even close. So it behaves much like rock does here on Earth. We do see signs of some tectonic and maybe volcanic activity, some icy flows of material that have occurred in the past. Ganymede likely also has a liquid water interior. It's also interesting in that it has a magnetic field. And you remember a magnetic field required some kind of liquid material that could conduct electricity. Now on Earth that was the molten outer core. On Jupiter it was metallic hydrogen. On Uranus and Neptune we wondered if it might be some kind of a slushy mixture that was going there. Could Ganymede be something similar to that or is there something else happening with Ganymede as well? But we do know that there has to be some kind of molten interior, some kind of material there. Now we can take a little bit closer look at Ganymede and as we see it rotate here we can actually look at the whole thing. This is taken from a number of images put together and we can see all the different structures lighter and darker regions. So the very bright spots we see are impact craters. So those are relatively fresh impact craters that dug into the surface. It also picks up darker materials, some generally carbon compounds that look a little sooty and those are the darker regions that we see. So those are some that are a little bit been there a little bit longer and we can see the very young craters that have just been exposed. Now that doesn't mean last week, last month that means over the last few million years or 10 million years. It takes a long time for the moon to pick up that much material. Now let's go ahead and look at the last moon of Jupiter that we're going to talk about here and that is Callisto. Callisto has a very old heavily cratered surface and in fact an icy surface much like Ganymede, lots of impact craters. You can see all of those bright spots here. So it's very similar to the lunar highlands. And again, to remember, ice in the outer solar system is much like rock. So we get volcanoes, we get volcanoes of ice. Now one interesting thing we note about Callisto that is not completely understood, it is not fully differentiated. So most of the large objects of things of this size differentiate so that the densest materials go down to the core. For whatever reason, Callisto has not done this, making it different than other large objects that we see. Now, that's the four large moons around Jupiter. There are two more in the outer solar system. First of those is Titan, the largest moon of Saturn and is the only moon with a significant atmosphere. And that makes it difficult to study because we cannot see its surface. Its atmosphere is comparable to Earth in terms of pressure, and it is primarily made up of nitrogen. It's actually about 50% more atmospheric pressure than Earth, not a lot more. It is the only object in the outer solar system to date that has been landed on. And we have been able to study the surface by the lander that landed there as part of the Cassini mission. We had the Huygens lander that landed on Titan and gave us some images of its surface. Now, the rocks that we see here, these are not made of rock. These are chunks of water ice. So we see lots of water ice, but what we are seeing is that we also will see lakes and rivers. So we can look at those. In a radar image, radar is very good, reflects very well off a rough surface and does not reflect so well off smooth surfaces. So the dark regions here are lakes of methane. So Titan is also unusual, and that is the only other planet that has a cycle on it. We have a water cycle here on Earth where you can have water that will rain and then run to flow to rivers and to lakes, and then will be evaporated and the process continues. So Titan has the exact same type of thing with methane and that gives us ideas that maybe Titan could have some form of life. Maybe there could be some life based with methane as the liquid instead of water. And we'll look at that again later in the course. I will talk about that in a future chapter. So let's move on to our last moon to look at here, and that is the large moon of Neptune, Triton. Triton is the largest moon of Neptune. It has ice volcanoes. Now we talk about this. The lava that we see is a water or a water-ammonia mixture. So that behaves much like the molten rock we have on Earth. You have this slushy kind of mixture that erupts up through the volcanoes. When we look at the icy surface, you can see that it has all sorts of structures to it. So very unusual areas. So a lot of this has probably had material flow on it and wipe out craters. We know that it is not an old surface because it is not covered in craters. The icy surface itself has a mixture of water, nitrogen, methane and carbon monoxide. We're getting out to the very depths of the solar system and temperatures are so low that almost everything is frozen. We also note that it has some unusual surface features. And that means that it's active because we see the lack of craters. Why does it have this activity? Well, some guesses, tidal heating from Neptune, radioactive decay from materials from its formation, some kind of icy greenhouse effect where the icy materials help absorb that material. These are all good questions, but you have to remember, the only visit we had to Triton was in 1989 when the Voyager craft flew by Neptune. So we can't really study it in detail from this far, so there's still a lot of questions about this large moon. So let's go ahead and finish up with our summary. And we talked about the six large moons in the outer solar system. Most of them are icy, and most of them sow some kind of geological activity based on ice, just as we have with rock in the inner solar system. The tidal interactions between moon and planet can really play a large role in developing these surface features. Those tidal interactions can keep the planet active, the moon active, even though it would normally be very cool and dead otherwise. So that concludes this lecture on the large moons of the solar system. We'll be back again next time for another topic in astronomy. So until then, everyone, have a great day, and I will see you in class.