 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the medium-sized moons in the Solar System. We've looked at the large moons. Now let's look at those that are not quite as large. And again, we're not going to really talk about the small moons. There's a lot of them, but not many of them well-studied by spacecraft, and in fact we'll start to see that even with the medium-sized moons that not all of them have been well-studied. Now just as a reminder, looking at moons in the outer Solar System, there are 200 moons in the Solar System. Only three of these are in the inner Solar System, our moon and the two small ones of Mars. There are six more large moons that are comparable to Earth's moon in size. There are the medium moons, which we will look at this time, and there are small moons, which are often captured objects. So let's go ahead and start out, and we'll work our way out in the Solar System, starting with Jupiter. Jupiter has two medium-sized moons, as you may recall, Jupiter had four very large moons that we've talked about previously. This is the medium-sized moon, so these are things that are in this case a few hundred kilometers. Amalthea is 200 kilometers, discovered in 1892, and was the last satellite to be discovered visually. That means by looking through a telescope, so not recording it on a photographic plate or a CCD, not recording an image of it and finding that the moon was there, but to actually see it. It was the last of the moons that was discovered visually. It has a red surface and is heavily cratered. Likely, possibly part of this is sulfur from Io. It's a very inner moon, but a very old, inactive surface. So it's a very small air and not going to tend to be very active. Amalthea, the second one, has not been closely studied by spacecraft, so I don't even have an image of it to share with you. Saturn had only one large moon, but has a lot of medium-sized moons, so we'll see several things in the few hundred kilometer range that we look at. First we have Mimus, which is a heavily cratered moon, and we see that here, including a very large impact off to the right-hand side, sometimes giving it the nickname of the Death Star moon after the Death Star from Star Wars. And it somehow maybe resembles that, although we didn't actually discover that moon until after the original Star Wars movie came out. So that's a very heavily cratered moon, very old surface. And Solatus is a little bit larger, 500 kilometers, has a very icy surface, so we see already very big differences between these two. One was very heavily cratered, and this one has a mixture of some cratered regions in the northern section up here, and some regions with very few craters down towards the middle and lower sections. So it is a mixture of old terrain and young terrain. And we believe this is because it has subsurface water. We've actually seen water plumes erupting from the south polar region. So there is water down below, now that would explain why we see very few craters. Remember that water like this would then fill in craters and freeze. And that's what we're seeing is the frozen water from the interior that has covered some of the lower areas of the moon. So we know that there is some subsurface water, likely giving it its unusual surface features. These aren't the only two moons, there's actually several larger ones too. We have Tethys, over 1,000 kilometers, twice as big as the other two, which is a combination of icy cratered with some faults crossing its surface. We have Dione, which has, again, icy with a subsurface ocean down below. We have Rhea, heavily cratered surface. So again, we see that they're all very different and that there are a mixture of cratered surfaces and some very younger surfaces as well. Remember, even the heavily cratered surfaces still are all ice, so we're still looking at ice here. Eopetus is the two-faced moon, has a light half and a dark half. So you can see part of that here, you can see this whole section is covered in kind of a sooty carbonaceous material and the rest of it is very light and bright. And it may be due to the fact that this is the way it orbits around Saturn, remember the moons are always locked so one side stays facing Saturn. So the darker side could be the leading edge traveling around in the orbit and therefore picks up more material debris from other areas and maybe it's been able to pick up some debris over time and given it that darker half of a surface. So Titan, while Saturn had the one large moon, Titan that we looked at previously, we get to see six that are classified in the middle-sized range here, four in the 1,000 to 1,500 kilometer range and a couple in the few hundred kilometer range. Now one planet we did not talk about with large moons was Uranus. Uranus does not have any large moons but does have several small, some medium moons and some small moons. So we have Miranda here, Miranda is a very unusual moon in that it is the smallest of the five moons of Uranus. It is mostly water ice but it has signs of intense geological activity. Now it's only a few hundred kilometers, less than 500 kilometers across but there has definitely been some faulting and shifting of material and perhaps material coming up from inside. So it's a very unusual, most of the smaller size of the medium moons we'd expect to be heavily cratered. This one is not. Now we also have Ariel about twice, more than twice as big and again this might expect a little more of the icy geology that we see. We get some of those tectonic cracks that you can see stretching across the moon and icy flows from the interior that have filled in and wiped out craters. Now there's a couple more, there are five of these moons to look at and let's take a look at the others. Here we have Umbriel, a very dark surface, the darkest surface among these moons. It is icy and we can see some of the impacts there. And we have Titania, again the largest of these moons at about 1500 kilometers, and signs of impacts and again more icy volcanoes. We can see those scarring impacts and the bright colors there but also some signs of other activity so even though these are not the largest moons we still see some signs of activity within them. And finally we have Oberon. Oberon is, has a dark reddish surface and is primarily impact so less volcanic activity. So the question could be why are some of these volcanically active and others not, it may have to do with the tidal forces or other things that are going on with these moons that turn them up inside because some of them are very active whereas others are very inactive and heavily cratered. Now you do have to recall that for Uranus and for Neptune coming up we have only looked at these with one spacecraft and you'll note that not all of them are really high detailed high resolution images and that's because their images of the configuration when Voyager 2 happened to pass through in 1986. So those are the only images we have to study these moons. We've noticed that the moons have changed significantly since then but we don't have detailed high resolution images as we might have had with other the moons closer in. So finally let's look at Neptune here. Neptune has two moons, Nereid, which has not been well looked at and you can see that here again. It is a highly eccentric orbit but not very, very well studied. Probably about 170 kilometers across and then we have Proteus and again a heavily cratered likely a captured object probably did not form with Neptune. So not as well here we did look at the one large moon of Neptune but there are several medium sized ones as well and again with all of these there are small moons too that were really not going to get into in these lectures. So where did the medium moons come from? We think perhaps the large moons formed with the planet and the small moons were captured. So small moons were captured objects from the asteroid belt or the Kuiper belt depending on the planet. Medium moons maybe at some kind of combination maybe some of these are captured maybe some of them formed with the planets themselves but it's still a good question as we look at these we have a pretty good idea for the large and small moons but less of an idea of how the medium moons may have formed. So let's go ahead and finish up with our summary and what we've looked at this time is the medium moons of the solar system show a variety of surface features wide variety some are heavily cratered while some show signs of volcanic type activity with ice. Many of these could be captured like some of these may have been captured like many of the smaller moons some of them may be natural satellites that formed with the planets. So that concludes this lecture on the medium moons of the 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.