 Greetings and welcome to the introduction to astronomy. In this video we're going to talk about the medium moons of the solar system. So large moons were some of the very largest moons and there were seven of those present in the solar system including our own moon. There are far more medium moons that we will look at. So let's look at overall the review of the moons here that we see and what we see is that there are more than 170 moons in the solar system and only three of these are in the inner part. That would be the Earth's moon and the two small moons around Mars. Those are the only moons in the inner solar system. The other nearly 170 would all be part of the outer solar system. Among these are there are six large moons for around Jupiter plus Titan and Triton. Titan around Saturn and Triton around Neptune. These are all comparable to the Earth's moon in size. We also see the medium moons which is what we're going to concentrate on today. Those are around Uranus, around Saturn, and some around Jupiter and Neptune as well that we will look at. Now overall there are also small moons which are tiny captured objects in general and those do account for much of the 170 moons that are known in the solar system. So let's take a look at these. We see first of all working our way out through the solar system. We see the medium moons of Jupiter and those are two of them, not very many around Jupiter. Jupiter has the very large moons and lots of small moons but not a lot of medium size moons. Amalthea is one that is 200 kilometers in size. It was discovered in 1892 and was the last satellite to be discovered visually and that means it was not discovered by photograph but was discovered by someone actually looking through a telescope at Jupiter and noticing another moon there. And that was the last time this was done back in the 1890s. It does have a heavily cratered and a reddish surface and a lot of that is due because it is in and it is sulfur from Io which is a very volcanically active moon. So that debris from the volcanic activity, the volcanoes are enough and the escape velocity from Io is low enough that volcanic material is able to be expelled out into space. Otherwise, Amalthea has a very old and inactive surface so we can see some images of it taken here and you can see definitely some craters and it's not a very detailed close-up image but we can see that it is just a good size rock in space orbiting around Jupiter. Now the other medium moon of Jupiter, Amalthea, is about about the same size and has not yet been closely studied by any spacecraft so we don't really have any details of it. So not a lot in terms of medium moons of Jupiter although we have a lot in terms of the larger moons that we can look at. So let's look at the next planet as we move outward. We will look at Saturn. Saturn, the medium moons of Saturn, there are several. Mimus is one of those at 400 kilometers and we see that here it is very heavily cratered including a large impact which gives it the name of the Death Star Moon. So we can see a very large impact that would have been close to similar to things that we saw on our own moon that eventually formed the maria. So large impacts not necessarily just in the inner solar system they occurred all over the place in the solar system. Now the next moon of Saturn would be Enceladus and we see that here it is about 500 kilometers in size. It has a very icy surface but a big mixture of terrain. You can see up in one side lots of craters signifying an older terrain. However other parts of it have few or no craters saying that it is a relatively young terrain. So there is some material that has been some parts of the surface that have been reworked very recently and other parts that have been around likely for a billion years or more because we see the very high density of craters on some parts of its surface. We also know that Enceladus has subsurface water and with plumes erupting from its south polar region. So plumes have been discovered and those are traced back to water below the surface working it way out in a form of icy volcanic activity. So icy volcanoes are actually common in the outer solar system. Now there are several others that we can look at too. Let's take a look at some of the other ones briefly. We have Tethys here at a thousand kilometers in size which is icy and cratered but does have some faults crossing its surface. So it does show some signs of activity but not near as much as some of the others that we've looked at. Dione seen here has is again icy and has a subsurface ocean but does again have some signs of structures on the surface. So some things that we've been able to see that are possibly tidal forces with Saturn that have caused some heating inside and it created some icy volcanic activity. So we do see some of that here. Now the next one to look at would be Rhea and Rhea here at 1500 kilometers again is a very heavily cratered surface with less signs of activity. You can see that this looks a lot more like our moon. You've got some very old craters that have been bombarded by lots of little craters inside them. So much looks much like our own moon in many ways. So we see a variety of activity levels on these medium-sized moons. The last one to look at for Saturn is Eopetus which is 1400 kilometers in size and this is sometimes called the two-faced moon because it has one side that is very dark and another side that is very light in color. So it's very distinct coloration difference between the two sides. The leading edge is dark as it moves through the moves around Saturn and that might be possible that it is picking up a sooty materials, carbon materials as it orbits around and maybe just in the right position to be able to pick up those far more. We do see impact craters on it and well in fact some large impacts seen here and many other small impacts so again like the others it has some kind of combination of activity that we see. Now moving a little bit further out we have the medium moons of Uranus. Uranus does not have a large moon but it does have several medium-sized moons including Miranda at 450 kilometers. This is the smallest of the five major satellites. It is primarily made up of water ice but there are some signs even though this is a very small moon. For a medium-sized moon it is there are signs of intense geological activity and we can see that in the structures that there are some cratered terrain but other things have some really strange properties to them and show signs of maybe some intense geological activity that has occurred in the past. Now as we look a little further out we have aerial. Aerial a little bit bigger than Miranda and shows again an active icy geology so even out in the depths of the solar system we start to see things with various cracks in the surface that are possibly tectonic or in some kind of plates or some kind of volcanic activity that is occurring and then icy flows that well up from the interior to wipe out craters so we don't see a whole lot of craters on the surface. There are certainly some but not as many as we saw on some of the other less active moons. Now three more to look at here for Uranus we have Umbriel which is about 600 kilometers and that has a more heavily cratered icy surface and is the darkest of the five uranium moons that we see that has been studied. You can tell from this image it was not seen up as close when the Voyager craft flew by it. The only craft that has been able to study these was the Voyager spacecraft which flew by Uranus back in 1986. It happened to be closer to some of the moons and not as close to others so some of them we don't have as much detail of. But this is the darkest surface and you can see again very heavily cratered regions on the surface. Now next outward is Titania. Titania is the largest of the moons of Uranus at about 1500 kilometers in size. There are signs of impacts and icy volcanoes in it. So again it's a mixture not completely heavily cratered but also has some icy materials and icy volcanic and tectonic features perhaps that we see as well. And finally as we work our way out here we have Oberon. Oberon about the same size as Titania has a very dark reddish surface and is primarily shaped by impacts. So vast majority of what we see are impact features and not a lot of volcanic icy features. Now the last set to look at would be the moons of Neptune. So Neptune has two medium sized moons and that includes Nereid not studied very well. This would be it at the Voyager 2 spacecraft in 1989 when that flew by. It has an extremely eccentric orbit meaning that it's very elliptical comes in close to Neptune and then goes out very far away. So a very highly eccentric orbit and we see one other moon around Neptune. So let's take a look at Proteus here at about 400 kilometers inside size very heavily cratered and we don't believe it actually formed with Neptune. So it may have been a captured object that formed. Remember that Neptune is right on the edge of what we call the Kuiper Belt at the very edge of the solar system. There are a whole lot of icy bodies out there. So maybe as Mars and Jupiter have been able to capture objects from the asteroid belt Neptune has been able to capture objects from the Kuiper Belt. But again heavily cratered but again both these cases not very well studied the only visit and look at these was the Voyager 2 spacecraft in 1989. So finishing up here let's look at the origins of these moons. Where did they come from? And how do we think the moons formed? Well the large moons we think might have been formed with the planet much as mini solar systems much as planets formed around the Sun, moons, large moons formed around the planet. We think that the small moons were likely captured objects and did not form with the planet itself and that leaves us with the question of what about the medium moons and perhaps at some of each some of these may have formed with them some of them may have been captured objects and it is something that we don't quite understand. We do note that there is a mixture of cratered regions which are old and we have of volcanic regions which are relatively young and we see those on many of these moons. So some of them show both signs some of them show primarily volcanic activity some of them show primarily just the remnants of impact craters. So trying to learn more about these medium sized moons kind of bridges the gap between the largest and the smallest moons in the solar system. So let's finish up with our summary and what we have is first of all the medium moons that we look at in the outer solar system have a varied distribution of surface features. So not they're not all the same we have some that have volcanic activity some that show primarily cratering, other show signs of other activity and we think maybe some of these were captured like most of the small moons of the outer planets but perhaps some of the larger ones also formed with the planet. So it's still something in terms of trying to understand how the moon formation occurs around these planets. So that concludes our lecture on the moons, us medium moons 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.