 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about rings in the different planetary systems and we will see that all of the outer planets do have a ring system, although there are some significant differences between each of these. So, what do we see? Well, here is Saturn, of course, the great ringed planet with the most prominent and best known ring system, but we know that all four of these Jovian planets do have a ring system. And while they are similar in that they are rings, there are significant differences between them. Jupiter is essentially a dust band. Saturn's rings are highly reflective and very prominent. Saturn's are very narrow and dark, and Neptune's have gaps in their rings. So, they are all a little bit different and we are going to look at each of them in a little bit more detail over the coming slides. So, let's first of all look at ring formation. What are these rings made of? Well, they are essentially like billions of tiny moons orbiting the planet. And the ring particles will interact with each other gravitationally and through collisions. And how they are distributed, if we look at how these are set, there is a limit, and this is the tidal stability limit. That is where a large object would be torn apart if it was any closer than that. So, smaller objects could exist, but a large moon would not be able to exist inside that region. So, you can see some smaller moons in there, but the very large moons tend to be outside of that, such as the large moons of Jupiter, Titan, the large moon of Saturn, and others. So, it's very difficult to have those, but you notice that the rings are all inside this stability limit. So, things that are too close together could get ripped apart by these tidal forces. So, if a body gets too close to them, it could be torn apart or prevented from forming altogether. It's also thought that they could be particles from shattered moons that collisions with a moon could have formed the ring particle. So, if some object smashed into the moon, it could send a lot of debris into the system, forming a ring system around each of these planets. Now, let's look at each of these ring systems in turn, and we have Jupiter's rings, first of all. They were discovered in 1979. They were actually the third set of rings to be discovered. And by this point, we started looking for them, because we'd known of rings around Saturn for a long time, and a couple years previously we had detected rings around Uranus. These are very faint and dusty rings, small particles, and they are best seen in scattered light from behind. So, note here how we have the spacecraft behind Jupiter looking at the unilluminated side, and we see the ring here in scattered light. So, looking at it from the front, it's very hard to see. Looking at it from behind, it illuminates it a little bit better and gives us a better view of Jupiter's ring. But it's not a very prominent ring as we see around Saturn, and as we will see, rings come and go. So, it's possible that Jupiter had a nicer ring system in the past and may have one in the future. So, Saturn's rings were seen by Galileo, but were not actually recognized as rings at the time. It was not until the 1650s, a few decades later, that Christian Huygens recognized these as rings, and here we see his drawings of Saturn orbiting around with the rings in a tilt. And when they're tilted, depending on the orientation of Earth relative to the ring plane, we sometimes see the rings almost invisible. Here the rings just a thin line cutting across the planet and essentially invisible. At other times, such as these two positions, we see the rings tilted, and we're looking at either the top or the bottom of the rings very clearly. So, that depends on the specific orientation of them relative to Earth. Now, it was Cassini who recognized that there were distinct rings, and we see them labeled here. They were labeled the A, B, and C ring, where the first three primary rings to be discovered. The D ring is very faint interior. That ring is exterior to these, and is very thin ring. But Cassini noted that there were distinct rings and gaps between these rings, and the largest of these is called the Cassini division, and we see that here between the A and B rings. So, they're not just one single ring, but there are lots of rings and billions of individual particles that are orbiting. Now, Saturn's rings are extremely wide and thin, and if you could scale them down to a piece of paper, they would be thinner than that piece of paper, and that's why they disappear when we see them edge on. Now, note, they are not thinner than a piece of paper. They are many kilometers thick, but if you scale them down relative to their diameter, then the thickness to that scale would be extremely thin. We note that the rings are designated by letters. The A, B, and C rings are the widest and most prominent, and we're the first ones to be discovered. We saw the F ring, which was very narrow, and we see that there are multiple ringlets within each ring. If we look at them in more detail, the rings have more and more detail as we look at them closer. So, there are lots of little gaps and lots of regions where there are brighter concentrations of material, so they are nowhere close to being uniform. There are thinner areas where there are hardly any rings, and there are areas where there are a lot of ring particles. So, what are they made of? Well, they're primarily icy, and in fact, primarily water ice is what we see. They are highly reflective, but the sizes vary. You can have very tiny particles, and you may have things up to maybe a meter or so in size. So, things the size of a human would be about as large as you would get there. They can clump together as we see here. Now, this is not a photo of Saturn's rings. This is actually an artist's conception of what you might see based on our understanding of what the rings are like. Now, how about the other planets? Well, we looked at Jupiter and Saturn. How about Uranus? Uranus's rings were discovered in 1977. They were the second set of rings to be discovered, and they were discovered quite by accident. Astronomers were watching the occultation of a star, and it was noted that the light from the star dimmed multiple times before the actual planet passed in front of it. An occultation occurs when a planet or other object passes in front of another astronomical object. Well, this was a star that Uranus was going to occult, and as we see here, with the rings, if one of these ring particles then would fall in front of that star, the star would dim before it got to the planet. And in fact, it was noted that there were five dimmings for five different rings. And of course, the image was left... the experiment was left going afterwards, and we saw the repeated astronomical object. And we saw the repeated of this in the reverse order, which confirmed the existence of rings around the planet Uranus. However, we note that these rings are very narrow compared to what we looked at with Saturn. And we see that they are confined by shepherding satellites. So there are small satellites that orbit around, and what that satellite will do is pull particles into specific rings. So as particles start to deviate and change their position, the gravity of the moon will pull and accelerate them and put them in a different orbit, or will pull and decelerate them into another orbit. So it confines them and allows the rings to stay narrow over much longer periods of time. Normally, over hundreds of millions of years, even with this, the rings should disperse. So we're going to have to look at later how rings may be replenished. So what are these rings made up of? Well, these are extremely dark, so very different than the rings of Saturn. So it's not a surprise that we didn't detect them right away. These are as dark as coal. So we think they may have carbon or hydrocarbon compounds around the icy material. They are also better seen in forward-scattered light. So here we see the rings in looking directly on towards the planet, and here we see them looking from behind. And you'll note that there's a lot more detail to be seen when we look from behind. So much like Jupiter's rings, we want to look back toward the Sun. In order to do this, of course, we have to have a craft out beyond Uranus to be able to see them. So this is a view we cannot get from Earth. Now how about the rings of Neptune? Well, we didn't know about the rings of Neptune. They were discovered by Voyager in 1989, but by this point we knew of rings around three of the giant planets, so we kind of expected to see something when we got to Neptune, and Voyager was going to look for those. In a way, these are similar, very dark rings similar to those of Uranus. However, they are different in that they have ring arcs. That material is concentrated in various areas, not just to and from the planet, but within the individual ring. There are areas of higher concentration, as we see in these couple, and then there are other regions which have much lower concentration of ring particles. So as far as we know, this is unique to Neptune. We do not see this very significantly in any of the other planets that we look at with ring systems. Now, how do these work? How do these interact with each other? Well, we find that the moons and rings are interconnected. So without the moons, there would be no rings. And the moons also create structures in the rings. They keep them confined by shepherding satellites, and here we see that, here we see the ring here, and we see one satellite on one side of it and one satellite on the other side, and those satellites will travel around and will keep particles from straying too far from the ring. These keep the ring particles in place. We find through models that the rings would dissipate over astronomical time scales if they were not resupplied. So perhaps new particles from impacts on the moons throwing new material into the ring system and keeping them replenished for longer scales of time. Now, as I said, the moons also create the structures within the rings, and this is because of resonances. So if a ring particle orbits twice in the time that a moon takes to orbit once, then they will keep lining back up and there will be a constant tug of gravity and that will clear out, giving us some of these gaps. And we see several of these gaps here, here, labeled, and those are all due to resonances with the various moons of Saturn. Certainly a bigger moon will do more than a smaller moon, but it also depends on the distance as well. So what it does is, so here, example, the moon orbits once, the ring particle orbits twice, they line up again, and this enhances them so certain areas get an excess of ring particles because they have a lack of this resonance and they're depleted in others, such as those that I've marked that have gaps in them, and that's again because of those resonances. Now, we will see this again when we look at the asteroid belt. The asteroid belt will have the same kind of patterns and we will look at that in coming lectures when we talk about the asteroids. So let's go ahead and finish up with our summary and what we've looked at this time is that all of the Jovian planets have a ring system and the ring particles vary in composition, but each orbits the planet like a tiny moon, so billions of moons around each of these planets. The moons interact with the rings to give them structure and to keep the rings confined and in place and allow them to exist over much longer time frames. So that concludes this lecture on planetary rings. 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.