 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about planetary rings or rings that exist in the solar system. Now the best known of those are around Saturn but there are a number of planets that have rings as well. So let's take a look at what we see here. And we look first of all here's our image. This is the planet Saturn and you can see its ring system stretching out around it. But it is not the only planet with a ring system and in fact all four of the Jovian planets will have a ring system around them. However there are differences between each of these. Saturn's is highly reflective and prominent, very easy to see and we can see that here in this image and in fact Saturn's were the first rings to be detected. The next set to be detected were those of Uranus. Uranus has very narrow rings and very dark rings and they were detected when a star was occulted by the rings or the rings blocked out the light of that star and caused it to dim. Jupiter's were the next detected which are a very faint dust band, some very minor rings but there is at least some portion of a ring there and Neptune's are unusual in that there are gaps in the rings. So they are not nice smooth rings here but there are actually areas where the material is denser and areas where material is less dense. So let's look at ring formation. Where do rings come from in the first place? Well first of all what are the rings made up of? They are billions of tiny particles each orbiting the planet. You can think of them each as a small moon. So there is a small moon orbiting each planet and each of these is very small objects. They may only be a few centimeters in size. They may be up to a meter or so in size but they are not gigantic objects by any sense. The ring particles do interact with each other by gravity which keeps them together and through collisions. So the particles will collide together as they orbit and that will give us the structures that we will see within the rings. Could these be a shattered moon? A collision with the moon might have formed the ring particles so if a large object crashed into a moon and recall that these jovian planets all have lots of moons that could have expelled a lot of ring particles out into space that could have been captured in a ring. So perhaps some sort of collision with the moon or tidal forces as well an object that gets close enough to the planet could have been torn apart by its tidal forces. The gravity on one side of the object, the near side, being stronger than the gravity on the far side. Because the gravitational force depends on distance. If you're close enough to the object it's quite possible that the object will not be able to hold up structurally to a significant force on one side and a very small force on the other. Essentially you're stretching the object apart. And in case if particles are just that close it may just have been prevented from forming in the first place. So let's look at the rings of each of these planets. So starting off here with Jupiter's rings which were discovered in 1979 by the Voyager 1 spacecraft. They are very faint and dusty rings made up of lots of small particles and are best seen in scattered light from behind. So we don't see them well from earth because they're not reflecting a lot of the light but they do scatter the light through and we look from behind Jupiter as seen by the Voyager 1 craft. We can see the dark unilluminated side of Jupiter here and then we're looking through towards the sun and we can see the very thin illuminated ring part right here going around Jupiter. So Jupiter does have this very faint ring but not a lot of detail there to learn about. So what we really want to look at are the rings of Saturn. Rings of Saturn have a lot more detail to them and we can see some of that here. They were actually seen by Galileo but not recognized as rings only as a blob on each side of the planet that sometimes disappeared. By the time telescopes got a little bit bigger just a few decades later in the 1650s, Huygens was able to recognize them as a ring system and around that time or a little bit later Cassini recognized that there were distinct rings not just a single ring and gaps between them. So gaps in between the rings the largest of these is now named the Cassini division in his honor. So here we see the A, B, C, D and F rings out here. Primary ones visible that you'll see are the A, B and C. D ring is a lot fainter and harder to see. F ring is a very thin ring out beyond the A ring and also a little bit harder to see. So when we look at the structures of these rings we see that they are extremely wide and thin. In fact they are thinner than a piece of paper if you took them to scale. So if you scaled them down to a size of paper they would be incredibly thin, much, much thinner than that piece of paper. We designate the rings by letters there are the primary ones are the A, B and C rings the first three to be discovered and those are the primary widest rings that we see and those are the big bright ones there and you also have the Cassini division the gap in between those rings too. So there are gaps and not just the one not just the Cassini division but you can see that there's another one out here and there are other gaps and structures within the rings as well. So the rings are not just a single ring but actually have multiple ringlets within each ring and if we look a little bit closer here zoom in on the rings essentially we can see that there are all kinds of structures here little gaps and areas where there are a lot more particles brighter areas so big detail towards what the rings are there's a lot of structures there and when you think about within each of these are just billions of particles all orbiting around Saturn and kept there so why do they have this detailed structure we'll come back and look at that shortly. So let's look here at the compositions what are these made up of Saturn's rings are primarily water ice which makes them highly reflective so why do we see them prominently from earth because they're reflecting a lot of light the size of the particles varies maybe up to about a meter in size and some of them will clump together now this is an artist's conception of what the rings might look like if you were sitting there inside the rings of Saturn themselves so you'd have clumps of material and these would likely stick together they're not strong enough to really hold themselves together gravitationally so as objects collide they would stick together and other times they would be pushed apart now Saturn again has the primary and most detailed rings but the other objects have rings as well so let's look at Uranus here Uranus's rings were discovered in 1977 a couple of years before Jupiter's were discovered although hundreds of years after the rings of Saturn were known they were discovered during the occultation of a star what happened was Uranus was going to pass in front of a star so astronomers wanted to study how its light dimmed as it passed in front of as the star as it passed in front of the star but before the planet passed in front of the star we saw that the light dimmed multiple times and this repeated after the occultation in the reverse order kind of confirming that the rings did the rings did exist so the rings as the star was passing the rings would pass in front of the star and that would dim the light a little bit as each of these rings passed in front of the star in turn and then you can imagine on the other side you went the other direction and it would pass through the rings in the opposite order rings are extremely narrow compared to the rings of Saturn so we see the rings of Uranus here very very thin these are not the wide broad rings that we saw of Saturn they are actually confined by shepherding satellites little moons that will orbit just on one side and just on the other side and keep all those particles in line so essentially as one moon orbits and the other moon orbits their gravity will keep those very well confined now what is the composition of these rings well they are very dark particles again why do we not see them from earth they're not icy like the rings of Saturn they are in fact as dark as coal so we don't really see them in forward in looking forward we do see them in forward scattered light which means we're looking back towards the sun so if we look at the image here you know there's what we see of the rings from the earth but if we were behind them looking back we would see all sorts of light scattering through these rings so there are a lot more structures there that we don't actually see from earth but that we had to get around behind and the only object that really went close to this was the Voyager 2 spacecraft that actually went past Uranus and was able to turn back and look back to the sun but it's a very different structure you get if you're looking from the sun versus if you're looking from behind when you look from behind other than the composition we know is different we see very similar structures to what we saw on Saturn now the last of the objects to look at with rings would be Neptune Neptune has a very very dark rings in many ways similar to those of Uranus and they are quite dark and we can see them here this is blocking out the light from the planet which would overwhelm the very faint rings so they're there but we've got to block out the planet's light in order to even be able to see them they are seen as arcs sometimes so if we look here another image of it and we see that there are denser areas of the rings and there are areas where there is less ring material so unlike the rings of Saturn and Uranus which seem to be relatively smooth the rings of Neptune are sometimes seen as partial rings or even just ring arcs so there's a little bit of a difference there in Neptune are these relatively young or relatively old rings something we really do not know now the last thing we want to look at is the interactions between the moons and the rings there is an interconnection between these and in fact without the moons the rings would not exist where did the rings come from probably from particles blasted off the moons in impacts and that would then give the rings particles a supply of material the moons also keep the rings confined and that's what we call the shepherding satellites which we see here orbiting around and keeping this the ring particles in this very narrow ring well confined so they keep the ring particles in place the rings would dissipate over astronomical time scales if we run models of all these particles orbiting we would find that they would slowly dissipate out into space there has to be some form of resupply and that would be from the moons that impacts on the moons could actually throw more material back into them and replenish the ring particles so where do we get those new particles from impacts on the moons the moons also create all of the structure we see in the rings the rings would be much smoother without the moons but the structures that we see are different patterns that we see and if we look real close at one of the rings here we can see all sorts of structures that are present in the rings of Saturn the moons create those by things like resonances when the orbital period of a ring particle is a multiple of the orbital period of a moon what does that mean well that means the moon orbits once the ring particle orbits twice and that causes particles to be enhanced in some areas and depleted in others so we get these very distinct gaps because of resonances with the moons narrow rings are defined very well defined because of shepherding satellites and all of the other structures that we see here are also likely interactions with various moons of the planets so let's finish up with our summary and what we find is that yes all jovian planets have a ring system but the ring particles can vary in composition and each orbits the planet like a tiny moon the moons interact with the rings to give them structure and to help keep the rings confined so it is because of the moons that we see the structure and the well-defined small rings and the real defined edges of other rings so that concludes our 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