 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss the structures of the outer planets and what we understand about how they are composed inside. And that can be difficult as it is with the inner planets because we cannot directly explore these areas. We do not cannot go down inside Jupiter and see what the interior structure is like. So we have to look overall at other properties that we see and use that to interpret what the interior of these objects will be like. So let's start off with some general properties. What do we know about these giant planets? Well first of all they're all far from the Sun. They are all very large and that means in size and in mass. All of the terrestrial planets that's Mercury, Venus, Earth and Mars would fit inside one of the smaller Jovian planets. They are low density meaning they have a higher concentration of things like ices meaning water, methane and ammonia and gases than the inner planets. They all rotate very quickly. They all have a large number of moons and each of these has a ring system. Only Saturns is pictured but Jupiter, Uranus and Neptune all also have ring systems. Now we differentiate between the giant planets or the Jovian planets and the terrestrial planets but even within the Jovian planets we see two distinct types. So what do we see here? Well we see that Jupiter and Saturn the two larger of the Jovian planets are primarily a gaseous and liquid hydrogen and in fact primarily especially in Jupiter what we call liquid metallic hydrogen. Now how is hydrogen a metal? We all know hydrogen is a gas but under very high pressures hydrogen can actually begin to behave like a metal. In a metal electrons are able to freely flow between atoms. Normally hydrogen does not allow this the hydrogen sorry the electrons are bound to each individual atom. However under extreme pressures the hydrogen is not only liquefied converted into a liquid but it also is converted to a metallic form where the electrons are free to flow between the different molecules of hydrogen and that allows for electrical conductivity which will be important later on. But this liquid metallic hydrogen layer is this darker area and is a very thick, thick part of the construction of Jupiter a much smaller section of Saturn and not present in Uranus or Neptune. So you need the very high masses high pressures that occur in the two larger planets. On Uranus and Neptune on the other hand the interior is icy. So we have things like water, ammonia and methane ices which are the primary components when you get down inside these smaller of the Jovian planets. Each of them does have a central core of rock and ice which would be several times would be more massive than the earth and we see that down at the center they would be highly compressed by all of the other material on top of them. So let's take a look at what we understand of the internal structures of these planets and first of all let's look at Jupiter and Saturn. Jupiter and Saturn would have a rock and ice core and that would be the material that we are used to that would include rocky and metallic material way down here in the center so we would see things like rocks, metals and even some ices because of where it formed. Remember that ices here are not necessarily icy as we think of them but are actually just the compounds of water, methane and ammonia and could still be present here deep down in the core. The large layer for Jupiter is the liquid metallic hydrogen and again the metallic part is the key that will be very important because it gives away for Jupiter to have an extremely strong magnetic field. Up a little further we would get just plain liquid hydrogen so not under the extreme conditions where it is compressed into a metallic material but where it is still liquefied so the vast majority of Jupiter's interior is this liquid. Then comes the part that we see the gaseous hydrogen atmosphere that is the portion that we see of Jupiter, the outer layers and then the structures in the various materials in the various clouds that occur on Jupiter. So again we have that rock and ice core that is way down inside, very thick layers of different liquids primarily hydrogen and a gaseous hydrogen atmosphere. Saturn on the other hand is really just very similar to Jupiter except it has less mass and that means that it has less pressure and forms less of the liquid metallic hydrogen and if we come back to this when we look at magnetic fields later on we will see that that is one of the reasons that Saturn has less of a magnetic field, a less strong magnetic field than Jupiter. Now the two outer Jovian planets have a slightly different structure and we'll look at those here this applies pretty much to both Uranus and Neptune. We do see the rocky core so rock and metal down here at the center. We do see a mantle note here how thick the mantle is by comparison we have a very very thick mantle as compared to what we saw with for example the earth and a gaseous atmosphere. So that is hydrogen, helium, and then methane gases and methane is what gives these planets their distinct color. Methane absorbs red light, so light from the sun the red and oranges are absorbed by the methane leaving only the blues and greens to be reflected back. So there is a structure to these again it is very similar you have something more massive than the earth at the center compressed down that is made primarily of rock you then have an icy mantle around that and then you have the outer atmosphere layers. Now the Jovian planets or at least most of them do have an internal heat source so they have an internal source of heat and most of them are quite significant. What this means is that they give off more energy than they receive from the sun so a certain amount of sunlight reaches them warming them up but they're actually giving off more energy than they are receiving. There are a couple of different reasons for this that can be for example for Jupiter considered energy of formation when we collapse an object down from very large distances we take potential energy, energy of position convert that to kinetic energy as material falls down inward and then that is converted into heat so there Jupiter still has some energy left over from its formation. There also can be energy from differentiation which is a separation of of materials by density and that happens in Saturn's atmosphere. Saturn is raining out helium so helium is going down to the down towards the inner layers whereas hydrogen rising to the surface and again that can cause an increase in the in the heat there as that energy that is formed that potential energy being converted to kinetic energy and then becoming heat warming up the interior of the planet so both Jupiter and Saturn have significant internal heat sources and give off a lot more energy than they receive from the Sun. Uranus and Neptune are more confusing Neptune does have some source of internal energy we can measure that it's giving off more than it can possibly receive from the Sun but Uranus does not and the big question then would be why? Why is this the case? So why does Neptune have a source of energy and Uranus does not? So that is something that is still has to be studied. Now I mentioned magnetic fields let's take a look at those Jupiter does have the strongest magnetic field and that is because of that liquid metallic hydrogen that spinning very fast will then generate a magnetic field currents in that metal as it spins will generate electric fields which spinning electrical fields will generate magnetic fields much as it does in the earth in the metallic outer core in that case made of iron and nickel but in this case we are looking at hydrogen that is doing the same thing and will give Jupiter the strongest magnetic field that we have seen that means that Jupiter will have aurora on it just like the earth and in fact any planet that has a magnetic field and has an atmosphere will have an aurora so that means that all of the outer planets will show auroral activity and we can see that with Jupiter here right around the north magnetic pole of Jupiter where we can see the particles from the Sun have excited and caused this to caused it to glow so just like the aurora we'd see here on earth we can see those on Jupiter Saturn Uranus and Neptune as well now Saturn by comparison has a weaker magnetic field less metallic hydrogen so it does not have as much of a concentration of that metallic hydrogen as Jupiter had so its magnetic field is going to be weaker Uranus and Neptune also have magnetic fields but the source is different remember they do not have the metallic hydrogen so there they would not be able to form a magnetic field because of that and we also note the little unusual is that their magnetic fields are actually offset from the center most magnetic fields are centered on or very close to the center of the planet well as we see here this we would have for this would be for Uranus we can see that there is the rotational axis right here this is the magnetic axis they're not aligned up for the most part they are lined up with other planets at least close to within a few degrees not almost 60 degrees apart and it also is not centered at the center of the planet which would be right at the middle of this circle instead it's about a third of the radius out so it's about a it's centered about a third of the way out from the center of the planet so it's not aligned with rotation and it is offset from the center that is something that we have to try to understand and we may wonder that it's maybe it's caused by something in the mantle of the planet instead so maybe those ices form some kind of electrical currents under high pressures remember that ices water methane and ammonia are present in great quantities in the mantles of both Uranus and Neptune so if it is something occurring off in the mantle maybe that would account for why it is not centered on the center of the planet and maybe something different about how that is generated gives us the fact that it's also not aligned so let's finish up here with our summary and what we've seen is that there are two distinct groups of planets in the outer part of the solar system we call them as a group the jovian planets but we have the liquid and gas giants which are Jupiter and Saturn and the ice giants which are Uranus and Neptune composed primarily of ices we know that the jovian planets have sources of internal heat with the exception of Uranus the other three have very significant sources of internal heat either left over from their formation or from differentiation or maybe from some yet unidentified source we do note that every single one of the jovian planets has a magnetic field but formed by different processes Jupiter and Saturn by the liquid metallic hydrogen Uranus and Neptune by some not completely understood process perhaps something in the icy mantle that would account for the differences between their magnetic fields and those of the two larger planets so that concludes our lecture on the interior structures 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