Okay, so at this point we're left with a theoretical understanding of what might be in the inside of Jupiter. We're left with some pretty big uncertainties, one of them being, is there is extra stuff besides hydrogen and helium on the inside? If there is, where is that extra stuff? We don't even really know what the ratio of hydrogen and helium is. We'll talk about that more in a little bit. But the question for today is is there another way to find out something more about the inside of Jupiter? Today we're going to talk about a subtle measurement, then can make out some incredibly important details about what's going on. Okay, imagine that Jupiter's here, great red spot and all, and that you are a spacecraft flying around. And as you fly around, you are in orbit and you can measure the gravitational field of Jupiter at all different points along the orbit. You measure the gravitational field of Jupiter not by sitting there measuring the gravitational field at an important time, because you’re in free fall, you’re not actually feeling any gravitational force as you’re falling through space. But, somebody over on the ground, here at Earth, is tracking you with a big radio antenna and watching your velocity as you go and circle around in your position. And you can calculate from that what the gravitational force at every single location we felt was very precisely. This is great, except that one of the things we learned even as early as Newton was that if we have a big spherical planet like Jupiter, it's exactly the same as having a point source with all our mass at that point. Gravitationally it's exactly the same. So our space craft could not tell the difference between just this point source with mass M, MJ, and a nice spherical Jupiter with mass MJ. They would seem exactly the same to our space craft. It's not true if Jupiter is not spherical, however. And there's one good reason why Jupiter is not spherical. And that's because it's spinning. It has a rotation period of 9.925 hours. We'll talk about that and how we know it in more detail next time. Rotation period of 9.925 hours is a pretty fast rotation. That pretty fast rotation turns Jupiter from a sphere, which it would be in the absents of rotation, to something more oblong. Okay, so this is exaggerated. Jupiter is not as flattened as this. But this is the type of shape that it gets from it's very fast rotation. Now as the space craft goes in orbit around Jupiter, it feels a different gravitational field from just the point source gravitational field. That would've felt. When it's over the pole, for example, it feels less attraction, because some of the mass of Jupiter is way over here, pretty far away, pulling at this direction, and this direction, as opposed to pulling it straight down like it would've been pulling it before. Likewise, when it's closer to the equator, It feels the stuff that's closer to it than it use to have close to it and it feels a stronger pull in. Those are generally subtle effects, but the spacecraft tracking is so good that you can often determine these sorts of things from Close spacecraft fly bys. Now, if all we could tell from these data was that Jupiter was oblong like this, that wouldn't be very interesting, but imagine now one of those cases that we were talking about is that Jupiter has something like 15 Earth masses of material, of heavy material, on its inside, like this. What's going to happen? First off, it's not going to elongate as much, it's got more mass. Centralized, pulling it back in so it's not able to flatten out like this. So, first off we have to draw it slightly less elongated. I don't know if I drew it slightly less elongated or not, but let's pretend like I did. Second, the difference is that that spacecraft felt would be different. Here, remember when it was sitting here looking down, it was feeling when it was sitting here and looking down It was feeling less force. Here it would feel proportionately more because more of the mass of the planet is concentrated in the center. More of it is like a point source pulling it in. You can figure out the details of these gravitational forces using spherical harmonics that we're not going to talk about. The mathematical details are a little bit tricky to work out, but you can just see conceptually that a spacecraft that's far away, now a spacecraft that's far away really doesn't have the ability to measure much besides the overall mass of the planet. It can't tell any deviations from this non-vehicle major. As the spacecraft has an orbit that's potentially closer in. It might feel the difference between something at the poll and something at the equator and start to measure the first order effect of this elongation like this. Even closer in you have the chance of measuring even more subtle variations. In real life it's not going to be that there's dense material here and everything else is exactly the same. There's going to be increasingly dense hydrogen compressed in through here. Less dense up through here, more dense up through here. Super dense material in through here. And the closer the spacecraft gets, the more subtle those variations become between the elongation of these layers, and the gravitational pull that they feel For now, we haven't had space crafts efficiently close to really do this, to measure what's going on in detail in what's called the density profile of Jupiter. There's a mission that's at Jupiter right now will do precisely this. We'll talk about that at the end of this unit on giant planets. Even this Cassini spacecraft, which has been in orbit around Saturn for such a long time, has been in a distant orbit unable to measure much other than the very highest order effects. But as Cassini Gets to its ends of its life, they will eventually orbit it really close to a Saturn before it plunges into the atmosphere. In those really close orbits will have the opportunity to really see a detail what the Dead Sea structure is like. For now with the modernly good measurements we have I will tell you that it is consistent with the idea that Jupiter has. Heavy material in it just like we said before and that that heavy material is concentrated in a core. Remember the alternative was that heavy material was throughout Jupiter like this. This core of material on the inside of Jupiter, this 15 Earth mass core Is a huge clue into how Jupiter formed. How it assembled itself from the original material of the solar system. We'll talk in detail about that at a later class, but as we talk about that, in that later class, remember that this measurement of the core on Jupiter is still probable, but not certain. And when we talk about different ways of forming it, remember that it might be the other way around where this material is distributed much more uniformly throughout the planet. And that's one of those key things that the Juno spacecraft, its really big solar panels is orbit around Jupiter to try to find out.
