Density is perhaps the most fundamental measurement you can make about an object in the Solar System. It's the one that you measure to first get an idea of what that object might be made out of, and a lot of effort has been put into measuring not just the density of planets, but densities of satellites. Densities of satellites are hard. You can only find densities of things that have something going around them, or perturbing them, but we've sometimes had spacecraft go to satellites. We have density measurements now of things like asteroids. We'll talk a lot about these objects in the next unit in the class, and these come because those asteroids have a satellite. But I am going to talk now just about the planets themselves. This is a picture that shows the planets with their correct relative spacings and their correct relative sizes. Which can't relative spacings and relative sizes at the same time because you would end up having little dots that you couldn't see at all. So the sizes are greatly exaggerated but they at least are correct in a relative sense. And you very quickly notice just by looking at this that we have some major differences going on in the planets, just by size. There are these tiny ones here in the interior, all grouped together. There are these two that are more or less the same size that are here in the middle part of the solar system. And Uranus and Neptune, although sometimes we think of them as the giant planets like Jupiter and Saturn. They're actually quite a bit smaller and quite a bit less massive. And we'll see in a minute, that they clearly have different properties. If you were to divide the solar system into parts, simply looking at this, you would probably divide it into these three parts. Although maybe you'll be tempted just to do gas giants and terrestrial planets, as we call them. But you would really know that you were onto something when you put the densities of all these objects together with their sizes and locations. First thing I want you to look at is the terrestrial planets. They all are high-density. Remember what I said about densities? I said that the density of rock is something like three grams per centimeter cubed. And ice or water is something like one gram per centimeter cubed. And iron another main component is close to eight grams per centimeter cubed. And you can quickly see from these high densities of Mercury, Venus and Earth that they probably have something more than just rock in them. If you look on the surface of the Earth and I go around and pick up a rock. It's going to have a density of somewhere around three grams per centimeter cubed, but there is heavier material underneath. And there's enough heavier material underneath that it can't just be due to the compression of these rocks. As one of the many different reasons we know that there is a core higher density and iron, in through there. Venus and Earth have very similar densities, they have very similar sizes. Mercury is actually quite a bit smaller than Venus and the Earth, and yet the density is as high. That means that there's not nearly as much compression on the inside, that means to have the same density as Venus and Earth, there must be even more high-density material, and it's true. We know that there is a substantial iron core inside of Mercury, much bigger relatively than that of Venus or the Earth. Let's move out here to what the point here was supposed to be, which is Jupiter, 1.3 grams per centimeter cubed. And it's huge. Whatever it's made out of, it's highly compressed on the inside. So the material it has much be much less than 1.3 grams per centimeter cubed. Is there any rock? Well, certainly not much if any. Iron, no hint of it. Ice, well even ice would be compressed so much that it would have a higher density than this. This is good indication that Jupiter is almost entirely made out of gas. Very much like the sun. The sun is an interesting one to look at too. 1.4 grams per centimeter cubed. It is of course entirely made out of gas, and it would, it's compressed because it's so huge. But there's another interesting thing going on with the sun. Because the sun is so hot, it expands, and the density goes down. There was some speculation early on that maybe this 1.3 grams per centimeter cubed was because Jupiter was indeed made out of the same materials as, as the Earth. But that it was super hot on the inside just like, like the sun and that heat caused it to expand and be less dense. People tried to measure the heat coming off of Jupiter for a long time. If you remember, we talked about that same process on Mars. In the 1950s, people had tried to measure it coming off of Jupiter much earlier than that. And one way that they had that they tried to do it is that they would take a telescope and point it at Jupiter and inside the telescope, you know there's a mirror light comes to a focus at a point right here. And they would put a small disk that would be deflected by the pressure of the, the heat, the emission, thermal emission coming through there and they would try to measure that deflection. No successful measurements were ever made, but the way they calibrated it was with candles. They would put candles at certain distances away, and they would write papers about how they lit the candle, waited for the flame to develop, and they developed this entire thing called standard candles. I'm just saying this because it's funny for, for any of you who follow astronomy. Standard candles is the term that astronomers use all the time for anything in the Universe that has a brightness that you know how bright it is. And therefore, which if, if you put it further away, you know how far away it is because you know how bright it intrinsically is. I, I've heard that phrase all of my astronomical career and it never occurred to me that actually they started these out as candles. The first standard candles were candles that were standard. In any case, the temperature of Jupiter was eventually measured the same sort of way that Mars was measured. And it was found that it's quite cold. Something like 130 degrees Kelvin. Certainly so cold that this 1.3 grams per centimeter cubed mean that Jupiter has to be predominantly made out of gas. While we're here, let's look at the other giant planets. Saturn, even lower density than Jupiter. Now, it's not as big as Jupiter, you can't quite tell that, but it's not as big as Jupiter, so there is less compression, but still the lower density of, of Saturn is a bit astonishing. But the more interesting thing, I think, are these two outer giant planets, Uranus and Neptune, with densities of 1.2 and 1.6 grams per centimeter cubed. Now you might argue that 1.2 is kind of close to 1.3, but Uranus, now you can see Uranus is so much smaller than Jupiter that there is much much less compression going on here. So there are significantly more dense materials in Uranus than Jupiter. And by the time you get to Neptune at 1.6 there is a ton more. And so, we actually often call Uranus and Neptune ice giants, with the idea that a lot of their interiors are things like actual ice or liquid water or some weird high pressure combination of the two of those. We'll talk about that more in detail. We'll go back now and examine what this 1.3 grams per centimeter cubed for Jupiter's density tells us about our central question, what's on the inside?
