Okay, I'll admit that as much as I love the site of comets in the sky, this actual image of a comet nucleus from the Rosetta spacecraft is one of the most spectacular things I've ever seen. Now of course, we can send a spacecraft to a comet to try to figure out what it's made out of. We can actually land on the surface and see what the materials are. But early on we of course couldn't do that so we needed to find other clues to what the comet was made out of. Comets are visible when they get a coma. Coma is that big glowing thing that is around the comet. Coma is Latin or Greek for hair and that's of course what these things look like, something with all this hair coming out of it. That is the characteristic of a comet, something that has a coma. And you see the comet when it has a coma. As we will subsequently learn, comets themselves are actually really quite small. And the only reason that we see them, you would never see them unless they had all this material expanding away from them reflecting the sunlight. A much larger surface area reflecting sunlight than the tiny little comet itself. And an interesting thing was noticed about comets early on, which is when they were discovered, when you first started to see comets. Well, okay, why are they discovered? They are discovered when they're bright. So that sometimes happens when they're close to the sun and they're coma gets really big, because there are a lot of things coming off of the comet. You sometimes see them when they're close to the earth. But you almost never saw comets further away than something like 3AU. Interestingly, you never saw comets further away than 3AU. But if you looked at the orbits of comets, the orbits of comets are usually very eccentric and they would come in close to the sun, and go out sometimes either really far. And we'll talk about that in a subsequent lecture. Sometimes, they would go out on more just orbits out by maybe Jupiter or something. And you would see them when they got to 3AU, or you would quit seeing them when they went past 3AU. Never saw them out through here, or at least almost never saw them out through here. There are a few spectacular exceptions. Halebop actually being one of those that was seen really far out and coming in. 3AU, what's so special about something like 3AU? Well, you could imagine that it's because there's something on the surface of the comet that is beginning to evaporate at around the temperature that it becomes, it's something like 3AU. Let's first calculate what that temperature is. Remember, it's the same way that we did it for Mars. We say the sunlight, this is the Earth solar constant. We're going to divide by the distance of 3AU squared. So divided by 3 squared, so that's the amount of sunlight that's seen by this comet at 3AU. We're going to divide by 4 to get that typical average temperature as the thing is rotating around. And that's equal to sigma T to the fourth and the way that we always do this, this is the black body emission of the object, so it's a good approximation for what it is. We didn't include an albedo here, we should include an albedo, a one minus the albedo because some of the light is reflected but we're not actually going to worry about it because as it will turn out comets are pretty dark and so they absorb most of the sunlight that hits them. So we can do this calculation very quickly and we find that the temperature at about 3AU is something like 160 degrees Kelvin. Or if you prefer, somewhere around minus 110 degrees Celsius. Okay it's cold, of course it's cold. It's 3AU away from the sun. It's get nine times less energy from the sun than the Earth does. It's going to be a cold place, colder than Mars. Not as cold as Jupiter but nice cold place. So the interesting question to ask yourself is there something we could find that starts to evaporate strongly at around 160 degree Kelvin? How do we answer that question? Well, if you remember when we talked about Mars, and the polar caps, and the evaporation to polar caps, the key quantity that we're interested in here is the vapor pressure. If you remember, vapor pressure is a function of temperature. As the temperature increases, the vapor pressure over here, the vapor pressure increases. And the vapor pressure is if you have ice right here, and if we had a gas on top of it, the vapor pressure tells you when you evaporating ice here is in equilibrium with gas that's being re-condensed on the surface. So what that's really telling you is the rate of the evaporating ice. So vapor pressures are low for low temperatures, of course, and they're high for high temperatures. As you heat an ice up, it evaporates more and more. We're not talking about melting now. We're well below the melting temperature of these ice. But it's the sublimation that moving to the gas phase directly from the ice phase. So what sublimate strongly at 160 degree and not very much below and a lot above? One substance that we think of as very common in the solar system has precisely that property. Here's a plot of the vapor pressure of ice. And when I say ice, I mean H2O ice. And okay, here's some temperatures. Remember we're interested in a temperature of something like 160 degrees Kelvin. That's right here. And you can see that well, there's a nice gentle curve since this is going low vapor pressures here, high vapor pressures here. But notice the pressures over here. Here's pressure in Pascal. This is in a logs scale. So this is 1 Pascal is right here. And this is 10 to the minus 10 Pascals over here, 10 to the plus Pascals over here. Vapor pressure changes ridiculously fast with temperature. And so you can almost think of vapor pressure as something that turns on and turns off as you dial the temperature up and down. And in fact, at 160 degrees, you are going from this relatively flat region in through here decreasing to this relatively fast decrease in through here. It's almost a wall that's hit. If you go from 160 degrees to 150 degrees you have dropped by a factor of ten in vapor pressure. If you go down another 10 degrees you drop by another factor of 100 in vapor pressure. And as you go through here your vapor pressure hits up at incredible amount. No other common seeming Ice in the solar system has this inflection right at around 160 degree, right at around when comets turn on. There's a pretty good indicator that water ice plays a big role in what's really going on with comets. Many people think of the year 1950 as the barrier year for figuring out what the heck is going on with comets. And it's all two papers, we'll talk about the second paper in the next lecture but this is critically important one. Okay, it was received in 1949 but it comes from this issue of the astrophysical journal 1950. And it's a fairly unassuming seeming paper when you read it the first time around. It's a comet model. The acceleration of comet Encke. Comet Encke is called a short period comet. We'll talk about the distinction between comets in the next lecture. It's a short period comet and it had some strange properties. Well, one critical strange property is It did not just simply mind its business going around the sun in a perfect orbit the way everything else is supposed to do. The orbit changed. The orbit changed by not huge amounts, it didn't go from going like this to suddenly going like this to suddenly going like this. But it was enough that it was measurably changing in ways that it should not have been. And this led Fred Whipple to try to figure out what was going on, and he proposed a relatively simple and ultimately correct idea for what's going on with comet Encke and comets in general. You can read right here, the nucleus is visualized as a conglomerate of H2O, NH3, CH4, CO2 or CO, C2N2, other possible materials that are volatile at room temperature, that means they evaporate at room temperatures, at temperatures that comets get. But the other critical part about it is that they are mixed in with, he calls them meteoritic layers. Basically, rocks, other things besides just these ices. And you can imagine what happens if you'd had a bunch of ices and a bunch of rocks going around here at first. The ices start to evaporate and you're left with a layer on top of just the rocks. And eventually the rocks will heat up and the ices will break through and evaporate through there and get this jets. You might have all seen jets in comets by now and that's these ices from down below heating up and eventually breaking through. What Whipple proposed very simply was that the fact that the ices are underneath this mantle of rock and that the comet is rotating. Imagine the comet is rotating this way, the sun is over here. Is that there's a lag between when the heating happens, the heating happens now but as the comet rotates around it takes awhile for the heat to get transmitted in to the interior. So the jet might be off in this direction and jetting in this direction as opposed to jetting uniformly. Jetting in this direction is almost like a rocket engine on the back of the comet. With a rocket on the back of the comet is going to do this. The comet is rotating in the other direction, the jets would come out off on this direction and it will slow the comet down and lead to different orbital changes. And that was the inherent observation that Fred Whipple made and suggest that comets are in the phrase that everybody associate with this paper dirty snowballs. Dirty because they have all this rocky meteoritic material mixed in with the ice, snowballs. Because yes they're made of H2O, although he was very clear that these other components are an important part of what's going on in the comet too. These days we now know a lot more about what comets are really made out of. And we can make detailed tables that look like this. These are the abundance of materials in comets relative to water percent, relative to water, why? Because water ice still is the dominant component in the comets and so this is plotted at 100% H2O. What's next? Well, this shows you the range of different observations in comets in the green and the amount that's here. Over here is the number of comets in which it's been observed. Some things have only been observed once and usually when it's once, it's because it was Hale-Bopp. Hale-Bopp was so bright that it was easy to observe. Some things are so bright that we see them all the time, greater than ten, greater than ten. And what is the most abundant thing, after the H2O? Well, CO and CO2. Remember, Whipple said CO or CO2. He was right on both counts. There's the methane that Whipple proposed, and then some acetylene and some ethane, and then some pretty complex hydrocarbons in through here. CH3OH is methanol, but these other ones are ones that you might not recognize. There's ammonia, these are things that have nitrogen in them. Ammonia is the main nitrogen bearing species. There's sulfur, H2S. There are all of the materials that we find on the Earth, sulphur, nitrogen, carbon. But now rather than being locked up in rock form or being a gas, these things are found as ices. And I mean we know about them because we take these ices that would be hard to detect otherwise and we bring these ices in close to the sun and Evaporate them and make them very easy to see. The existence of all these gasses is a big clue to what might be going on with these comets. First off, they can't have formed very close to the sun, because as soon as they get close to the sun they start to evaporate like crazy. We actually see comets eventually peter out and die when they get too close to the sun. The recent one, Comet ISON, got so close to the sun, it was never seen again after we had a close passage. So we know that they must have formed further out. Not only that, we know the existence of something like the water line we've talked about before, where frozen water becomes important. Well, this thing had a lot of water in it. So these things probably formed beyond the water line. But they also probably form beyond most of these other important frozen ices there own lines. Everybody has their own equivalent to the water line. So we have clues that these things formed in the outer part of the solar system and are emissaries from this outer part of the solar system that we get to study up close when we bring them in. Like I said at the beginning comets are fascinating thing, there's so much more we could talk about, about the compositions of comets. About the fact that it looks like some of them have different compositions from others, which is telling you maybe about where the different types form. We haven't even talked at all about the dust in the comets, that thing that's making them so bright for much of time. They're fascinating stories, indeed they are. Go out and see a comet if you can. There's not a particularly bright one in the sky right now, but pay attention, there will be. Go see one, they're something that's worth seeing. Everybody should see at least one real comet in their life, well, because they're cool.
