 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about cratering and very specifically impact cratering and how that works in our solar system. Now there are other types of craters that you can get. Not all craters are due to impacts. You can get volcanic craters as well as seen here in Crater Lake in Oregon where we have a crater that is just the top or the caldera of the crater. So top of the volcano has actually been filled in to become a lake in this case. So there are volcanic craters that you can get as well and you can also get impact craters such as the meteor crater in Arizona. And we can see that here. You can see this. It looks a lot like many of the impacts that we would get on the moon. You can see how it's got the edge around here and it's been hollowed out by a very massive impact. This occurred in Arizona so fortunately it was out in the desert meaning that it is very well preserved because erosion is not near as much as it would be here. Typically, we can find the impact craters on the moon. On the earth, we find few impact craters and we do find some volcanic craters so we get a mixture of both on the earth. How does an impact crater form? Let's take a look at that. And what we have is when an impact crater form there are a couple of different stages. The first stage is the impact stage. That is the object striking the surface. And you can see that in the first section up here. The material has hit the surface, those shock waves down below pushes the material, some of it downward, some of it wells back upward along the sides of it. So that is the initial contact and compression of the material by the large impact striking into the surface. Now really what happens then is the excavation where the material is then thrown out of it. So stage two here would be the excavation stage where material thrown out of the crater and that is caused by the shock wave. Essentially it's a massive explosion when this meteorite depending on its weight could be various sizes but when it strikes it vaporizes itself. It could be moving at tens of thousands of miles per hour. It vaporizes itself in the surface and that explosion then hollows out the crater. So why do all impact craters look circular? As you look at them almost every impact crater will look circular. That's because it is not the impact itself. You can imagine throwing something in the mud or the dirt at an oblique angle here on earth and getting an impact that is not very circular that will be oblong in shape. In this case because it's the explosion it has nothing to do with the angle of the impact so the object could come straight down or could come at an angle and you're still going to get the same type of crater. So excavation is that second wave when that shock wave expels the material outward. Then we have some modification as material collapses back into the crater. So material collapses back down along the sides here. Material wells up in the middle. That gives us sometimes the central peak of the crater that we can see in the final crater here. As the material kind of slides back in and settles down from the explosion then we get the final stage which is weathering. Weathering effects will modify the crater. So we can get things like caused by gravity. Gravity can cause the material to slump downward. We'll eventually pull it down towards there. That will happen a lot on the moon. We can get weathering effects here on the earth that would be wind and water that would wear away the craters and wind them down as well. For an impact a good rule of thumb is that the diameter of the impact will be about ten times the object size and the depth will be about twice the object size. Now that really depends on exactly what the type of material was. Metallic meteorite may carry more energy than a rocky meteorite so it can depend a little bit. But overall that's a pretty good rule of thumb that if you see a mile-wide crater then the object that formed it was probably about a tenth of a mile in diameter. Now let's look at the details, some details of an impact crater here. So let's go to the moon and look at an impact crater we see here on the moon. Some of the parts that we see coming out are the ejecta blanket. So right around the edge we'll get ejected material that will, material is thrown out by the explosion. And further out we will see crater rays. And you can see these extending outward away from the crater. All of these rays that are ejecta that extends out very large distances across the surface of the moon. So the ejecta blanket is the material right in by the crater. The crater rays are the material thrown further out. And then you'll get many of the rocks that come out will form secondary craters. So smaller craters around the initial crater will be from the debris that was thrown out in the mass of explosion will actually form secondary craters as well. Now we want to take a quick look at determining ages here. So how can we determine ages of craters? Well we look at some examples here. First of all we use primarily relative ages is the easiest way to do this. And it has to do with what is on top. So if you look at an old crater here, perhaps this large one in the middle, we know that it is older than many of these other craters because craters like this one here and this one and even these ones inside all overlap it. We know that these craters could not have been here first because then this crater would have wiped out parts of them or wiped them out completely. So craters formed on top of another crater must be younger. The younger object is always going to be on top. We can also use crater counts to estimate ages. The fewer impact, fewer craters means a younger surface and we can use this across the solar system. This doesn't work just on the moon or the earth but we can look across the solar system and we can look at other planetary surfaces and moons of other planets and use the number of craters on them. When we see a lot of craters we know it is a very old surface. When we see very few craters then we know it has been reworked relatively recently. So things like the bit of the moon here are very, very old surfaces because there are lots of impact craters in fact to the point where they overlap each other. Now if we want to determine the actual ages we need to get samples of those. So some of the Apollo missions or all of the Apollo missions brought back samples of the moon to be able to be radioactively dated to determine the ages of those sites. So that gives us a precise age but looking at crater counts works for anything that we can look at and we can always determine relative ages, what is younger and what is older by looking at what is on top of something. So let's look a little bit finally at what the cratering rates have been and what we find is that cratering rates first of all are not, we are not completely, have not been constant across the history of the solar system because if you use those crater counts and other measurements we would find out that the highlands of the moon are older than the entire universe which obviously would not be possible. So it tells us that cratering rates have not been constant over time. Probably in the history of the solar system cratering rates were higher, lower means they were lower cratering rates today. So while they have not been constant the cratering rates have been consistent meaning that they've been about the same wherever you were in the solar system. So that we can look at the number of craters on widely separated objects things like the moon and say Io, one of the moons of Jupiter and we can use those to compare the relative ages. We find that the moon has lots of craters and Io has none so we know that the moon is older and that Io is younger. So just by looking at them we can get the relative ages of objects that are nowhere near each other in the solar system. So let's finish up here as we do with our summary and what we found is first of all we talked about two types of craters we had volcanic and impact craters. The impact craters are created when an object slams into the surface at very high speeds and explodes and we find those on almost every astronomical object. Certain ones do not get impact craters such as the gas planets because there is no solid surface to impact but every solid surface has some amount of craters with the exception of Io. Io has the youngest surface in the solar system and has no impact craters and we can use that to tell us that it is very young because the number of impact craters on a surface tells us about the relative age, the more craters we see, the older the surface, the fewer craters we see, the younger the surface. So that concludes our lecture on impact cratering. 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.