 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about meteors and meteorites, bits of debris in the solar system, and some of those that actually land here on Earth. So let's start off with a few definitions. First of all, we have a meteoroid. This is a bit of material out in space, perhaps the size of a pea or a grain of sand, so something relatively small compared to the much larger objects we've been looking at. We have meteors, which are also sometimes called shooting stars, and this is a meteoroid, which has then struck Earth's atmosphere and vaporizes, leaving a visible trail behind. And then finally we have meteorites. This is material that survives the trip through Earth's atmosphere to reach the ground. So these are the pieces that we actually can have and touch here on Earth. So let's look at when meteors occur. We often get a meteor shower, and most meteor showers are associated with comets, which we've talked about previously. Comets leave debris behind in their orbits. Material is vaporized from their surfaces, and they will leave material behind in their orbit around the Sun, and it will continue to follow that same path. Now when the Earth crosses that orbit, as shown here at a couple of points, one point here and one point here, when the Earth happens to cross the orbit of the comet, then that little bit of debris picks up, and that's where we will get meteors. And when we see those, they seem to come from a single point in the sky. We call this the radiant point of the shower. Now the radiant point is where they appear to come from. It does not mean that meteors are coming from the same point. It simply means they are following parallel orbits in space and coming in parallel to each other. And parallel objects will seem to converge off in the distance, much as railroad tracks do. We note here that the railroad tracks appear to converge off in the distance, even though we know that they are still the same width apart here as they are in the often much further away. They will still be the same distance if we were to measure them. So it is just an optical effect that causes them to appear to come from this same radiant point. And that is how we name meteor showers. When they occur, they are named after the region where they appear to come from. So it may be a constellation. It may be near a star if there are specific more than one associated with that constellation. Meteor showers occur around the same time each year and are generally associated with a specific comet. For example, we have two associated with Halley's Comet, one in early May and again in October. Those are the two times that Earth passes through the orbit of Halley's Comet. In these cases, the number of meteors that are seen varies quite a bit. In a good shower, you might get one or two per minute. That's a pretty extreme good shower. Something like the Perseids is often that intense. Others, you might only see a few per hour, a handful per hour. If we get a meteor storm where we happen to be going through an increased amount of debris, we may get even more than that one to two per minute. But you won't just see meteors flashing through the sky constantly. You do have to be patient and sit there and watch for them. You can also see meteors sporadically that will occur that are not necessarily associated with the shower itself. So here we see those that are associated with the shower. They're all coming from the same direction, following the same orbit in space. Now, when some of the debris lands on Earth, we have meteorites. Now, a meteorite can be difficult to identify. If you don't know what you're looking for, many of them just look like ordinary rocks. So if you just find a rock there, there's little to tell you that it is a meteor or just an ordinary Earth rock. It would take specific testing to find out whether what kind of material it is and where it came from. Now, when we look at these, we talk about two different types of meteorites. We have the falls and the fines. Falls are when you actually see the object falling from the sky, you see it, you see where it landed, and you're able to recover it. That makes it very easy to identify it as a meteorite. Fines means it's found on the ground long after it fell. Might be years, decades, centuries after it fell on the ground, after it fell, or even more, and then is later identified as a meteorite. Now, when we look at these, we can see the classifications that we'll look at as to the different types of them, and we'll look at these in a little more detail coming up. But you have stony meteorites, you have iron meteorites, and you have stony iron meteorites. When we look at the percentages, we see that they're quite different. This is an example of a selection effect. When we look at the falls, those that were seen to fall, the vast majority are stony. We see this, 96% of them are stony. Those that are found, much later, only 53% of them are stony. Well, why does this occur? Well, it occurs because the other ones, such as the iron meteorites, are far easier to identify after the fact. Finding a lump of iron is unusual, and that would mean that you might be more likely to get that checked and tested as to what is this object. Whereas a stony one, especially after it's been here for thousands of years, will be well eroded and will look just like an ordinary earth rock. So an example of a selection effect, when we're finding them here on earth, it's much more likely to find an iron meteorite even though those are very rare among the meteorites in general. So the falls, the ones that we see, are probably a better overall estimate of what the distribution of meteorites are like. Now, where can we find meteorites? Well, there's a couple places that you can do this. First of all, it's easy to identify if they are seen to fall, or if you find a metallic meteorite, those kind of things stand out. Many meteorites are found in Antarctica, as shown here, because the meteorites are easier to identify on the icy surface. Now, that doesn't mean that they just fell, they may have fallen long ago, and as the glaciers move, they may churn up material, meaning that when we find these, it's now more likely that they are something that came from space. It's certainly not a guarantee, and they have to be checked and tested, but a lot of meteorites can be found in Antarctica this way. Another one that sometimes uses a desert area, where the rocks will stand out against the rest of the terrain. So sometimes finding that rocky material, it's not a guarantee that it's a meteorite, but it means that it's more likely to be a meteorite than it would be with just a random rock. Now, we can classify these meteorites and we do that in as by their type, and we have the stony meteorites, which are made of rocky materials. So same kind of things that make up ordinary earth rocks, that's what makes them hard to just identify when you see one on the ground. You wouldn't necessarily know that it is a meteorite, but they are primarily stony material. We also have iron meteorites, which are essentially pure iron nickel, and we also note, one of the things interesting in them is that they have this Widman statin patterns, which we see here in this cutaway. Now, you wouldn't normally see that. That is after you have cut them open and treated them to be able to bring out the patterns of crystal formation. And what that means is that they cooled slowly. They didn't just cool all at once, exposed to space and just cooled. They cooled over a long time, meaning they had to have formed within a larger object. So an object that differentiated formed a solid metal core and a rocky crust and was then broken apart somehow. And we see the evidence of that in these types of patterns. And then you have the stony iron, which are a mixture of these two. They have stony and iron material, likely where things are still differentiating. Objects had not completely differentiated at this point. So we'll see kind of a mixture of metallic material with still inclusions of stony material as well. Now, how about the ages of meteorites? How old are these? Well, we can date them because we have samples, so we can get absolute dates using radioactive decay methods. Crater counting methods would not be very useful here. The ranges are about four and a half to four, a little over four and a half to a little over build that billion years. We also see what we call primitive meteorites. And primitive meteorites are very interesting because they are unchanged since the formation of the solar system. So why do we want to study these? Well, these are ones we look at because they are bits of debris left over from the kind of material that formed planets. So how did the earth form? Well, it formed from things that originally would have been like the primitive meteorites. These are the debris left behind that still exists in the solar system from that material which made earth. We also see things like differentiated meteorites which have changed significantly. And those are things like the metallic meteorites that we have looked at. They are not the way they originally formed. They had to be part of a larger object at some point. Studying those primitive meteorites helps us to study the very early history of the solar system. And we can then learn what our early solar system is like. And we specifically have what are called carbonaceous meteorites which are enriched in carbon compounds. So they have an enrichment of these types including organic compounds and amino acids building blocks of life that are not of terrestrial origin. So does life exist elsewhere? Possibly. Does it exist on meteorites? Probably not. However, certainly the building blocks of life the fact that we can find them there means that the building blocks of life are relatively easy to form. And we will look at that in a little more detail in a later chapter. So when we look at some of these again we see the various different inclusions that we see on these types of meteorites. They are ones that have not been melted and separated into there and differentiated like a much larger object. So we still have a chance to study this type of material left behind from the origin of the solar system four and a half billion years ago. So where do these come from? Well I want to kind of differentiate the meteors and the meteorites. While they sound the same and they have really vastly different origins. A meteor is generally a bit of a comet. These things are fragile and they burn up in Earth's atmosphere. So these are what we see as meteors are shooting stars. They don't make it down to the ground. They're fragile compounds. They break apart very easily and don't make it to the ground. Meteorites on the other hand are generally bits of asteroids. And again they're much larger objects. Little bits of debris even if it is an asteroid will still break apart. But a larger object can survive the trip through the atmosphere and land on the ground for us to study. So let's go ahead and finish up with our summary. And we've looked at the terms meteoroid, meteor, and meteorite which are really each a very specific definition in astronomy. Meteor showers are generally associated with comets. There are a few exceptions but for the most part they are associated with comets. We looked at the three types the stony iron and stony iron meteorites that we see. And we talked a little bit about primitive meteorites which give us clues to the early history of the solar system. So that concludes this lecture on meteors and meteorites. 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.