 Greetings and welcome to the Introduction to Astronomy. In this lecture, we're going to talk about two different types of objects, and those are meteors and meteorites. Now, they have very similar names, but they actually refer to quite different things as we'll look at and discuss in this lecture. So, let's go ahead and get started. And what we want to look at first are some definitions. First of all, what do we mean by these different definitions? A meteoroid is the first one, and that is the bit of material when it is out in space. These can be very small, things like a grain of sand or a pea, small pebbles, very tiny objects out in space we call meteoroids. So, we have no way of detecting any of these. They're way too small to ever see unless they happen to strike the Earth's atmosphere. And when that happens, the meteoroid can become a meteor. We also call this a shooting star, and that is the meteoroid that strikes the Earth's atmosphere and vaporizes, and it leaves a trail that we can actually see. So, when you see a shooting star, it has nothing to do with the star. It has everything to do with small solar system debris that is burning up in the Earth's atmosphere. And finally, we have a meteorite. A meteorite is material that survives the trip through the Earth's atmosphere and lands on the ground. So, it actually is able to be here to be studied, and we'll look at some of the different types of meteorites. Now, this means that most meteoroids never become meteors. For the most part, most of those meteoroids just float out in space and are completely unknown to us. A small percentage of them will become meteors, will strike the Earth's atmosphere and give us a shooting star. And even smaller percentage of those will actually make it down to the Earth's surface and become meteorites. Only the larger ones, things like grains of sand, peas, will actually burn up in the Earth's atmosphere. So, let's look a little bit about where these come from and what we have. And we have what we call a meteor shower. So, meteors, most meteors are associated with comets. When comets are traveling through the solar system, especially when they come close to the Sun, they have material vaporized off their surface. And that causes them to leave debris behind in their orbits. And that material continues to travel along the orbit of the comet, as you can see here. So, as the comet travels, it left some material behind, and that can continue along the orbit and will follow exactly the same orbit that the comet did. And that will happen each time it passes close to the Sun. And that means when we look at the Earth's orbit here, there can be a point where those two intersect. And that is when we are going to get a meteor shower. So, when the Earth crosses the orbit of the comet, the debris will strike through the atmosphere, hit it at very high speeds, and become vaporized. So, this is what can happen. Now, this doesn't mean the comet has to be anywhere near us. The comet can be gone. The comet could have long since vaporized or anything else. The material will continue to orbit, and the meteor shower can go long after the comet is actually gone. Now, meteor showers. Also, the meteors, when we see them, they all seem to come from the same point in the sky. So, if we look here, we can see all of these meteors, and if we kind of trace them back, they seem to come from one point in the sky. As you trace all of these images back, they would seem to come from one specific point in the sky. And we call that the radiant, or the radiant point. And that is where they appear to come from. That is not that they're coming from anything. It is that they are, it is essentially an optical illusion. So, they all seem to, they're all traveling through space together, parallel to each other. And when we see them coming in, they all seem to come from the same direction. It's the same optical illusion that we would see for looking at telephone poles or anything else, railroad tracks that stretch out into the distance. They seem to come together at very large distances. However, if you actually traveled out there, you would see that they are still exactly the same distance apart. And that's what we would see here. We would see that all of these meteoroid pieces are still at the same traveling in the same orbit. Now, meteor showers do occur on a regular basis, and there are annual meteor showers, that occur around the same time each year. So, these are some of the better known ones here. Two of these are actually associated with Halley's Comet, which is one of the best known comets, and we can see those here. And those occur in May, and then again in October, just about six months apart, when we pass through those two portions of Halley's Comet's orbit. Now, Halley's Comet at this time is nowhere near the inner solar system. So, we still see those because we are passing through its orbit in debris left behind, which could be hundreds or thousands of years ago through previous orbits. How many meteors we see varies greatly, and in fact a very good shower you might see one to two per minute. Now, that would be a really good shower and seen from a very dark site. So, not always going to be able to see those, but one that is very well known for having lots of meteors is the Perseid meteor shower. Usually has a pretty good show. Some of the other ones you might only get five or ten per hour, and if you're in a brighter area then you would not see as many, because many of the faint ones would be drowned out by the sky brightness itself. We can also get a meteor storm, which is when we get far more objects. You can actually get significantly more than this if we happen to pass through a really intense portion of the comet's orbit where there are lots and lots of particles. Now, what we really want to look at here is the parts that actually make it down to the Earth. So, these are what we call meteorites. These are pieces that we could actually touch. So, stones from the sky that we have seen, pieces of material that have fallen from space. Now, they can be difficult to identify. How do we know whether a rock that we see like this comes from space or if it's just a regular Earth rock? And it can be very difficult to tell. Typically, you have to actually have someone who is experienced with meteorites to be able to study it and tell whether it is really something that came from space. This is especially true once things have sat on the Earth for a while. Old rocky meteorites that are still sitting there can have been worn down and degraded by weathering effects and may no longer look any different than a typical Earth rock. Now, we have to differentiate these into two different types. We have what we call falls and finds. And what they are, the falls are objects that were actually seen to fall from the sky. So, you actually watch this, you saw it fall and strike the ground, and it was then seen, so you knew that it fell from the sky. Finds are objects that were found on the ground long after it fell and then identified as a meteorite. So, it fell early on, long ago, hundreds, thousands, a million years ago, and is then later identified as a meteorite. Now, when we look at the table here, when we look at falls and finds, we see a very big difference in what is the numbers that are seen. Falls are dominated by stony meteorites that are very primitive, whereas finds are dominated by iron meteorites. This really doesn't mean, this really means that the finds are simply identifying objects that don't look like Earth rocks. When you find a hunk of metal, it's more likely to be taken to be examined than a simple rock. So, the finds look much less likely to be stony meteorites. So, this is an example of a selection effect. And what that means is that what we're observing is what we're seeing in the finds is not a really good example. The falls would be a good example of the types of meteorites that we get, because it is a completely random sample. We also find this in Antarctica, and you'll note that these two are actually very similar. There's a little bit of a bias here, a little bit more in the differentiated stones, but overall we see a lot of stony meteorites and very few iron meteorites, quite different than what we see in the finds. So, in the Antarctic, again, these are pretty much fine, these are all finds as well. We are see, or falls, I'm sorry, these are all finds, but we're seeing a little bit different perspective on them here, because regular finds will get worn down more, and here we're actually getting a search for specific meteorites. Now let's look at where we find these meteorites and where we're going to find them really. In Antarctica, we really know is one good place that I've mentioned previously. First of all, they're easy to identify when they're seen to fall. If you actually see it falling, then you know that it is a meteorite. When you find a metallic piece of rock, that tends to stand out and is something that is more likely to be identified as a meteorite. When we look in Antarctica or desert areas, the meteorites will stand out against the icy surface or the sandy surface that we see in a desert. So that makes them much easier to identify and stand out from other rocks. Whereas finding something in a typical rocky area, that rocky meteorite is not going to stand out from the other materials. Now we classify meteorites in a couple of different ways, and in fact there are three types. There are stony meteorites, which we see here, and these really don't look any much different than an earth rock. So there's what we call silicate meteorites or stony meteorites, and they are simply chunks of rocky material that fall from space. There is little difference between those and ordinary rocks that we see here on the earth. Now some of the others we see are the iron meteorites, and the iron meteorites do stand out. These are pure iron and nickel. This is not the kind of rock that you would typically find on the earth. So when you find something like this, especially you'll note that it's extremely heavy by comparison to a typical rock, we see, and we see when we cut it open, we get all sorts of crystallization patterns that occur that occurred as the metal cooled. And what this tells us, and we call these the Widman statin patterns, they mean that the meteorite had to have cooled slowly, and that means it probably came from inside a larger object because out in space a hunk of metal that was molten would freeze very, very quickly to a solid, and you would not get these patterns. Since we see these patterns, that means it must have cooled slowly, and we then see that in these patterns. So it tells us something about the formation of the metal meteorites, meaning that they formed within a larger body that then broke apart. Now the other type that we see is a mixture of these two, which is the stony iron meteorites. So stony iron, they're just a mixture. You see stony material in some areas, and you get some inclusions that are metallic. So this could be formed also in a larger object, that was in the process of differentiation. So things had been molten and material was in the process of separating, and maybe around the border, material had not finished separating as it was broken apart or as it solidified before differentiation fully occurred. So we do get some metallic inclusions where we'd see some chunks of metal within the overall stony structure of the meteorite. Now how can we determine how old a meteorite is? And the main way to do that is through using the radioactive decay methods. And when we look at those ages, we find that the vast majority, especially the primitive meteorites, are about four and a half billion years old. So they date the time of the origin of the solar system. So these are actually parts that we see now that were part of material that formed the planets many billions of years ago. We can separate those into primitive and differentiated meteorites. The primitive ones are essentially unchanged since their formation. These ones are quite interesting because we're able to then study them and learn what the solar system was like while the planets were forming. The differentiated meteorites have undergone significant changes, meaning that they've been melted and that more volatile materials like ices have been released and things like the metals have actually condensed out. So when we find a metallic meteorite, that's what we'd call a differentiated meteorite, and that has changed. So just like the planets have changed, some of the meteorites were probably formed within a larger object and have changed as well. So let's look at these primitive meteorites because they are very interesting ones, and they do help us to study the earliest history of the solar system. We see what we call carbonaceous meteorites, and these are one of the primary kinds that we see here. They are enriched in carbon compounds, and in many of the cases, they are very dark. So we see one here, a very dark object that has lots of carbon compounds, including organic compounds and amino acids. So we get a darkish color from these carbon compounds, giving it the darker color, and we also get amino acids, and amino acids are the building blocks of life, and they're different than the ones that we get here on Earth. So these are ones that are not of terrestrial origin. Now remember, organic compounds does not mean that these are life. They are simply complex organic molecules based on carbon. There is nothing in these that shows that life exists out there, but shows that the building blocks of life have formed quite a bit out in the solar system. So origin of these, where do these meteors and meteorites come from? Well, even though they have similar sounding, they really have very different origins. Meteors are bits of comets that are fragile and burn up in the Earth's atmosphere. So the vast majority of meteors we see are little tiny objects that are just bits left behind by a comet. These never make it down to the Earth. Meteorites, on the other hand, are generally larger bits of asteroids. They are large enough and sturdy enough to be able to survive the trip through the Earth's atmosphere and land on the ground. So they're ones that actually land here on Earth and that we can study. So let's finish up here with our summary, which says that we have three different types of objects, meteoroids, meteors, and meteorites. Each of these has a very specific definition and meaning. The meteor showers are associated with comets. There were three types of meteorites, the stony, the iron, and the stony iron. And finally, primitive meteorites, ones that we like to study because they give us clues about the early history of the solar system. So that concludes our 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.