 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about large impacts and those effects that we've seen and those impacts that we've seen here on Earth specifically. So we want to know what the evidence is that impacts have occurred on the Earth. We see craters on the Moon. What do we see on the Earth that tells us that the Earth has been impacted just as much as the Moon? So let's get started today. And what we have is we do have evidence of impact craters. Are there craters on the Earth? And so let's get started. And what we have is the evidence for impacts on the Earth. And one of the big pieces of evidence is that yes, there are indeed craters on the Earth. And in fact, there are 190 confirmed craters that are at least 100 meters in size. If you think about that, that's about the size of a football field. So there are at least almost nearly 200 craters that are at least that size. So why does the Earth have so few craters when the Moon has so many? Well, there are a couple of different things that can happen. First of all, there are erosional processes that can occur that can wear down a crater over time. So craters like the one pictured here occur on the Earth, but they get worn down. And even if they only get worn down a very small amount each year, just a couple of millimeters, when you're talking about thousands and tens of thousands and hundreds of thousands of years, those craters get completely wiped out. So craters on the Moon, we can see craters that formed a thousand years ago, and we can see craters that formed a million or a hundred million or even a billion years ago or even four billion years ago. So they're all still there, whereas erosional and geological processes, such as tectonics, creating new crust, volcanic activity, wipe out the craters here on Earth, leaving us with just a few of the more recent ones that are still present. Now also a difference between the Earth and the Moon is that small objects do not make it through the atmosphere. So smaller craters on Earth do not form as often because the atmosphere serves to burn up all those small objects. Meteors on the Moon smashing into the Moon will crash into it of any size, whether they be gigantic things that are miles across, or whether they be just microscopic pieces of dust crashing into the Moon. They all impact the Moon's surface. The smaller ones on Earth get burned up in the atmosphere. And of course, most of the Earth's surface at least is water. And when a meteorite smashes into the water, it does not leave an impact scar, so there's nothing to see behind it in those cases. So together all of those things give us a good reason why we do not see so many impacts on the Earth, but we do see enough to know that the Earth has been significantly impacted in the past. So let's look at how often some of these occur, how often does a meteorite impact occur, and we have our table here that will show us, depending on how often that things will strike us. So first of all, we do know that small meteors will burn up in the atmosphere every day. We see meteor showers when there are larger numbers of them, but certainly we are constantly being bombarded by small objects from outer space. Over decades' worth of time, meteorites will strike the Earth's surface and do some damage, and we can see here that things like a high altitude breakup of material can occur on the time scale of just a few decades. So those are ones that break up high in the atmosphere and explode, causing significant damage. Over tens of thousands of years, you will get major impacts that will cause localized devastation. So that's the range here, things in the 300 or so to 600 meter size range that will occur over tens of thousands of years. And when we get to the highest level, over hundreds of millions of years, there will be the extinction level events. So those will occur on 100 million year time scales. Those are objects 10 kilometers in size. So the very localized are these small ones. They will only damage the area where they happen to hit, and things far away will not really notice anything in the very small regions here. When you get things a little bit larger, they will have some global effect, but not as large as could be. And the extinction level events are the absolute strongest ones that can do the most damage. So let's look at a couple of these impacts that have occurred in the past. And one that is well known is the Tunguska event that occurred in Siberia in 1908. And in this case, the object exploded up high in the atmosphere, five miles above the Earth's surface, and flattened an area of a thousand square kilometers of forest. So we can see the image here. All these trees that are lying down, if you look at them from above, they're all radiating away from the central portion where this explosion occurred. So this is a tremendous amount of damage. All these trees are just laying flat down from this massive explosion that occurred. Now there have been more recent ones, smaller ones, one that occurred in Russia a few years ago that also had some damage. It was not near as big as the Tunguska level one, though. This would, if it were to occur over a major city, would devastate the city. Now the big things we get are the mass extinction, the rarest levels, and what we call the mass extinction events. When we look at those, those have occurred at least five times in just the last half a billion years. So when we see those five times in about 500 million years, it leaves us an average of about every 100 million years there is an extinction level event. So the big ones are shown here with the yellow triangles are the big extinction events that have occurred. And we see how many species were eliminated each time. You can see how the number of species drops very drastically at each of these extinction events. Some of them are a little more, some of them are a little less. Certainly the one right here is pretty drastic. Some of the others are pretty big jumps as well that have occurred. The most recent one occurred about 65 million years ago, and that is related to the extinction of the dinosaurs. But it was not only the dinosaurs that went extinct at that time, about 75% of the species that existed at the time became extinct. So lots of them were lost. Now how do we know this occurred and how do we know it was an astronomical event? Well there's two pieces of evidence that we see. First of all, there is a crater off the Yucatan Peninsula that can be detected through a gravity maps of the Earth's surface. And that is one piece of evidence and that dates to about the time of 65 million years ago. There is also a layer of iridium in the crust that dates to this time. And interestingly enough we find dinosaur fossils below the iridium layer, but not above it. And it dates to the right time, and the fact that iridium is very rare in the Earth's crust, so finding a concentration of it is interesting. But we also know that iridium is more common in meteorites, so it lends another piece of evidence to the idea that the extinction of the dinosaurs was caused by a massive impact. So what about future impacts? So what might happen in the future? Well certainly they will occur. There are future impacts. I gave you a table that showed the probabilities about how often these will hit. That of course is no guarantee as to when they will actually hit. A probability might say things might hit every 10,000 years, but you could get to only 1,000 years apart and then you might wait 30 or 40,000 years for the next one. It is all random and we are only looking at averages. So when will the next large event occur? Is it a decade from now, a century, a millennium, or even longer? When will that occur? We do know that there are thousands of objects that are known to cross the Earth's orbit. The vast majority of these are small, dark, and very difficult to detect. They do not reflect a lot of light and it is very difficult to know about them until they are almost on top of us. And it does not take a gigantic object to cause significant damage to the Earth. Something even just a few kilometers across would be a devastating impact and have significant effects on the surface of the Earth. And of course there are also many more. Some of these are ones that we've known. There are many more that we have yet to learn about and do not even know of their existence. So let's finish up here with our summary. What have we learned in this section? First of all, we do know that the Earth has been impacted by objects from space in the past. We can see that evidence. The larger impacts occur less frequently and the smaller impacts more frequently. But massive impacts can cause very significant damage including the extinction level events such as that that killed off the dinosaurs. So that concludes our lecture on large impacts here on the Earth. 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.