Welcome back. So now, let's talk a bit about craters. Here is the Barringer crater in Northern Arizona. Over a kilometer across here. This enormous crater formed when a fairly small meteor struck the earth. As you saw in your problem that you just worked through, the energy released when a meteor strikes the earth or the moon is quite large and it will excavate quite a large region. We can look in more detail about what happens when a crater comes in. We have this projector coming in at very high velocity. It has a number of impacts. It melts the rock around it. A shock propagates as the energy is deposited into the earth or the moon. It creates fracturing in a large region of the surface. And then, some of this energy will go into ejecting material outwards. An experiment you can do at home is take a bucket of water and throw a rock in it. And you'll see, even in water, quite clearly this process of evacuating a region and ejecting material outwards as when you throw your rock into the water you'll see a splash occur and material come out of the water bucket. And that's what happens when a meteor strikes the moon or the earth or the surface of any planet. It produces both an impact crater and then sends ejecta material outwards, and we find outside of the crater a lot of ejecta. Some of the ejecta can travel at fairly high velocities. As we talked about in the Mars lecture, some of the ejecta carries enough energy that it could actually escape from the surface of the planet and some of the rocks thrown up from a collision like this can escape, say from Mars, and reach Earth. We can, the cratering rate was once much higher. We have occasional meteors strike the Earth the today We had a very dramatic example of one not long ago, in Russia. Where we saw a meteor come in. It was filmed on many, many people's cameras on their cars. I encourage you to go to YouTube, and look at some of the images of, what is a very small meteor striking the Earth. In the past, this rate of meteor collision was much higher. This plot here shows the rate of infall from meteors striking the Earth as a function of time. This represents when the solar system formed about 4.5 billion years ago. We're off the graph here. You'll notice that the rate drop very dramatically, remember that picture I showed at the simulation. The Earth and the surrounding solar system was once made up of, we had all these rocks floating around, the remnants of the formation of our solar system, these planetesimals and meteors rained down on the Earth. There was an enormous infall rate early on. We think, and this is based on the rocks we brought back from the surface, that there was a bump here called the late heavy bombardment. Possibly due to the giant planet orbits rearranging themselves somewhat. And in doing so, driving more material into the inner solar system, where it rained down and struck the Earth and the Moon. And then this rate has been falling off over time. Now this drop you see here is enormous. Let's look at this axis. There's a factor of 100 in rate between here and here. So you can see the rate drop by a factor of 100 here, and it has dropped another factor of 100 between the rate even at the end of the late Heavy Bombardment today. So we had a very high rate of collisions here. One of the ways we infer this is by looking at the rocks we see of meteors, we rocks we've brought back from the lunar surface. We are able to date those, look at the age of craters as a function of time, and put together a coherent picture of the history of the lunar surface. One of the ways we can date things is by superposition. We think that this crater here happened after this crater here. Right. Something came in. This is a later event, where this whole crater was cleared out and then this guy came in. So we can date things by superposition and get relative ages just by an image. But by landing on the lunar surface and bringing back samples from different regions, we're able to infer the whole history of a bombardment on the lunar surface, and then use that to extrapolate to other places in our own solar system. And one of the things you can see by dating is just look at the relative ages, say of the low land regions here. These are the lunar mares where there are lava flows on the lunar surface. And here is the highland region. And you'll notice this highland region has, more craters. Therefore, it's older because it experienced a longer fraction of this bombardment period. So the integrated exposure, the number of things that hit it, is larger here in the highlands. So this is going to be older when you see more cratering. And here, where the Moon surface is smoother, it's going to be younger. We can apply this to any planetary system that we can image. And infer at least the relative ages of different portions of the system. When we think about things like this late, heavy bombardment, we can think about what it means for the Earth. In the case of the Earth, that enormous rate implies there are something like 22,000 craters over 20 kilometers and larger that formed during this period of the late heavy bombardment. So there was basically a major meteoritic event happening every 100 years. This made earth a very unpleasant place to be, probably led to melting of its crust in many regions often. Tremendous protubations to its atmosphere. Made it a very difficult place for life to form and thrive, and it was only after the bombardment did earth become a more stable environment for life. And this story is probably be going to be true for most solar systems. We think that this picture of how our solar system formed a gas and dust disk forming around the sun, the dust condensing, merging, forming planetesimals. These planetesimals assembling into planets, the planets being bombarded from meteors and other leftover material from the early solar system including comets, which will contain ice and water, that that's the basic history of most planetary systems. So early on, many planets may be an unpleasant place to be and it's only when the system settles down that they become stable enough that life can really grow and thrive. So, now what I'd like you to do is use these concepts of cratering to date planetary systems. And remember that when you see more cratering, that means there's been more time to be hit by things. That means the system's going to be older. So let's think about a couple applications and then we'll come back and talk some more.
