 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the origin of our moon. So we'll be looking at more moons in the solar system and we'll also kind of discuss where they come from. But our moon is unusual in many ways and that makes us question how it might have formed. So let's look at some of the things that we know about our moon. Of any of the planets that have moons, our moon is the largest satellite relative to the size of the planet. Yes, there are several moons that are larger than our own, but they are orbiting much larger planets. So the moon is one fourth the diameter of Earth. Now there are comparable things, say in the outer solar system with the dwarf planet Pluto, which has a large moon as well relative to its size. But among the planets themselves, our moon is by far the largest. It's also the only large moon in the inner solar system. Among the other planets, Mercury and Venus have no moons between them. Mars has two moons, but they're very small asteroid, very tiny relative to Mars's size, and they're probably things that were captured from the asteroid belt. When we also look at the moon, we find that there are similarities and differences which need to be explained and our model of how this formed has to be able to explain these. For example, why is the moon so much less dense than Earth? Why is there a complete lack of icy material on the moon as compared to Earth? So those are things that we have to look at and that any model explaining how the moon formed has to be able to explain. So let's look at some of the early theories and some of the early theories that we can look at were fission theory. That says that the Earth spun really fast and a blob of material would have come off at the equatorial region and become the moon. Well, first of all, how could the moon have started spinning fast enough? Sorry, the Earth spinning fast enough is a very good question. However, it would explain some of the things. It would explain why the moon is much less dense than Earth. And that would be because it comes from the mantle and crust of Earth which are mostly rocky and that would give it a lower density than the Earth which has a large metallic core. Another previous theory was what they call the sister theory which means that the Earth and the moon just formed together in space. Well, couple questions. First of all, why are they so different than in composition? There are in terms of the density and other materials and in addition, why if this is the case, if it forms naturally then why do the other terrestrial planets not have relatively large moons? And then finally, the capture theory. That perhaps the moon formed elsewhere. Well, that could explain why it has a different composition but it is very difficult to capture an object into an orbit. It can be done. We believe it happens with small moons in the outer solar system but for an object this large to be captured by Earth into a nearly circular orbit would be extremely difficult. It would have to lose a lot of energy in order to do this. So what do we believe? Our current theory states that it was a giant impact instead that struck the Earth. In fact, a Mars-sized object strikes the Earth which disrupts Earth and throws a large amount of material from the Earth. This material would then condense to form our moon. So we had an impact such as shown here, a massive object very early in the history. So just as Earth was forming, this material smashed into Earth and this can help to explain a number of things in terms of the properties. And here we see a little bit of a video showing how this might have occurred. There's the impact. Material gets spewed out. The Earth there, the cores collide together and merge together. That gives the Earth a larger density than it otherwise would. And you can see the other object forming there which would be the moon. So one object comes way back to Earth and then the other will become the moon. So again, this is one way of simulating how this might have occurred in a very large impact. A lot of the material would have fallen back to Earth but what would have left Earth would have been material that was mostly crust and mantle material. The cores would have collided together and then the other material would fall back to Earth or be dispersed. It would also explain why there's no icy material there and that's because the lack of icy material would have been vaporized. Any icy material would have been vaporized in this impact. So this is one way to be able to explain it and it also explains why the other planets do not have moons large like ours. The other terrestrial planets do not have this and that would be because it's a very rare thing. It happens to be a collision, a very massive collision between two objects which is a catastrophic hypothesis which we usually don't like but they're worthwhile to explain things that are unusual when an unusual thing has happened like this. So let's look a little bit at how this helps with the properties that we look at of the Earth and Moon system. What do we see? Well, the Earth has a high density, the Moon has a lower density. So the Moon was formed from mantle material that gives the Moon a lower density and the metallic cores were not disrupted in fact they combined together making the Earth unusually dense and the Earth is denser than it should be for its place in the solar system. The Moon has little in the way of what we call volatile elements things such as icy material. These would have been vaporized. Anything with a low melting or boiling point would have been completely vaporized and lost in this impact. We also see that isotope ratios looking at the different isotopes of various elements in lunar and terrestrial rocks are roughly the same and that gives, says that there should be a common origin between the rocks we find on Earth and the rocks we find on the Moon. And these are things that the collision theory can explain. So is it perfect? Not really, but with any model it is our current best estimate as to how Earth would have formed a large Moon if the other terrestrial planets did not. So let's go ahead and finish up with our summary and what we've looked at again we've looked at multiple theories to explain the origins of Earth-Moon. The currently accepted theory is the giant impact hypothesis that is currently fits the best and can explain a large number of the properties of the Earth-Moon system. So that concludes this lecture on the origin of the Moon. 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.