 Greetings and welcome to the Introduction to Astronomy. In this lecture we're going to talk about the origin of the moon or where did our moon come from? And here we are specifically looking at the Earth's moon. So where did that moon come from? Now why is that unusual? Well our moon is the only large moon in the inner part of the solar system. And among the eight planets it is the largest moon relative to the planet's size. Not the largest moon in the solar system, there are several that are larger, but in terms of comparison to the planet it orbits, it is only about a quarter of the diameter of the Earth, which is far larger than the moons of any of the other eight major planets. So there's always been different theories as to where the moon came from. So we can look at some of those, early theories, and what we see is that there were several different ones. One was the fission theory that the Earth was spinning and spun fast enough that a chunk of material spun off of it and eventually formed the moon. The big question or problem with that is how why was the Earth spinning fast enough for material to be able to come off of it. It would have to have been spinning incredibly fast, so it's not really known how that could have happened. Another is what we call the sister theory that the Earth and the moon just forms together. Now we kind of think that this happened for some of the outer planets, especially their major moons. For example, the Galilean moons of Jupiter are believed to have formed around Jupiter much as the planets formed around the Sun. So that's a possibility for the Earth. Big questions are, first of all, why is the Earth the only place this happened in the inner solar system? Mercury and Venus have no moons and Mars has two small ones, which may be captured asteroids. So why is the Earth the only place that this occurred in the inner solar system? And also, why are there differences in composition? The composition of the Earth and the moon. The moon is rocky while the Earth has more metal to it. And if we look at densities, the moon's density is about 3.3 grams per cubic centimeter. And the Earth's is about 5.5 grams per cubic centimeter. So there's a big difference in the densities. And you would think if they formed together, they would be essentially the same, that they would have the same proportions of rock and metal because they formed at the same distance from the Sun. Third theory that was once thought would be the capture theory. Well, it's difficult because how could something get captured? It is very difficult for one object just to happen to capture another gravitationally. We can certainly do it when we send objects into orbit to explore another planet, but to actually have it just randomly happen that it's coming at the right of speeds and velocities to get it into a circular orbit is very difficult. So it's a big question as to how this could have happened. And it would need to lose a lot of energy to get into that kind of orbit. So that leads us to our current theory, our current model of how, why the Earth has such a large moon. And that is what we call the giant impact hypothesis. So what we hypothesize is that a large object here, maybe about the size of Mars, crashed into the very early Earth. So as the Earth was forming four and a half billion years ago, and it was still collecting planetesimals, there was a large massive impact. This would have been again a Mars sized object that strikes the Earth, disrupt the Earth and threw a lot of material into orbit around the Earth. So the material would have then orbited the Earth. And that material would have condensed to form the moon. So this large impact explains a couple of things. First of all, it would explain why the Earth is the only object in the inner solar system with a large moon. Because it just happened to be the one that had the impact that struck it just right. Now you have to imagine it has to strike things just right. Because otherwise, if it was just a little bit off, then this object would miss the Earth altogether, or maybe just glance off the top of it. And if it was a direct head-on collision, they may have just coalesced together. So what does this explain about some of the various properties of the Earth in the moon? And let's take a quick look at those. Well, it can explain why the Earth has a high density and the moon has a lower density. Essentially, the moon was formed from mantle material. And the metallic cores were not disrupted but coalesced together at the core of the Earth. So it explains why the Earth has an unusually high density, or an unusually high amount of metal, for its location in the solar system. And since the moon is being made up of the crust of the Earth and this other object, that would explain why the moon has such a rocky composition and so little metal. It also explains why the moon has very little in terms of what we call volatile elements. Volatile elements are things like ices. And they are objects that would have been vaporized, things with low melting or boiling points. And they would have been vaporized in the impact. So the material that formed the moon, that would then end up in orbit around the Earth that formed the moon, would then not have contained lots of ices. So the moon, unlike the Earth, has very little of these volatile elements. And that would be one way to be able to explain that. It also explains why isotope ratios in lunar and terrestrial rocks are similar. The surface rocks would need a common origin. So the material that came from the Earth and this object would have then been part of the material, would also form similar crustal materials on the Earth and the moon, and would explain why those ratios are roughly the same. So there are some things that the Earth and the moon have in common in terms of these that can be explained by the large impact theory. We can also explain why the Earth has such a high density and the moon a low density, and why the moon is lacking in volatile elements. So let's finish up here with our summary. And what we find is that there have been multiple theories that have been proposed to explain the origin of the Earth's moon. Why is there only one object in the inner solar system with a large moon? The currently accepted theory is the giant impact hypothesis, which suggests that a large Mars size object crashed in to the very early Earth or proto-Earth as it was forming. And this theory can explain a number of the properties of the Earth-Moon system that are otherwise difficult to be able to explain. So that concludes our 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.