 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to look at the structure and features that we see on the Moon, our closest celestial neighbor to us here on the Earth. So we'll want to look at some of the various properties and what we see, features that we can actually see on the Moon and the type of rocks that have been explored and brought back to the Earth from the Moon by the Apollo astronauts. So, let's get started here with some of the basic properties of the Moon. First of all, what do we know about the Moon? Well, its mass is much less than that of the Earth, a little over one-one-hundredth the mass. Its diameter is a little more than a quarter of an Earth diameter. Its surface gravity is only about 17% that of Earth, so you would weigh a lot less on the Moon than you would on the Earth. So, why do images of the astronauts look like they're bounding around, even though they're carrying weight of several hundred pounds on them, is because the surface gravity is so low, and that is therefore makes it very easy for them to be able to move. Now, the density is also a lot less than the Earth. The Earth is about five grams per cubic centimeter. The density of the Moon is about 3.3 grams per cubic centimeter, meaning that it has a higher concentration of rock and a lower concentration of metals than the Earth. And its rotational period is 27.3 days. That is how long it takes the Moon to spin on its axis. It is also how long it takes the Moon to orbit once around the Earth, and that is why one side of the Moon always faces the Earth. Now, exploration of the Moon. Of course, the Moon is the object we've been able to study for the longest time. However, the far side of the Moon is not something that we've known about because one side is always facing us. That means that we really don't know about the other side of the Moon and have not until very recently. In fact, it was not until 1959 that the Luna 3 probe brought back the first view of the far side, which is pictured here. Now, it looks quite different than what images that we see of the near side of the Moon. The near side of the Moon is dominated by the dark areas that we call the Maria. And on the far side, it is not. There are just a few darker areas, and most of it is a much lighter area. But it's just interesting to note that until 1959, we did not even know what the far side looked like. So while this image might not seem like anything spectacular, it was amazing to astronomers back in the late 1950s who were getting their first view of the other side of our nearest celestial neighbor. Seven years later, in 1966, we had the first soft landing on the Moon. We'd have previous landings were all what we'd call impactors that just crashed into the Moon and sent data back until they were destroyed impacting on the Moon. Between 1969 and 1972, we had the Apollo landings where we landed on the Moon a number of times and were able to bring back direct samples. But with that, we did not stop exploration of the Moon. Even in 2009, the Lunar Reconnaissance Orbiter was put into space and put into orbit around the Moon and gave us some more detailed images of the Moon. So here we see the Apollo site, one of the Apollo landing sites. And you can see the lander here. This is one of the rovers that was used in the last three Apollo missions. Of course, the astronaut there standing by the flag that is put up. And you can see a number of their footprints scattered around the surface of the Moon. Now, the Reconnaissance Orbiter gave us some more details and allowed us to see some other parts of the Moon and in fact has mapped the Moon in great detail over about a decade's worth of exploration. It's been able to resolve things down to just a few meters to see details. And in fact, this is an image of it looking at one of the lunar landing sites. And you can see where the lunar module would be present here. Discarded materials and various equipment that was left in one part of the Moon and a camera over on the other side. So various other objects can be imaged by the Reconnaissance Orbiter which is giving us much more detail than we've ever had before. In 2013, we had the U2 rover, a Chinese craft that was actually a rover on the Moon. Now, we hear about rovers on Mars all the time. We don't hear about them on the Moon because it is a much more difficult thing to send an unmanned rover to the Moon because of the long day-night cycle on the Moon. You will have two weeks of daylight where things are baking in the heat of the Sun because there's no atmosphere to modify the temperatures, no clouds to protect you. And then for the next two weeks, you will have complete darkness and temperatures will drop by hundreds of degrees. So not like here on Earth where it gets a little bit colder at night but where it goes from well above. Very high temperatures to very, very low temperatures in a very short period of time. And of course, that will play havoc on the equipment and they usually can only survive a couple of day-night cycles. So let's look at what we know about the Moon. And first of all, what is the composition? Well, it is primarily rock. It has less metal than the Earth. It has no atmosphere to speak of. But it does have some water ice that has been detected in craters. Now, it's odd to think that there would be any water ice on the Moon. Any water placed on the Moon would either freeze solid, would be frozen, or would be vaporized in the heat of the Sun. So during the daytime of the Moon, that would vaporize any water converted to vapor, and that water vapor the Moon would be unable to hold on to. However, near the poles of the Moon, there are some craters that are permanently shadowed and has been found that water ice exists in some of those. Now, let's look at the different parts of the Moon. And what we have the different regions of the Moon include the maria, or singular, one of those is mare for seas, not because they have anything to do with water, but because perhaps to ancient astronomers, they might have looked like the watery areas on the Moon. We now know that they are not. They are very dark colored regions, and you can see some of them here, darker areas on the surface of the Moon. They have very few impact craters. If you look at some of these lighter areas, you see lots of craters, but when you look at the maria, you see far fewer craters. So these are younger, and the number of craters that we see on an object tells us something about its age. The lighter colored areas are the highlands, and those have many, many impact craters, so they are significantly older parts of the Moon. Now, all of the Apollo missions did bring back a lot of Moon rocks, and we've been able to study those to learn about what the Moon is like. So let's take a look at some of those, and here's an example. We find three general different types of rocks. We find basalts that are formed in the maria. They are very dark colored volcanic rocks, so they are formed through volcanic activity or cooled from molten lava. We also see a lot of them that we call vesicular. That simply means that they were trapped air bubbles when they cooled, and when the air bubble's air was released, they have all of these air bubble air pockets left in them. So material that was left there, and now we have these bubbles that appear on them. But these are dark colored areas that give the maria its distinct dark color. Anorthocytes are another type. Those are lighter colored volcanic rocks, and we can see one here. These are found primarily in the highlands, and these are a lower density rock, and lighter colored. So they give the highlands its distinct color, and they are lower density, so they are the highest portions of the Moon. They are the ones that would be pushed to the top if the Moon was molten. The third type that we want to look at briefly are what we call the breccia. A breccia, looking something like this, is a rock that are created by impacts. The impacts will melt and fuse materials to make a new rock. So essentially you have rock that was there, and that massive impact will melt some of the rock and cause different bits of different types of rocks to then fuse together in a conglomerate like this, and that gives us a whole new type of rock that we call a breccia. And that is a completely different type of rock from the others that we looked at. It's actually some of those different types that can be cemented together. Now one of the things on the Earth we get are erosional processes that wipe out craters. On the Moon we see far fewer craters. So we want to look at what are the erosional processes that may occur on the Moon. Well, the erosional processes on the Moon are quite different than what we see here on the Earth. There is no wind and no water on the Moon. So we cannot get things that are eroded by wind or things that are eroded by water as happens here on the Earth. However, the Moon does get something that the Earth does not, and that is micrometeorite impacts. Those all burn up in the Earth's atmosphere, but they do strike the ground on the Moon and cause a slow erosional effect. And that gives rise to the powdered soil on the Moon. So if you look here at an astronaut's footprint on the Moon, this that is over 40 years old is still present on the Moon because there are no erosional effects. How many footprints are going to last on the Earth for decades? So here is one that we look at, and we see as a footprint, and it shows you the powdery surface on the Moon. That has been done by micrometeorite impacts over billions of years. All of those things that we see as shooting stars are constantly pounding the Moon's surface and pulverizing it into this fine powder. Now the astronauts' footprints will eventually wear away, but it can take millions or tens of millions of years for those micrometeorites to slowly repulverize that material and to slowly churn it up. So eventually these will disappear, but if we go back to the Moon in the next few hundred years, we will still see the astronauts' footprints left much as they left the Moon back in the 1960s and 1970s. So let's finish up here with our summary, and what we find is that our Moon is our closest neighbor to us, but it is also very different from the Earth. It has a lot of differences that are quite important when we look at it. It is divided into two distinct regions, the Mariah and the Highlands, which each have their own properties. The Mariah being a lighter, a younger area, and the Highlands being a... I'm sorry, the Mariah being a darker, a darker, less cratered area, and the Highlands being a lighter, heavily cratered area. And finally, we have different types of rocks that have formed on the Moon. One thing we do not see, as an example, is a sedimentary rock. We do not see sedimentary rocks on the Moon because those would imply water, and there has never been liquid running or standing water on the Moon that would have been able to create sedimentary rocks. So none of those have ever been found on the Moon. All of the rocks that we do see are from volcanic activity. So that concludes our luck lecture on the structure and some of the surface 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.