 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to go through and start talking about the Earth and most specifically the interior structure of the Earth, so what the Earth is like inside. Now that is something that we cannot see directly. We cannot drill down to the center of the Earth to learn what the interior is like. So we have to use some indirect methods to be able to figure out what is really going on inside the Earth. And we can then use that information to be able to help us better understand the interiors of other planets by making some comparisons. So let's start off by looking at some basic properties of the Earth and those are, we have some basic sizes here. The diameter is 12,756 kilometers. That would be the distance from one edge of the Earth to the other straight across that. The diameter of that would be 12,000 kilometers. The mass of the Earth is about 6 times 10 to the 24th kilograms and that is how much material there is and that's one of those kinds of numbers that is really beyond our direct comprehension. So when we want to compare other objects to how massive they are compared to the Earth, we consider this number to be one Earth mass. And what we might do is then say that another planet has half the mass of the Earth or has two times the mass of the Earth and those are the numbers that we can comprehend. Ones and twos were very good with. Things that are here, this would be 6 times 10 to the 24th would be a 6 followed by 24 zeros. That amount of material is just simply beyond our comprehension. So we compare them relative Earth masses. And the density of the Earth is about 5.5 grams per cubic centimeter. That means that the Earth is made up of a mixture of rock and metal. So looking at the density we can learn something about the interiors and we can use that to compare other objects as well. The rotational period of the Earth, 23 hours and 56 minutes, how long it takes the Earth to spin once on its axis and that is very much related to our day of 24 hours. But do recall that that also requires that the Earth is moving around the sun and the little bit that the Earth moves to get the sun back in position for our day takes that extra four minutes that gives us our 24 hour day. And the escape velocity, 11.2 kilometers per second, how fast you need to be going to be able to launch something off the surface of the Earth. So it would not, if you travel any slower than that, you would not be able to leave the Earth's surface. It would not have enough velocity to be able to get away in an object with less velocity than that would go up and turn around and come back by the Earth's gravity. If you had enough velocity then you'd be able to launch a spacecraft, for example, off into space if you're able to get it going at greater than the escape velocity. Now let's look at some of the unique properties that the Earth has as well. The Earth is unique in many ways, it has several different things, including the only planet with liquid water on its surface. Now Mars has had liquid water in the past, so there is evidence that other objects have had liquid water on them, but Mars has no current liquid water and if any that it does is really not water in the sense that we are used to. But we do see evidence that Mars had liquid water in the past. We also know that some objects in the outer solar system, some of the moons like Europa, have a liquid water ice, liquid or other icy surface, and do have liquid water down below the surface. So we do know that liquid water is present in the solar system, but only one object has liquid water on its surface right now and that is our Earth. It's also the only object with oxygen in its atmosphere. We have other planets with atmospheres, none of them have oxygen at all. So no oxygen on Venus, no oxygen on any of the large moons, like Titan that have an atmosphere or even in the atmospheres of the giant planets. So that is one thing that is unique to Earth, and it's the only planet known to have life. Maybe life existed on Mars in the past, maybe there's some kind of simple life today. Maybe life existed on Europa in the past or exists there now, but the only place that we know of right now that has life is the Earth. So that is the only place right now that we know of that supports life. Now let's get to our subject for today, which is really looking at the interior of the Earth. So what do we know of the interior? Well we divide it into four layers, and the first is the crust of the Earth. That is the outer very thin layer right here, and that's it. That is the area that we have explored. So we can look at the crust, those are the very lightest materials that have floated to the top when the Earth was molten. So these are the lowest density rocks, and we also get things like waters and ices and the atmosphere that are up above that, that are lower density objects. Everything down below it will be a higher density. So when we go down a little bit further, the next area we get is the mantle. The mantle is this region here, and that contains some molten or semi-molten rocky materials. So it's still rocky, and these materials are a little bit denser than those on the crust. So a little bit denser materials in the mantle that would have separated down, and they're kind of in almost a plastic-y state that they can move very, that they can actually still move and deform very easily compared to rocks that we are used to. Now when we get down a little further, we get down to the core, and the core is divided into two parts. There is a liquid outer core here that has molten iron and nickel, so it's almost solid metal. When you get down here, this is a solid chunk of metal at the center of the Earth. The outer portion is liquid, and behaves much like liquid metal here on Earth, except that it is under much higher pressures and temperatures than we would be used to. Deeper down, under even higher pressures, we have a solid inner core, which again is just solid iron and nickel. So all of the iron and nickel that help form the Earth is down at the core there. Very little of it remains on the surface. So this is the solid material here, solid core. This is the outer core, which is molten, and then a plasticky area in the mantle that is really denser, much denser rocks. Now the question is, how do we know all of this? So let's look a little bit at how we can determine that this is what the Earth's interior looks like, because as I've said, we cannot drill to the center of the Earth. It is simply not possible. How can we get down to the center? Well, we can't. Our deepest drilling may be about seven miles down. The distance to the core is about 4,000 miles. So we've only drilled a tiny fraction of the way. We've barely gotten through in the outer portions of the crust, so we have no way to get down to the core. So in order to learn about this, we need to use indirect observations to figure out what is going down on down further in the Earth. And we can do that with earthquakes. When an earthquake occurs at some location on the Earth, it then sends out seismic waves, and those travel through the interior of the Earth and reach the other side and then could be detected by seismic stations at various locations across the Earth's surface. So in other points around the Earth, we can then detect these. And mapping out what types of waves we get and when they are received allows us to then determine what the interior of the Earth is like. Now, there are two types of waves that form. There are P waves and S waves. S waves cannot travel through very liquid material. So no liquid here. If they try to travel through liquid, they get damped out very easily. So an earthquake occurring at this location up at the top of the image then would send P and S waves through, and over in this portion, you would be able to observe both P and S waves. Here, you would only be able to observe P waves, and here you observe nothing. No waves at all. Now what that tells us is that on this side of the Earth, these detectors only detect the P waves, meaning that those waves had to have traveled through a liquid to eliminate the S waves. On this section, the P and S waves are both detected so that we're able to get, things did not have to travel through anything very liquid, so we can actually see both P and S waves. The fact that we get nothing in what we call the shadow zone here tells us something about how things are bent, and that certain areas here, the waves are getting bent by the changes in densities of material, and at the border, they're bent even more. So the waves coming through here that actually reach the core get bent significantly and actually end up coming out way over here. So as these waves travel through the interior, we use those patterns to interpret the Earth's interior structure. So what does that tell us? We cannot learn the interior structure of the Earth by a single earthquake, but when we look at tens and hundreds and thousands of earthquakes that occur all around the Earth, we can then use those to put together a detailed map of the interior, because we can then make estimates as to what the sizes of the core, inner core and outer core and mantle are, and we can then use that to predict what will occur in each earthquake, and we can use multiple earthquakes to continually refine our measurements of what the sizes and what the density variations are in the interior of the Earth. And after looking at thousands of these earthquakes, we've been able to get a very good model of what the Earth's interior looks like, even though we've only been able to actually drill and explore a very small section of the Earth's interior using their seismic waves, allows us to really learn what the Earth is like inside, and then to be able to use that to interpret and look at what other planets might be like inside. Again, we cannot study those directly either. So the last thing to look at here is what happens when metals spin and they generate a magnetic field inside the Earth. And it is generated by electrical currents forming in the outer core of the Earth. So the molten outer core is a metal, that's metal, iron and nickel, and as it spins, it generates electrical currents and spinning electrical currents will give you a magnetic field. This magnetic field will then extend out into space, giving us a magnetosphere, which is where the Earth's magnetic field is dominant. And that means that it protects us from charged particles from the Sun. So as solar particles come through, they strike the Earth's magnetic field and they're deviated around the Earth. So materials then go around the Earth instead of striking us directly. Now the Sun emits many high-energy charge particles, so we would not want to be constantly bombarded by those. So the Earth's magnetic field serves as protection for us from those. It also gives us things like the aurora as some of these particles come down and do strike the Earth's atmosphere around the poles and give us then the aurora as they excite atoms in the atmosphere, giving off that distinct greenish glow that we associate with the aurora. Other charged particles can get trapped and form radiation belts around the Earth. But most of the particles just get deviated away and travel off into space and out through the solar system and therefore the magnetic field serves to protect us. So finishing up here with our summary, let's look at what we've gone over this time, and we did talk about how the Earth is a unique planet in many ways. We talked about water, only planet with liquid water on its surface. We talked about oxygen, and we talked about life as several things that are very unique to the Earth. We learned how we can map the interior structure of the Earth using seismic waves and we divided the Earth into four parts across the part we live on and can explore, the mantle, the outer core, and the inner core. And we learned that the liquid metal core and currents in that are what generates the Earth's magnetic field and protects us from charged particles from the Sun. So that concludes our lecture on the interior structure of 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.