 Greetings and welcome to the introduction to astronomy. In this lecture we are going to begin our discussion of the outer planets. We've looked, previous lectures, looked at the inner terrestrial type planets, those similar to Earth. Now we're going to look at planets that are very different than our own. So let's look at these in an overview here. What do we know about the Jovian planets? Well, we know that they're larger. We know that they're composed of ice and gas primarily, and those ices that include things like water, ammonia, and methane. So it does not mean just water ice. And while they are much larger than the terrestrial planets, you could fit all of the terrestrial planets with inside Neptune. Still, they are incredibly tiny as you see here relative to the Sun. These have no solid surface. There is no way we will ever land on these. These will never be landed on because there is no place to land. However, we can land on their moons, and we will look at the planets and their moons in coming lectures. So let's look a little bit about what these planets are made up of, and it's different than what we looked at in the inner solar system. If we're looking by mass then hydrogen makes up 75% and helium 24%. If we go by number, it's more like 90% and 10%. So because the helium atom is more massive, it contributes more to the mass, but the hydrogen atom by number, if you pick an atom at random out of Jupiter, for example, you got a 9 in 10 chance that it will be a hydrogen atom. So we also again have traces of the ices, water, methane, and ammonia, and yes there is a small amount of rock and metal, but it's very small percentage that we will see of those. And the only reason the percentage is so high is again because they're heavier. They are much more massive, so they'll get weighted a lot higher when we look at the measurements by mass. Now let's look briefly at each of these planets. Let's start off with Jupiter, which is the largest of the giant planets and is the largest planet in the solar system. As you recall, we put these relative to Earth, so instead of giving you a diameter in kilometers, we say that it is 11 times Earth's diameter, meaning that you could fit 11 Earths across Jupiter. Its mass is 318 times Earth's mass. So if you could take 318 Earths, you would need that much to make one Jupiter. Its rotation period is incredibly short at 9 hours and 55 minutes. That is the fastest rotation of any planet in the solar system, and it's less than 10 hours. So if you could somehow sit in the atmosphere of Jupiter and rotate around with it, the sun would rise and 5 hours later it would set and 5 hours later it would rise again. The orbital period is nearly 12 years and the semi-major axis, 5 astronomical units, 5 times further away from the sun than Earth, its density a little greater than the density of water. So it has some very dense interior, but it has very low density gases on the exterior that average it out to be a very low density overall. Now we're going to see that Saturn is a little bit smaller, but has some similarities here. Saturn being the ringed planet, its diameter about a little less than 10 times Earth's diameter, and its mass about 95 times Earth's mass. Its rotational period is a little bit longer than Jupiter's, and its orbital period is a lot longer, going to 30 years from 12. It is nearly twice as far away from the sun as Jupiter is, and its density is even less. Its density is less than that of water, meaning that if you had something full of water large enough and could somehow set Saturn in it, Saturn would float. So it is less dense than water. Next we look at the two icy planets, and those are Uranus, which was discovered in 1781. So we have the first planets now that were discovered, and this is the planet that lies on its side. It is 4 times Earth's diameter, 14.5 times Earth's mass, so much smaller than Jupiter and Saturn. Its rotational period is getting back closer to Earth at 17 hours, and its orbital period of 84 years. So one year on Uranus would be 84 Earth years. Semi-major axis would be almost 20 astronomical units, and the density has increased a little bit. We're back to about the density that Jupiter had. And then finally we have Neptune. Neptune discovered in 1846, and it is the most distant planet. It is about the same size and mass as Uranus. Its rotational period is also similar, but its orbital period, of course, being further away, 30 astronomical units instead of 20, means it takes much, much longer to go around the sun once. 165 years. So since it was discovered, it is just starting its second orbit. So it's gotten back to where it started in its orbit, and it is just starting to work the first few years of its second orbit around the sun since it has been discovered. And its density, again, is comparable to Uranus and Jupiter's. Now all of these planets have been explored by spacecraft. Each has been visited by at least once. Jupiter and Saturn have had orbiting spacecraft, and around Jupiter we had the Galileo mission, and we have the Juno mission, which is ongoing. These were both orbiting missions, and around Saturn we had the Cassini mission. So those are orbiting missions. The rest of these were all flybys, and some of them were flybys to go someplace else. For example Cassini flew by Jupiter and used its gravity to help it onto Saturn. You'll see that Voyager 2 went from Jupiter to Saturn, to Uranus, and to Neptune. So the Voyager 2 craft visited all four planets, and is the only craft to visit Uranus and Neptune. So all of our detailed images of those come from the Voyager 2 spacecraft, which was able to make a grand tour of the solar system because of the alignment, the positioning of the planets in the solar system, when that craft was launched in the late 1970s. So each of them has been explored to varying extent. Jupiter and Saturn pretty well. Uranus and Neptune just a little bit. So what do we know? What else have we done with exploration? We've done a few more things, too, than just those. The Galileo spacecraft actually had a probe into Jupiter's atmosphere. So a probe that went into the atmosphere doesn't last very long. It will quickly heat up, and the pressures and temperatures will vaporize it. But we were able to get some measurements of the atmosphere as it plunged deep into Jupiter. Or at least the upper layers of the atmosphere. Then there is Juno, which is studying the polar regions of Jupiter. This is currently, as of this recording, an active satellite around Jupiter. It is in a polar orbit, meaning that it goes up and over the poles instead of around the equatorial regions. So we can see a little bit about what it sees here, and it sees a little bit different than what we're used to seeing in the equatorial regions of Jupiter. Very interesting patterns of storms that occur in those in the polar regions. And this is really the first time we've gotten to get good looks at these storms. Most of the other craft have come in the equatorial plane and given us great views of the equatorial regions with its banding structure, but not so much a good look at the poles. Juno is also in a highly elliptical orbit, meaning that it zips in close to Jupiter, comes in close, takes a quick look at it, and then heads back out. Remember Kepler's second law, and what that says is that the closer you are to the object, the faster you're going to move. So when it comes in close to Jupiter, it zips by and gets a few up close pictures, then gets out of there. Jupiter being very intense, massive, massive and intense, has a lot of radiation near it, so it doesn't want to spend a lot of time there, and it spends most of its time well away from Jupiter, but passes around to be able to get some close up shots. Then we have the Cassini spacecraft, which discovered a polar hexagon on Saturn. So a hexagonal pattern, unusual when we look at things in the universe. Usually we get things that are spherical, elliptical, ellipsoidal shaped, but some kind of really smooth curve like that. To get a hexagon, we tend not to see that. We don't see things like triangles or squares very often. Those would be very unusual. So the polar hexagon is something interesting that Cassini was able to find, and may have something to do with the fluid mechanics in the atmosphere near Saturn's pole. So we are going to look at all of these in a little bit more detail in coming lectures, but let's go ahead and finish up here with our summary. And what we looked at is the terrestrial planets that we've been studying are quite different than the Jovian planets that we are going to look at now. The properties are very different, but each of these has its own unique characteristics. So there are similarities between them, but there are also differences. Each of these has been visited by spacecraft, but to a much smaller extent than the number that we talked about for Mars and Venus. So that concludes this lecture on the overview of the giant planets. 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.