Lecture: Gas giant planet: Uranus: Narrator: Listen to part of a lecture in an astronomy class. Professor: So, we've been talking about the outer solar system and the planets Jupiter and Saturn. Now we turn to another gas giant planet – Uranus, which is one of the more difficult planets for astronomers to study because it's so far away from us. To give you an idea how far away it is, one orbit around the Sun takes Uranus 84 Earth years. Yes, Kevin? Kevin: Didn't the Voyager spacecraft get close enough to take pictures of it? Professor: Yes, the spacecraft did get close enough to take pictures and measurements. In fact, we've learned a lot about Uranus' moons and rings from the Voyager flyby. Nevertheless, you can study a planet without making close-up observations and we have been able to deduce quite a lot about Uranus from observing it from Earth. For example, when you look at Uranus through a telescope, it appears blue, like a blue-green disk. What does this tell us? Student: What it's made of? Professor: Well, no not exactly. It tells us the composition of the planet's atmosphere. It tells us that Uranus' atmosphere has methane gas in it. How? Well, the explanation for the blue-green color goes like this: methane gas absorbs red light, so as sunlight passes through Uranus' atmosphere and is reflected back into space, the methane gas absorbs most of the red portion of the light and lets the other light pass through. That's the blue-green color we see. Using indirect observations, we can make inferences about the composition of the planet's interior. Uh, for example, we know Uranus' mass, how much stuff there is, and its volume, how much space that stuff takes up. So, we were able to calculate its density. Knowing Uranus' density gives us clues about its chemical make up because the density of a substance is consistent and so planets that have the same density usually are made of the same material, but not always. For example, Uranus and Jupiter have almost the same density, however, we know that Jupiter has a a lot more material, a lot more mass than Uranus and all that mass presses on itself, increasing its own density. So, even though their density are almost the same, Uranus probably is made up of heavier elements than Jupiter. Student: So, is Uranus made of rock? Of iron? Professor: We're discussing gas giant planets, remember? No. Its density is much lower than that of rocky planets. Uranus might have a small core of rock or iron, but we think it's primarily composed of ordinary water mixed with methane and ammonia. Now, another feature of Uranus is its rotation axis, which is nearly level with its path around the Sun. No other planet in our solar system is tilted so far on its side. Student: Do we know why Uranus' tilt is so different? Professor: Well, some astronomers think it might be the result of a collision with another object early in Uranus' history, which changed Uranus' rotation. A result of this tilt is that the poles get most of the sunlight; half the time one pole and half the time the other within a year and remember, that's 84 years, so you get very long seasons. Student: It probably doesn't get warm anywhere there, so far away from the Sun. Professor: Right and not only is Uranus far away from the Sun, it doesn't generate any heat unlike the other giant planets which actually give off more heat than they get from the Sun. So, Uranus must have a very low core temperature and we don't know why. Some astronomers think it has to do with the unusal tilt. That the collision that knocked Uranus onto its side may have caused it to release much of its internal heat. But there're other theories. But getting back to your question earlier about the Voyager spacecraft, I don't want you to think that these space missions are somehow a propeller in terms of what we can learn about a planet. So for example, we know that you're in this has a magnetic field. And this is something that we can't learn from our vantage point here on planet earth. You have to get up close for that.