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Listen to part of a lecture in an astronomy class.
Professor: We’ve been focusing on the outer solar system and the planets Jupiter and Saturn. "Now let’s turn our attention to another gas giant planet Uranus, one of the most difficult planets for astronomers to study due to its distance from the Earth. To give you a hint how far away this planet is, one orbit around the sun takes Uranus 84 Earth years. Yes, Kevin?
Student: Didn’t the Voyager’s spacecraft get close enough to take pictures of it?
Professor: Yes, the spacecraft did get close enough to take pictures and measurements. Actually, we’ve learned a lot about its moons and rings from the Voyager flyby. Nonetheless, you can study a planet without making close-up observations. We have been able to deduce quite a lot about Uranus by observing it from Earth. For instance, when you look at Uranus through a telescope, it appears like a blue-green disk. What does this mean?
Student: What it’s made of?
Professor: Well, not exactly. lt tells us the composition of the planet’s atmosphere. It means that Uranus’ atmosphere has methane gas in it. Why? 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 then allows the other light pass through. That’s the blue-green color we see. Using indirect observation, we can make inferences about the composition of the planet’s interior. For example, we know its mass volume, so we are able to calculate its density. Knowing Uranus’ density gives us ideas about its chemical makeup, because the density of a substance is consistent, 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 lot more material, a lot more mass than Uranus, and all that mass presses on itself, increasing its own density. So even though their densities 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, aren’t we? 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 comprised 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 unusual?
Professor: Well, some researchers think it might be the result of a collision with another object in Uranus’ history, which changed its 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 the reason. Some astronomers think it is related to the unusual tilt, that the collision that knocked Uranus onto its side may have caused it to release much of its internal heat. Of course there are other theories. But getting back to your question earlier about the Voyager’s spacecraft. l don’t want you to think that these space missions are somehow superfluous in terms of what we can learn about a planet. For example, we know that Uranus has a magnetic field and this is something that we can’t learn from our standing point here on the Earth. You have to get up close for that.