TOEFL Listening: ETS-TOEFL听力机经 - 455I6TUIY0BOL2LS

Lecture: Saturn's Moons, Titan: Narrator: Listen to part of a lecture in an astronomy class. Professor: One reason that Mars is so interesting to study is that its valleys indicate that there may have once been water on Mars, which means that the atmosphere may have been thicker. You get a thicker atmosphere on the planet when a liquid such as water is present. Emma: So the atmosphere is different now than it was a long time ago. Professor: Absolutely. Here on Earth, too, is a matter of fact. And that's what's so important for us to understand. You see, Earth's first atmosphere was, well, nothing like it is today. One element especially was missing and that's oxygen. We think early Earth probably also had high concentrations of gases like methane and ammonia than it does today. Yes, Emma? Emma: I was reading something recently about how the early atmosphere of Earth may have been like atmosphere of one of Saturn's Moons, Titan, I think. Is that even possible? Professor: Actually yes. Now I know you are wondering how Titan, a moon that belongs to a planet as far away as Saturn, could be of any significance to us here on Earth. After all, Titan only gets about 1% the amount of sunlight we get here. And because it's so far away from the sun, it's obviously very cod. But Titan does have something that no other moon in the entire solar system has. And that's a think atmosphere. And that's similar to what we think Earth's early atmosphere was like. And Titan's the only celestial body other than Earth known to have constant periods with liquid on its surface. Male Student: But it can't be liquid water, right? Professor: Right. It's actually a liquid methane.Because of Titan's temperature, liquid water can's exist, only frozen ice. Methane, though, does behave on Titan how water behaves here on Earth. It goes through cycles of evaporation of precipitation, and as I said it forms pools, lakes on the surface of Titan. Male Student: So it's a moon with lakes on it? Professor: Very different from the Earth's moon. When we think of a moon, we immediately think of craters, craters from meteor impacts. Well the methane rain on Titan has caused erosion which has helped wipe away all those craters that our moon still has. Male Student: It's strange to think of cycles of evaporation and precipitation on one of Saturn's moons. Professor: Well, like I said, there are many similarities to Earth, especially earlier Earth. So you can see why astronomers are trying to learn as much about Titan as possible. Now speaking of those methane lakes,they change in depth just like lakes do here on Earth. Recently we've been studying the largest lake on Titan and discover that its liquid level has in fact fallen by four or five meters over a four-year period. Male Student: So does that mean that the lake is drying up? Professor: Actually you see Titan's largest lake is located in its southern hemisphere. It was summer there when information was collected. It's actually common for moisture to migrate from one hemisphere to the other, evaporating when it's summer and precipitating when it's winter. So the fact that there's less liquid methane in the largest lakes is certainly interesting, since some of the same weather cycles occur here on Earth. Emma: Okay. So the methane makes sense. And you also mentioned ammonia. Is that found on Titan? Professor: Yes, it is. It's actually being spewed from Titan's volcanoes. Emma: Wait a minute. Volcanoes? In a frigid environment like that? Professor: Well, these volcanoes don't spew molten lava. Obviously, they are old-weather volcanoes and they eject mixtures of water ice and ammonia. Emma: And so what does that mean for us? Like can these findings help us understand more about Earth' early history? Maybe have life started here? Professor: Well, let's put it this way. Ammonia we believe was a key ingredient in Earth's atmosphere when life first began on our planet, so, is this exciting? Absolutely. We need to find out, though, through further research if the chemical processes on Titan are in fact similar to the conditions under which life came to be here on Earth. Let me tell you, if that ends up being the case, who know how much will be able to learn.