【托福听力备考】TPO13 听力文本——Lecture 4.doc

【托福听力备考】TPO13 听力文本Lecture 4众所周知，托福TPO材料是备考托福听力最好的材料。相信众多备考托福的同学也一直在练习这套材料，那么在以下内容中我们就为大家带来托福TPO听力练习的文本，希望能为大家的备考带来帮助。TPO 13 Lecture 4 Astronomy Narrator: Listen to part of a lecture in an astronomy class.Professor:OK, I wanna go over the different types of meteorites, and what weve learned from them about the formation of Earth, and the solar system. Uh the thing is whats especially interesting about meteorites is that they come from interplanetary space, but they consist of the same chemical elements that are in matter originating on Earth, just in different proportions. But that makes it easier to identify something as a meteorite, as opposed toto just a terrestrial rock.So to talk about where meteorites come from, we need to talk about comets and asteroids, which basically.theyre basically made up of debris left over from the origin of the solar system 4.6 billion years ago.Now Im going a bit out of a order hereumIm not going to go into any depth on comets and asteroids now, but well come back later and do that. For now, Ill just cover some basic info about them.OK, comets and asteroids. It might help if you think of.remember we talked about the two classes of planets in our solar system? And how they differ in composition? The terrestrial planets-like Mars and Earth-composed largely of rocks and metals, and the large gas giants, like Jupiter. Well, the solar system also has two analogous classes of objects, smaller than planets-namely, asteroids and comets.Relatively near the sun, in the inner solar system, between Jupiter and Mars to be precise, weve got the asteroid belt, which contains about 90 percents of all asteroids orbiting the sun. These asteroids areuhlike the terrestrial planets, in that theyre composed mostly of rocky material and metals.Far from the sun, in the outer solar system, beyond Jupiters orbit, temperatures are low enough to permit ices to form out of water andand out of gases like methane and carbon dioxide. Loose collections of these ices and small rocky particles form into comets. So comets are similar in composition to the gas giants.Both comets and asteroids are.typically are smaller than planets. An even smaller type of interplanetary debris is the meteoroid. And its from meteoroids that we get meteors and meteorites. “Roids” are, for the most part anyway, they are just smaller bits of asteroids and comets. When these bits enter Earths atmosphere, well, that makes them so special that they get a special name. Theyre called meteors. Most of them are very small, and they burn up soon after entering Earths atmosphere. The larger ones that make it through the atmosphere and hit the ground are called meteorites. So meteorites are the ones that actually make it through.Now weve been finding meteorites on Earth for thousands of years, and weve analyzed enough of them to learn a lot about their composition, most come from asteroids, though a few may have come from comets. So essentially they are rocks, and like rocks, theyre mixtures of minerals. They are generally classified into three broad categories-stones, stony irons and irons.Stone meteorites, which we refer to simply as, uh, stones, are almost entirely rock material. They actually account for almost all of the meteorite material that falls to earth. But even so, its rare to ever find one. I mean, its easier to find an iron meteorite or a stony iron. Anyone guess why? Look at their names. What do you think iron meteorites consist of?Student: Mostly iron?Professor: Yeah iron and some nickel, both of which are metals. And, if youre trying to find metal?Student: Oh! Metal detectors!Professor: Right, thank you. At least thats part of it. Stone meteorites, if they lie around exposed to the weather for a few years, well, theyre made of rock, so they end up looking almost indistinguishable from common terrestrial rocks-ones that originated on earth. So its hard to spot them by eye. But we can use metal detectors to help us find the others, and theyre easier to spot by eye. So most of the meteorites in collections, uh, in museums, theyll be.theyre iron meteorites, or the stony iron kind, even though they only make up about 5 percent of the meteorite material on the ground.希望这些对你的托福备考有帮助，预祝大家托福考试能取得理想成绩。