【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

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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 we’ve

learned from them about the formation of Earth, and the solar system. Uh… the

thing is what’s 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 to…to just a terrestrial

rock.

So to talk about where meteorites come from, we need to talk about comets

and asteroids, which basically...they’re basically made up of debris left over

from the origin of the solar system 4.6 billion years ago.

Now I’m going a bit out of a order here…um…I’m not going to go into any

depth on comets and asteroids now, but we’ll come back later and do that. For

now, I’ll 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, we’ve got the asteroid belt, which contains about 90

percents of all asteroids orbiting the sun. These asteroids are…uh…like the

terrestrial planets, in that they’re composed mostly of rocky material and

metals.

Far from the sun, in the outer solar system, beyond Jupiter’s orbit,

temperatures are low enough to permit ices to form out of water and…and 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 it’s 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 Earth’s

atmosphere, well, that makes them so special that they get a special name.

They’re called meteors. Most of them are very small, and they burn up soon after

entering Earth’s 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 we’ve been finding meteorites on Earth for thousands of years, and

we’ve 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, they’re 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, it’s rare to ever find one. I mean,

it’s 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

you’re trying to find metal?

Student: Oh! Metal detectors!

Professor: Right, thank you. At least that’s part of it. Stone meteorites,

if they lie around exposed to the weather for a few years, well, they’re made of

rock, so they end up looking almost indistinguishable from common terrestrial

rocks--ones that originated on earth. So it’s hard to spot them by eye. But we

can use metal detectors to help us find the others, and they’re easier to spot

by eye. So most of the meteorites in collections, uh, in museums, they’ll

be...they’re iron meteorites, or the stony iron kind, even though they only make

up about 5 percent of the meteorite material on the ground.

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