TEXT: TPO 13 Lecture 4 Astronomy
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, butthey 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. (目的主旨题①)—
soto 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 justcover some basic info about them.
OK, comets and asteroids. It might help if you think of...remember we talked about thetwo 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.(比较细节题②-1)
Well, the solar system alsohas two analogous (对比逻辑)classes of objects, smaller than planets--namely, asteroids and comets.(比较细节题②-2)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 asteroidsare…uh…like the terrestrial planets,in that they're composed mostly of rocky material and metals.(比较细节题②-3)
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.(比较细节题②-4)
【主体段2】 前面分别介绍了asteroid 和comet，现在开始真正开始说meteoroid从哪里来，然后有区分了3个不同的和meteor有关的概念。
Both comets and asteroids are...typically are smallerthan 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. (内容细节题③)
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.(对比细节题④)
Nowwe've been finding meteorites on Earth for thousands of years, and we've analyzed enough of them to learn a lot about theircomposition,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. Theyactuallyaccount for almost all of the meteorite material that falls to earth.(转折细节题⑤-1)
Buteven so, it's rare to ever find one. I mean, it's easier to find an iron meteorite or a stony iron. (转折细节题⑤-2)
【主体段4】 主要在介绍为什么stone meteorite最少被发现
Anyone guess why? Look at their names. What do you think iron meteorites consist of?
Professor:Yeah… iron and some nickel, both of which are metals. And, if you're trying to find metal?(重听题⑥)
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. Butwe 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.