Official 53 Set 6

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Saturn's Rings

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What is the main purpose of the lecture?
  • A. To show the connection between asteroids and Saturn’s rings

  • B. To discuss theories about the formation of Saturn’s rings

  • C. To describe the composition of Saturn’s rings

  • D. To show how Saturn’s rings affect the planet’s atmosphere

显示答案 正确答案: B

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in an astronomy class.

    MALE PROFESSOR: Saturn’s rings have always baffled astronomers.Until about 30 years ago, we thought the rings were composed of particles of ice and rock that were left over from Saturn’s formation—extra material that never managed to form, uh, coalesce into a moon.As you know, it’s believed that Saturn, and all the planets in our solar system coalesced from a swirling cloud of gas some 4.8 billion years ago.However, if the rings are made of leftovers from that process, then they’d also be about 4.8 billion years old.The problem is that anything gathering space dust for that long would certainly have darkened by now.But Saturn’s rings—most of them, anyway—are pristine...so bright and shiny that they make Saturn the jewel of the solar system.

    So the hypothesis that the rings are just made of material left over from the time of planetary formation—that hypothesis must be wrong.Saturn’s rings are much younger than the planet itself.They may have formed only a few hundred million years ago—around the time the earliest dinosaurs lived on Earth.We realize now that the ring particles, which, uh, range in size from microscopic dust to boulders bigger than large houses.Well, a lot of these particles are eventually lost.We believe they gradually spiral down out of the rings and into the planet’s atmosphere.This occurs as a result of the planet’s gravity, and also because of the effects of its magnetic field ...Now, if material from Saturn’s rings is being lost and nothing new is added from time to time, the rings would be disappearing.

    But that’s not happening! So somehow, there must be new material feeding the ring system.Question is: Where’s this new material coming from? So we’re back to square one.But, instead of asking, “How did the rings form?” We should be asking… uh, anyone? Beth?

    FEMALE STUDENT: How do the rings form?<-MALE PROFESSOR:-> How do the rings form! Because they’re apparently replenishing themselves somehow.

    Uh, OK, here’s one possibility—the moons, the dozens of moons that orbit Saturn are providing raw material for the rings.A moon in a system as complex as Saturn’s—and Saturn has at least 49 known moons, which vary tremendously in size and shape.Um, a moon in such a complex system is not only affected by the gravitational force of the planet, but also by that of the other moons.

    FEMALE STUDENT: So the planet may be pulling a moon one way, and other moons may be pulling it...other ways?MALE PROFESSOR: Exactly. Such forces could actually alter a moon’s orbit, and as a result, there might be a collision—one moon might crash into another—and the debris from that collision could become part of the rings.Then there are tidal forces.A moon might get too close to the planet and get broken apart by Saturn’s tidal forces.

    FEMALE STUDENT: Excuse me. You mean, tidal forces, like high tide and low tide on the oceans?MALE PROFESSOR: Well, by “tidal force,” I’m referring to the gravitational pull of Saturn on its moons.Um, in the mid-1800s, a French scientist named édouard Roche was studying the effects of a planet’s tidal forces on its moons.

    Roche was able to show mathematically that if one celestial body—say, a moon—uh, if it passes too close to another—say, a planet—that has a gravitational force stronger than the force of self-attraction that holds the moon together, well, that first body, that moon, it’d be ripped apart.We call the distance at which this happens the “Roche limit.”So if one of Saturn’s moons reaches the Roche limit of the planet or even a larger moon, it would disintegrate—be torn apart, and thus add more material to the ring system.

    And there’s another way new material might be added to Saturn’s rings—an asteroid crashing into one of the moons.This hypothesis is supported by the fact that some of the many rings are a bit reddish in color. Uh, yes, George?

    MALE STUDENT: I’m sorry. I don’t follow the logic.<-MALE PROFESSOR:-> Well, this reddish coloration suggests the presence of complex organic molecules—uh, carbon-based molecules—mixed in with the water-ice.Remember, the rest of Saturn’s rings are made almost entirely of water-ice. And none of Saturn’s moons is red.But asteroids could be... and thus could end up contributing to the ring system the kind of carbon-based molecules we’re talking about.

  • 听一段天文学课的片段。

    Professor: 土星环通常让天文学家们很迷惑。直到30年前,我们认为这些环是土星形成后剩下的冰和岩石的颗粒组成的,这些剩余材料没能联合成一个卫星。你们知道的,所有太阳系的行星包括土星都是在48亿年前由气旋而形成的。然而,如果这些环是由这个过程中的残留物所形成,那么他们应该有48亿年了。问题是它们吸收宇宙尘埃这么多年现在应该很黑。但是大部分的土星环,还都很新,非常明亮,让土星成为“太阳系的珠宝”。

    所以有一个假设是这些环是在行星形成的时候剩下的物质组成的。这个假设肯定是错的。土星环的物质要比土星本身年轻很多。他们可能在几亿年前才形成,在最早期的恐龙生活在地球的时代。我们发现这些环中许多的颗粒,从微尘那么小到比房子还大的巨石那么大。很多这样的粒子最终都消失了。我们认为它们渐渐地螺旋下降出了土星环进入土星的大气层。这是因为土星重力的吸引。并且它的磁场也起了作用。现在,如果土星环中的材料失去了,并且没有新材料不断的补充,这些环就会消失。

    但这并未发生。所以出于某种原因,肯定有新材料在供养环体系。问题是,这些材料来自哪里呢?所以我们又要从头讲起。但是,与其问土星环在以前是怎么形成的,我们应该问... 有人知道吗?贝斯?

    Student: 环是怎么形成的?Professor: 环是怎么形成的!因为很明显他们在自我补给。

    好的,这里有一种可能性。卫星,几十个卫星,它们都环绕着土星,给土星环提供了原料。卫星在土星系统中的情形很复杂,已知的土星卫星至少有49个,大小和形状各异。卫星在这个复杂的系统中,不仅被土星的重力影响,而且还被其他卫星的重力所影响。

    Student: 所以行星把这个卫星拉向一个方向,而其他的卫星把它拉向另一个方向?Professor:完全正确。这种力实际上能够改变卫星的轨道,可能引起撞击,卫星就会撞到其他卫星。这些碎片就会变成环的一部分。还有潮汐力卫星距行星太近的话就会被土星的潮汐力破坏分解。

    Student:打扰一下,你的意思是,潮汐力就是海洋的涨潮和落潮吗?Professor: 这里的潮汐力,我指的是土星对其卫星的吸引力。19世纪中期,法国科学家爱德华洛希研究了行星潮汐力对其卫星的作用。

    洛希计算出了,如果一个天体,比如卫星,与另一个天体,比如行星离得太近了,另一个天体的吸力更加强大,比卫星本身维持自身的力更大。那么,这个卫星就会被撕裂。我们把能够发生这个现象的距离叫做洛希极限。所以说,如果土星的卫星到达了行星或者更大的卫星的洛希极限,它就会被破坏分解,给环系统补充更多的材料。

    还有一种增加土星环系统的方法,就是小行星撞击某一个卫星。这个猜想来自一个事实,土星环是有些发红的。乔治,有问题吗?

    Student: 不好意思,我不是很明白。Professor: 这些红色表明了有一些复杂有机分子的存在,碳基分子,与水冰混合在一起。记得吗,土星环中其他部分几乎都是水冰组成的。并且卫星都不是红色的。但是小行星可以是红色的。因此可以补给我们讲的碳基分子到环系统中。

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    题干分析:这篇讲座的主要目的是什么?

    原文定位:

    MALE PROFESSOR:But, instead of asking, “How did the rings form?” We should be asking… uh, anyone? Beth?

    FEMALE STUDENT:How do the rings form? 

    选项分析:很明显,文章从开头就引入土星环的形成,贯穿全文的是rings形成的多个假设(hypothesis),对应的正确选项是B

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