Official 38 Set 6

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The Formation of Gas Planets

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What is the main purpose of the lecture?
  • A. To describe two new theories about the formation of rocky planets

  • B. To discuss competing theories about the formation of gas planets

  • C. To compare the composition of Jupiter, Saturn, Uranus, and Neptune

  • D. To explain why young stars are often surrounded by disks of gas and dust

显示答案 正确答案: B

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

    MALE PROFESSOR:Last week we discussed the formation of Earth and the other rocky planets-the planets in the inner solar system.Uh, s-so what about the gas giants: Jupiter, Saturn, Uranus, and Neptune?Well, there's two theories, but first let's recap.We believe our solar system began as a huge spinning cloud of dust and gas, which flattened and eventually collapsed in on itself.The matter at the center condensed into a ball of hot gas and dust, eventually becoming our Sun.And... what happened to the remaining cloud, to the disk encircling the Sun when it was a young star?

    FEMALE STUDENT:The rocky planets were born. Um, dust- little grains of rock and metal within the disk collided with each other and stuck together.And this process sorta snowballed over millions of years... until the chunks grew into mini-planets-protoplanets.

    MALE PROFESSOR:Yes. Uh, this process is called accretion, and we call the disk an accretion disk.

    [Explaining] Now... think of it as two parts- an inner accretion disk and an outer accretion disk.In the inner part, once an object gets large enough, that object's gravitational field gets stronger, which speeds up the accretion process.You know, larger objects attract smaller ones and sorta gobble them up. And eventually you get a full-size planet in its own orbit.OK... That's how the inner rocky planets probably formed-by accretion. But what about those gas planets in the outer solar system, in the outer accretion disk?

    Well, the first theory says the accretion process was similar to the one that formed the rocky planets, with some key differences.Remember the gas giants are farther from the Sun, where temperatures are much colder.So, in the outer accretion disk, compounds like water and ammonia exist in frozen form.Closer to the Sun, they're more likely to be vaporized by solar radiation.What this means is that, in addition to rocky and metallic particles, there's be other solids, like frozen water and frozen ammonia.

    FEMALE STUDENT:[Following on]...so more solid substances are available to clump into protoplanets, right?

    MALE PROFESSOR:Precisely. So the solid cores of the gas giants could conceivably have formed by accretion.And once their mass reaches a certain point- around about five to ten Earths- what would happen?

    FEMALE STUDENT:Five to ten Earths? With a mass that big, I guess gravity would start to pull in more and more material faster, right?MALE PROFESSOR:Material, meaning gas. It would rapidly pull in more and more gas from the accretion disk.So, you end up with a solid core of rock, metal, and ice surrounded by massive amounts of gas. That's the core-accretion theory.Now-the other theory's called the disk-instability theory.

    The disk-instability theory holds that gas begins the planet-making process without a solid core.You see, most of the outer accretion disk would've been gas; we believe solid particles probably made up just 1 percent of the outer accretion disk.So, this theory suggests that the large planets-the gas giants- they develop from large clumps of mostly gas and some dust in an accretion disk.Um, outer regions of an accretion disk can be unstable, uh, gravitationally unstable, which is what causes these clumps to form and, in some cases, grow into protoplanets.Over time, dust particles within a gas clump coalesce-bond together-and eventually fall toward the center, creating a core.Once this happens, the gas clump grows relatively quickly, as its gravity pulls in more and more gas and dust particles.And this whole process can theoretically happen within 100,000 years.

    FEMALE STUDENT:That's amazingly fast! So, which theory's correct?

    MALE PROFESSOR:That's the debate. Most of my colleagues favor core accretion.Personally, I think the accretion theory works for the formation of rocky planets, but not necessarily for gas planets.A major problem is that gas giants like Jupiter and Saturn would take too long to form through core accretion.Core accretion would take several million years. But observations of other star systems indicate that a disk's gas disappears more quickly than that.Whatever's not drawn into planets ends up dissipated and evaporated by solar wind and radiation, from nearby stars.So basically, a baby Jupiter would run out of gas before it grew up.But the disk-instability theory... well, the timing's right. That process is fast enough to finish before the gas runs out.

  • 旁白:请听一段天文学讲座的节选片段。

    教授:上一周我们讨论了地球以及其他岩态行星的形成,这些行星位于太阳系内部。哦,那么,气态行星是怎么形成的呢?比如说木星、土星、天王星和海王星。现在有两种理论。但是首先,我们先简要回顾一下上节课的内容。我们相信太阳系始于一大团旋转的尘土和气体的云团,这一团云团最终变平并且自己瓦解。它的物质浓缩成了一个热气和尘土构成的球,最终变成了太阳。那么剩下的云层又变成了什么呢?在太阳还是一颗新生恒星的时候,作为环绕它的气体带?

    学生:岩态行星就形成了。嗯,灰尘、小的岩石碎片和气体带里的金属物质互相碰撞,彼此聚合,这个过程就类似于一场百万年的滚雪球,直到这些碎片变成了小行星和原行星。

    教授:是的。这个过程被称为吸积。我们把这个环叫做吸积盘。

    我们把它分成两个部分来考虑:内部的吸积盘和外部的吸积盘。在内部的吸积盘里,一旦一个物体变得足够大,它的重力就会变得更大,这就加速了吸积的过程。你知道,大的物体会吸引小的物体,把它们都“吞吃”掉。最终,我们就得到了一个有自己轨道的足尺的行星。好的。这就是内部的岩态行星怎么通过吸积作用形成。但是那些位于太阳系外部、处于外部吸积盘内的气态行星,是怎么形成的呢?

    第一个理论认为,形成气态行星的吸积过程与形成岩态行星的吸积过程相似,但是有几个关键的不同。记得吗,气态行星距离太阳更远,在它们所处的位置,温度是更低的。所以,在外部的吸积盘里,水和氨气的混合物是以冰冻状态存在的。离太阳越近,这种化合物就更有可能在太阳辐射下蒸发成气体。这就意味着除了岩石和金属颗粒,还有其他的固体比如说冰和固态氨气。

    学生:所以越来越多的固体物质可以聚成原行星,对吗?

    教授:很准确!所以可以想象的是气态行星的固态核心可以通过吸积作用形成。一旦它们的质量达到了地球的5到10倍,什么事情会发生呢?

    学生:地球质量的5到10倍的话……嗯,如果质量如此之大的话,我猜重力会越来越快地把其他物质吸引过来,对吗?教授:物质,也就是气体。这个巨大的行星会迅速地从吸积盘里吸引越来越多的气体。所以我们就得到了一个由岩石、金属和冰组成的核,这个核由大量的气体包围着。这就是核吸积理论。第二个理论被称为吸积盘无用论理论。

    吸积盘无用论理论认为,气体开始了行星形成的过程,这时候并没有一个固态的核心。你看,大多数的外部吸积盘的构成都是气体。我们相信,固态粒子可能只占外部吸积盘的百分之一。所以这个理论认为,大的行星,气态大行星,嗯,它们是形成于大团的吸积盘的气体和灰尘。吸积盘的外部区域是不稳定的,在重力上是不稳定的,这就导致了大团气体的形成,在某些情况下,这就导致了原行星的形成。经过较长时间,大团气体里的尘埃粒子聚合在了一起,最终向中心积聚,形成了一个核。这个过程一旦发生,大团气体就变大地非常快,因为它的重力会吸引来越来越多的气体和尘土粒子。这整个过程在理论上可以在十万年之间发生。

    学生:真是令人吃惊地快啊!那么哪个理论是正确的呢?

    教授:这就是争论所在。我的同事们多数支持核吸积理论。我个人认为核吸积理论适用于岩石星球的形成,但是对气体星球不一定成立。一个大问题在于巨大的气体星球,像木星和土星,如果通过核吸积的话耗费的时间就太长了。核吸积可能会耗费数百年的时间。但是对其他行星系统的观测证明盘状气体会更快消失。对于任何没有被吸引到星球上的东西都会被周围星系的太阳风、辐射分散和蒸发。因此,基本上一个在木星形成之前就会耗尽所有的气体。但是盘吸积理论,如果时间是对的话,这个过程会在气体耗尽之前就完成。

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    题型分类:主旨题

    音频定位:Last week we discussed the formation of Earth and the other rocky planets—the planets in the inner solar system. Uh, s-so what about the gas giants: Jupiter, Saturn, Uranus, and Neptune? Well, there’s two theories, but first let’s recap.

    选项分析:有last week和so要提高警惕,接下来讲gas giants形成的两种理论。

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The Formation of Gas Planets

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