GMAT阅读机经整理：月球形成的三种学说.

　　4月5日GMAT换库后，小编为大家收集整理4月份的GMAT阅读机经，这篇是关于月球形成的三种学说的文章，分享给大家，希望对大家有所帮助，仅供参考。

　　一、本月原始

　　月亮是怎么形成的。先提出三种理论：1. Moon formed from a part of the earth 2. The moon and the earth formed independently of each other, at the same time 3.The moon was‘captured’ by the earth’s gravitational pull。作者提出一些证据，说明这三种理论都是不太可能的。最后一段说了第四种理论：月亮是什么东西和地球碰撞后产生的，并且提供证据，说明这个最有可能

　　二、主旨

　　三种学说。拿出证据全部否定(全是细节题)。新的学说。

　　三、文章结构

　　« 文章(旧的假说—混合反驳—新的解释)

　　四、 段落大意

　　P1：早在Apollo登月之前，就对月球的形成有三种学说：

　　(1)分裂说fission：地球甩出去的

　　(2)同源说concurrence：和地球一起形成的，此理论成立的话月亮和地球应该有同样的materials

　　(3)俘获说capture：独立形成

　　P2： 登月后采集了样本数据和研究成果，发现三种说法都错：分别给出理由【运动和组成成分的证据】。

　　(1) 首先否定fission，月亮如果是地球分离出去的话，那么地月系统转的速度都要比现在快(in fission model earth and moon have much more spin)。

　　crater应该很厚。但是……：反分裂;

　　现在的地月系统应该转得更快(spin)。但是……：反分裂;(Q2)

　　应该还有很多小的卫星体在地球周围。但是……：反分裂;

　　如果是capture，月亮这个独立的星体应该运动的很慢，才会落入地系，反capture;(Q3)

　　(2)提出一个证据，发现月球与地球的materials不同。如果是concurrence，月球应该具有和地球类似的组成(materials)，但是发现月球上的活泼金属(volatile mental)，比如zinc，lead的含量比地球少很多。反fission、concurrence;

　　(3) 最后提出一个证据，说是月球的core size很小,和地球不像。月球的铁内核(iron)应该跟地球相似，所以应该比现在的更大。因此不可能是独立的小星星，也不可能是concurrence的，否定了capture和concurrence。 (Q4) (Q5)

　　P3：提出新理论：collision学说，合理的解释了一切之前的不合理。

　　(1)活泼金属少：因为地球和另一个宇宙中的物体擦肩而过，温度高而挥发;

　　(2)core小：因为撞偏了;

　　(3)转速：因为有角度撞击，所以变慢了。

　　五、题目

　　Q1. 主旨/结构

　　提出了几个存在问题的理论最后提出了一个较完美的理论。

　　Q2. 如果fission成立的话月球会是什么样子? /if the fission(分裂学说) model is true, the moon that differ the current is 。。(就是如果fission是正确的，月亮会与现在的有什么不同。)

　　A. Spin faster than now

　　B. Spin slower

　　C. 月亮周围会有很多Material.

　　狗主解释：几经上说是spin faster or slower. 当时做题的时候出现了spin slower，但是回文中定位发现in fission model earth and moon have much more spin. 就是说要快，所以这个不对，我选择的是月亮周围会有很多Material.因为定为第一段说fission使地球甩出去很多物质。

　　Q3. 如果俘获论是正确的话，如何加强?

　　Spin slower than now

　　狗主解释：就是找到第二段中的反对意见，改写成否定就好了

　　Q4. 以下哪个为真，会削弱the objection to俘获说capture?(注意!双否定!)

　　对：月球的core其实比科学家预测的更大一些

　　(错)科学家对月球表面元素的分析有错误;

　　(错)月球crater的厚度被低估;

　　Q5. 如果月球是和地球同时形成的，会怎么样?/哪个现象出现在同时形成这个学说中?/关于concurrence，一下那个观点成立可以削弱科学家的结论?

　　the size of moon core 比科学家原来预测的大

　　六、小编分析:

　　如果fission成立：

　　l 现在的地月系统应该转得更快(much more spin)(考点)

　　l crater应该很厚。

　　l 应该还有很多小的卫星体在地球周围

　　l 它应该具有和地球类似的组成(materials)(考点)

　　如果是capture：

　　l 月亮这个独立的星体应该运动的很慢(考点)

　　l 月球的core size比预测更大(考点)

　　如果是concurrence：

　　l 它应该具有和地球类似的组成(materials)

　　l 月球的core size比预测更大(考点)

　　七、备注

　　注意第二段基本上是一对多的反驳，即一个证据同时反对多个理论。

　　新观点没有考点，全是针对错误解释的改善题。

　　During the last two centuries, astronomers developed three different hypotheses for the origin of Earth’s moon, but these traditional ideas have failed to survive comparison with the evidence. A relatively new theory proposed in the 1970s may hold the answer. You can begin by testing the three unsuccessful theories against the evidence to see why they failed.

　　The first of the three traditional theories, the fission hypothesis, supposes that the moon formed by the fission of Earth. If the young Earth spun fast enough, tides raised by the sun might break into two parts. If this separation occurred after Earth differentiated, the moon would have formed from crust material, which would explain the moon’s low density. But the fission theory has problems. No one knows why the young Earth should have spun so fast, nearly ten times faster than today, nor where all that angular momentum went after the fission. In addition, the moon’s orbit is not in the plane of Earth’s equator, as it would be if it had formed by fission.

　　The second traditional theory is the condensation (or double-planet) hypothesis. It supposes that Earth and the moon condensed as a double planet from the same cloud of material. However, if they formed form the same material, they should have the same chemical composition and density, which they don’t. The moon is very poor in certain heavy elements like iron and titanium, and in volatiles such as water vapor and sodium. Yet the moon contains almost exactly the same rations of oxygen isotopes as does Earth’s mantle. The condensation theory cannot explain these compositional differences.

　　The third theory is the capture hypothesis. It supposes that the moon formed somewhere else and was later captured by Earth. If the moon formed inside the orbit of Mercury, the heat would have prevented the condensation of solid metallic grains, and only high-melting-point metal oxides could have solidified. According to the theory, a later encounter with Mercury could have “kicked” the moon out to Earth. The capture theory was never popular because it requires highly unlikely events involving interactions with Mercury and Earth to move the moon from place to place. Scientists are always suspicious of explanations that require a chain of unlikely coincidences. Also, on encountering Earth, the moon would have been moving so rapidly that Earth’s gravity would have unable to capture it without ripping the moon to fragments through tidal forces.

　　Until recently, astronomers were lt with no acceptable theory to explain the origin of the moon, and they occasionally joked that the moon could not exist. But during the 1970s, planetary astronomers developed a new theory that combines the best aspect of the fission hypothesis and the capture hypothesis.

　　The large-impact theory supposes that the moon formed from debris ejected into a disk around Earth by the impact of a large body. The impacting body may have been twice as large as Mars. In fact, instead of saying that Earth was hit by a large body, it may be more nearly correct to say that Earth and the moon resulted from the collision and merger of two very large planetesimals. The resulting large body became Earth, and the ejected debris formed the moon. Such an impact would have melted the proto-Earth, and the material falling together to form the moon would have been heated hot enough to melt. This theory fits well with the evidence from moon rocks that show the moon formed as a sea of magma.

　　This theory would explain other things. The collision must have occurred at a steep angle to eject enough matter to make the moon. The objects could not have collided head-on. A glancing collision would have spun the material rapidly enough to explain the observed angular momentum in the Earth-moon system. And if the two colliding planetesimals had already differentiated, the ejected material would be mostly iron-poor mantles and crust. Calculations show that the iron core of the impacting body could have fallen into the larger body that became Earth. This would explain why the moon is so poor in iron and why the abundances of other elements are so similar to those in Earth’s mantle. Finally, the material that eventually became the moon would have remained in a disk long enough for volatile elements, which the moon lacks, to be lost to space.

　　The moon may be the result of a giant impact. Until recently, astronomers have been reluctant to consider such catastrophic events, but a number of lines of evidence suggest that some planes may have been affected by giant impacts.

　　Question: Where did the Moon come from?

　　The Fission Theory: This theory proposes that the Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. This theory was thought possible since the Moon&aposs composition resembles that of the Earth&aposs mantle and a rapidly spinning Earth could have cast off the Moon from its outer layers. However, the present-day Earth-Moon system should contain "fossil evidence" of this rapid spin and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

　　The Capture Theory: This theory proposes that the Moon was formed somewhere else in the solar system, and was later captured by the gravitational field of the Earth. The Moon&aposs different chemical composition could be explained if it formed elsewhere in the solar system, however, capture into the Moon&aposs present orbit is very improbable. Something would have to slow it down by just the right amount at just the right time, and scientists are reluctant to believe in such "fine tuning". Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

　　The Condensation Theory: This theory proposes that the Moon and the Earth condensed individually from the nebula that formed the solar system, with the Moon formed in orbit around the Earth. However, if the Moon formed in the vicinity of the Earth it should have nearly the same composition. Specifically, it should possess a significant iron core, and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

　　There is one theory which remains to be discussed, and it is widely accepted today.

　　The Giant Impactor Theory (sometimes called The Ejected Ring Theory): This theory proposes that a planetesimal (or small planet) the size of Mars struck the Earth just after the formation of the solar system, ejecting large volumes of heated material from the outer layers of both objects. A disk of orbiting material was formed, and this matter eventually stuck together to form the Moon in orbit around the Earth. This theory can explain why the Moon is made mostly of rock and how the rock was excessively heated. Furthermore, we see evidence in many places in the solar system that such collisions were common late in the formative stages of the solar system. This theory is discussed further below.

　　背景知识：

　　一.分裂说。这是最早解释月球起源的一种假设。月球本来是地球的一部分，后来由于地球转速太快，把地球上一部分物质抛了出去，这些物质脱离地球后形成了月球，而遗留在地球上的大坑，就是现在的太平洋。这一观点很快就收到了一些人的反对。他们认为，以地球的自转速度是无法将那样大的一块东西抛出去的。再说，如果月球是地球抛出去的，那麽二者的物质成分就应该是一致的。可是通过对“阿波罗12号”飞船从月球上带回来的岩石样本进行化验分析，发现二者相差非常远。

　　二.俘获说。这种假设认为，月球本来只是太阳系中的一颗小行星，有一次，因为运行到地球附近，被地球的引力所俘获，从此再也没有离开过地球。还有一种接近俘获说的观点认为，地球不断把进入自己轨道的物质吸积到一起，久而久之，吸积的东西越来越多，最终形成了月球。但也有人指出，向月球这样大的星球，地球恐怕没有那麽大的力量能将它俘获。

　　三.同源说。这一假设认为，地球和月球都是太阳系中浮动的星云nebula，经过旋转和吸积，同时形成星体 celestial body (e.g. planet, satellite, etc.)。在吸积过程中，地球比月球相应要快一点，成为“哥哥”。这一假设也受到了客观存在的挑战。通过对“阿波罗12号”飞船从月球上带回来的岩石样本进行化验分析，人们发现月球要比地球古老得多。有人认为，月球年龄至少应在70亿年左右。

　　四.大碰撞说。这是近年来关于月球成因的新假设。这一假设认为，太阳系演化早期，在星际空间曾形成大量的“星子”planetesimal，星子通过互相碰撞、吸积而长大。星子合并形成一个原始地球，同时也形成了一个相当于地球质量0.14倍的天体。这两个天体在各自演化过程中，分别形成了以铁为主的金属核和由硅酸盐silicate构成的幔和壳。由于这两个天体相距不远，因此相遇的机会就很大。一次偶然的机会，那个小的天体以每秒5千米左右的速度撞向地球。剧烈的碰撞不仅改变了地球的运动状态，使地轴倾斜，而且还使那个小的天体被撞击破裂，硅酸盐壳和幔受热蒸发，膨胀的气体以及大的速度携带大量粉碎了的尘埃飞离地球。这些飞离地球的物质，主要有碰撞体的幔组成，也有少部分地球上的物质，比例大致为0.85:0.15。在撞击体破裂时与幔分离的金属核，因受膨胀飞离的气体所阻而减速，大约在4小时内被吸积到地球上。飞离地球的气体和尘埃，并没有完全脱离地球的引力控制，他们通过相互吸积而结合起来，形成全部熔融的月球，或者是先形成几个分离的小月球，在逐渐吸积形成一个部分熔融meltdown的大月球

　　以上就是关于月球形成的三种学说的GMAT阅读机经的全部内容，考生朋友可以有选择的看看，最后需要提醒各位的是，机经虽然会对我们解题有所帮助，但是在考场中即使题目很像也要避免秒选，最后祝大家都能考出好成绩。