GRE阅读机经 Passage 38
Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, wheretemperatures are close to absolute zero—must be pristine relics of the material that formed the outer planets. The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface. Nevertheless, astronomers still believed that when a comet approached the Sun— where they could study it—the Sun’s intense heat would remove the corrupted surface layer, exposing the interior. About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve.GRE阅读文章解析
第一句话：
Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where temperatures are close to absolute zero—must be pristine relics of the material that formed the outer planets. 白话版讲解： 研究行星构造的天文学家们曾经认为，因为彗星大都位处遥远的“Oort星云”（Oort Cloud），那个地方的温度接近“绝对零度”，所以彗星都肯定是构成太阳系中那几个outer planets的材料的原始的、未受破坏的残留。（意思就是有些材料构成了outer planets，没用完，剩了一些，又构成了彗星，这些彗星一直处于温度极低的“Oort星云”，所以彗星至今肯定保持了那些材料原始的、未受破坏的样子。（因为低温可以保鲜嘛。）这里还有一个言下之意，就是outer planets的位置不在“Oort星云”，而是到了离太阳近一些的地方，所以outer planets就不是pristine relics了。第二句话：
The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface. 白话版讲解： 到了1970年代，一些实验室仿真揭示，传到（处于遥远的“Oort星云”的）那些彗星的紫外线还是够的，这些紫外线足够让那些彗星的表面（因为被腐蚀而）变暗，传到那些彗星的宇宙射线也是够的，足够改变彗星表面物质的化学键和分子结构。于是，从1970年代开始，大家开始觉得“彗星至今肯定保持了那些材料原始的、未受破坏的样子”这个观点不对了。（业界认为彗星不是pristine relics）第三句话：
Nevertheless, astronomers still believed that when a comet approached the Sun— where they could study it—the Sun’s intense heat would remove the corrupted surface layer, exposing the interior. 白话版讲解： 不过，天文学家们仍然认为，彗星（在其轨道上）运行到接近太阳的位置时（彗星处于这个位置时，天文学家才可以仔细观察彗星），太阳照射带来的密集热量会把彗星被腐蚀的表面给熔化掉，彗星内部的原始物质又会露出来。（业界认为彗星还是pristine relics） 注解： study vt. 仔细观察或阅读 look at closely in order to observe or read第四句话：
About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve. 白话版讲解： 但是，科学家们又认识到，彗星（自身）可能包含一直在衰变（放出放射性射线）的一些同位素，这些同位素会使彗星内部温度升高到引起彗星内部发生演化的程度。（业界又认为彗星可能不是pristine relics） 注解： ●at the same time 仍然、不过 nevertheless, yet ●decay vi.（放射性物质、粒子等）衰变 (of a radioactive substance, particle, etc.) undergo change to a different form by emitting radiationGRE阅读题目解析
Consider each of the choices separately and select all that apply. 1. According to the passage, astronomers recognize which of the following as being liable to cause changes to comets? A. cosmic rays B. radioactive decay C. ultraviolet radiation白话版讲解：
问根据文章，天文学家们认为下列哪几个因素有可能会引起彗星发生改变？ 这道题有人想选AC，其理由是题干问的是astronomers，而第四句话中用的词是scientists，scientists应该是和astronomers不同的另一波人。 实际上，全文中的astronomers和scientists，都是在泛指研究行星构成的这个圈子里的大部分科学家。全文是在讲这整个圈子的看法的多重（多层次）转变，而不是在讲持有不同观点的两波人在debate。所以ABC都正确。 这里我们还要复习一个语法知识，可数名词复数前面不加定冠词，表示泛指、比较普遍的情况。 几个例句如下： ◆Cats are great pets! ◆You’re not talking about one specific cat or one specific pet. You’re talking about all cats and all pets in general. ◆I love reading books. ◆Women love it when men send them flowers! ◆Houses are expensive in that neighbourhood. ◆Americans drive big cars. 本题答案：ABC Consider each of the choices separately and select all that apply. 2. According to the passage, astronomers’ belief that comets are pristine relics was A. overturned by analysis of what happens when comets approach the Sun. B. supported by what observations revealed about the composition of the outer planets. C. based on consideration of the conditions that prevail where comets are located.白话版讲解：
问根据文章，天文学家所持有的“彗星是pristine relics”的这个观点……？ 根据文章第三句话，A选项的内容是支持“彗星是pristine relics”这个观点的，所以A选项中的“overturn”说反了。 文章没有提到the composition of the outer planets，所以B选项错误 根据文章第一句话，C选项正确，C选项中的“the conditions that prevail”就是指第一句话中的“temperatures are close to absolute zero”。 本题答案：C 3. The author suggests that the realization described in the final sentence of the passage had which of the following effects? A. It introduced a new topic for study by astronomers interested in planetary formation. B. It led astronomers to adopt a number of different strategies in trying to determine the composition of cometary interiors. C. It called into question an assumption that astronomers had made about comets. D. It cast doubt on astronomers’ ability to study the interior parts of comets. E. It caused astronomers to revise their account of the composition of the outer planets.白话版讲解：
问作者暗示，文章最后一句话中提到，“科学家们认识到……”这个事，有什么效果？ 文章提到，天文学家们之前认为彗星的位置在很远很冷的地方，阳光和宇宙射线够不到这么远的地方，所以就改变不了这些彗星。这说明科学家们实际上有一个前提假设（assumption），就是改变这些彗星，得靠外来的阳光和宇宙射线；内部的因素不会让彗星发生改变。而文章最后一句话，提到，科学家们认识到，彗星内部存在会衰变的同位素（衰变时会释放出射线），这些射线会从内部改变彗星。这就相当于质疑了“只有外部射线才会让彗星发生改变”这个前提假设（assumption）。所以C选项正确。 Consider each of the choices separately and select all that apply. 4. It can be inferred that the author would agree with which of the following statements about the “laboratory simulations”? A. The simulations showed that despite the low temperatures in the Oort cloud, there was sufficient energy there to alter comets. B. Astronomers were initially reluctant to accept what simulations showed about the composition of comets. C. The simulations themselves did not eliminate the possibility that comets contain pristine relics of material from the early solar system.白话版讲解：
问作者会同意下列哪个关于“实验室仿真”的说法？ A“仿真发现，尽管在Oort星云那块地方，温度很低，但是那块地方还是有足够的能量使彗星发生改变” B“起初天文学家们不愿接受仿真发现的关于彗星构成的情况” C“仿真并没有排除彗星包含有pristine relics的可能性” 根据文章第二句话，A选项和C选项正确。因为仿真只是说彗星会改变，并没有说彗星改变得一点relics都没有了，所以C选项正确。文章没有提到“天文学家起初不愿接受”，所以B选项错误。 本题答案：AC文章作者
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