GRE阅读Passage 20
In 1995 the Galileo spacecraft captured data about Jupiter’s atmosphere—namely, the absence of most of the predicted atmospheric water—that challenged prevailing theories about Jupiter’s structure. The unexpectedness of this finding fits a larger pattern in which theories about planetary composition and dynamics have failed to predict the realities discovered through space exploration. Instead of normal planets whose composition could be predicted by theory, the planets populating our solar system are unique individuals whose chemical and tectonic identities were created through numerous contingent events. One implication of this is that although the universe undoubtedly holds other planetary systems, the duplication of the sequence that produced our solar system and the development of life on Earth is highly unlikely. Recently planetary scientists have suggested that the external preconditions for the development of Earth’s biosphere probably included four paramount contingencies. First, a climate conducive to life on Earth depends upon the extraordinarily narrow orbital parameters that define a continuously habitable zone where water can exist in a liquid state. If Earth’s orbit were only 5 percent smaller than it is, temperatures during the early stages of Earth’s history would have been high enough to vaporize the oceans. If the Earth-Sun distance were as little as 1 percent larger, runaway glaciation on Earth about 2 billion years ago would have caused the oceans to freeze and remain frozen to this day. Second, Jupiter’s enormous mass prevents most Sun-bound comets from penetrating the inner solar system. It has been estimated that without this shield, Earth would have experienced bombardment by comet-sized impactors a thousand times more frequently than has actually been recorded during geological time. Even if Earth’s surface were not actually sterilized by this bombardment, it is unlikely that any but the most primitive life- forms could have survived. This suggests that only planetary systems containing both terrestrial planets like Earth and gas giants like Jupiter might be capable of sustaining complex life-forms. Third, the gravitational shield of the giant outer planets, while highly efficient, must occasionally fail to protect Earth. Paradoxically, while the temperatures required for liquid water exist only in the inner solar system, the key building blocks of life, including water itself, occur primarily beyond the asteroid belt. Thus the evolution of life has depended on a frequency of cometary impacts sufficient to convey water, as well as carbon and nitrogen, from these distant regions of the solar system to Earth while stopping short of an impact magnitude that would destroy the atmosphere and oceans. Finally, Earth’s unique and massive satellite, the Moon, plays a crucial role in stabilizing the obliquity of Earth’s rotational axis, this obliquity creates the terrestrial seasonality so important to the evolution and diversity of life. Mars, in contrast, has a wildly oscillating tilt and chaotic seasonality, while Venus, rotating slowly backward, has virtually no seasonality at all.文章解析
第一自然段：
In 1995 the Galileo spacecraft captured data about Jupiter’s atmosphere—namely, the absence of most of the predicted atmospheric water—that challenged prevailing theories about Jupiter’s structure. The unexpectedness of this finding fits a larger pattern in which theories about planetary composition and dynamics have failed to predict the realities discovered through space exploration. Instead of normal planets whose composition could be predicted by theory, the planets populating our solar system are unique individuals whose chemical and tectonic identities were created through numerous contingent events. One implication of this is that although the universe undoubtedly holds other planetary systems, the duplication of the sequence that produced our solar system and the development of life on Earth is highly unlikely. 白话版讲解： 1995年，太空飞船“伽利略”号记录到了关于木星大气的数据（原先科学家们预测木星大气中有很多水分，结果“伽利略”号发现其实很少），这就对一些流行的有关木星构造的理论提出了质疑。“伽利略”号的这一发现是出乎科学家预料的（科学家们觉得自己的理论很对啊，怎么可能不对呢），但其实这种事其实不是个例，而是一个pattern。这个patter是什么呢，就是科学家们提出了有关一些行星的构造和发展模式的理论，然后这些预测的理论预测某个行星上会是什么样，结果太空探索发现的实际情况根本不是那个样，这种事经常发生，都成了一个pattern了。科学家们对（太阳系外其他星系）的一些行星的结构也提出了一些理论预测，都预测准了，但是对太阳系内行星就都预测不准。（以至于现在科学家们都服输服气了，承认）太阳系内的这些行星，每一个都是独特的，它们的化学构成特征和地质构造特征都是多个偶然事件导致的。而因为地球也是太阳系内的行星，所以地球的化学构成特征和地质构造特征也是由多个偶然事件导致的。宇宙中还有其星系，也有和太阳系这样的一恒星加多行星系统，但是这些行星系统和太阳系行星系统一样，并也有一个地球一样能让生命存活的行星，这种可能性就非常小了。（太阳系中现在有一个地球这样的能让生命存活的行星，是发生了一连串偶然发生的好事才有的，运气太好了。其他星系不可能有同样的好运气。） 注解： dynamics n.成长、变迁发展模式或过程 a pattern or process of change, growth, or activity population dynamics sequence n. 一组连续事件（或动作，事物） a set of related events, movements, or things that follow each other in a particular order第二自然段前半部分：
Recently planetary scientists have suggested that the external preconditions for the development of Earth’s biosphere probably included four paramount contingencies. First, a climate conducive to life on Earth depends upon the extraordinarily narrow orbital parameters that define a continuously habitable zone where water can exist in a liquid state. If Earth’s orbit were only 5 percent smaller than it is, temperatures during the early stages of Earth’s history would have been high enough to vaporize the oceans. If the Earth-Sun distance were as little as 1 percent larger, runaway glaciation on Earth about 2 billion years ago would have caused the oceans to freeze and remain frozen to this day. 白话版讲解： 最近专门研究行星的科学家已经提出，几乎肯定（probably）需要先发生四个主要的偶然事件（前提条件），才能生出地球上这种适合生命存活的生物域（生物域包括地表和大气层）。第一个偶然事件（前提条件）是轨道（大小）得恰到好处。生命需要适宜的气候，适宜的气候需要地球的轨道（大小）在一个恰当的很窄的范围（parameter）内，大了一点不行，小了一点也不行。如果轨道（半径）小5%，地球历史早期，温度就会非常高，把海洋都给蒸发了（地球上就产生不了生命）；如果地球离太阳距离大1%（即轨道半径大1%），约200万年前迅速发生、不可控制（runaway）的冰河作用就会把海洋全部冻成冰，至今也不会化（地球上就也产生不了生命）。第二自然段后半部分：
Second, Jupiter’s enormous mass prevents most Sun-bound comets from penetrating the inner solar system. It has been estimated that without this shield, Earth would have experienced bombardment by comet-sized impactors a thousand times more frequently than has actually been recorded during geological time. Even if Earth’s surface were not actually sterilized by this bombardment, it is unlikely that any but the most primitive life- forms could have survived. This suggests that only planetary systems containing both terrestrial planets like Earth and gas giants like Jupiter might be capable of sustaining complex life-forms. 白话版讲解： 第二个前提条件是木星的存在。木星体积超大，如果不是它帮着挡着，很多朝着太阳飞的彗星会穿透内太阳系。如果不是木星当大盾牌，地球在历史上被彗星大小的天体撞击的频率会高一千倍；被撞频率提高这么多，即使地球不变成完全无法让生命存活的地方，也会变成只有最原始生命可以存活的地方（只会有些单细胞能存活，其他鸟兽虫鱼都无法存活）。要出现能养活复杂的生命形式生存的环境条件，要求一个星系里既要有像地球这样的岩石行星，还要有像木星这样的气态巨行星（帮岩石行星挡彗星）。 注解： terrestrial planets 岩石行星（也叫类地行星） gas giants 气态巨行星（也叫类木行星）第三自然段：
Third, the gravitational shield of the giant outer planets, while highly efficient, must occasionally fail to protect Earth. Paradoxically, while the temperatures required for liquid water exist only in the inner solar system, the key building blocks of life, including water itself, occur primarily beyond the asteroid belt. Thus the evolution of life has depended on a frequency of cometary impacts sufficient to convey water, as well as carbon and nitrogen, from these distant regions of the solar system to Earth while stopping short of an impact magnitude that would destroy the atmosphere and oceans. 白话版讲解： 第三，巨行星的引力盾必须非常高效地挡住彗星，同时偶尔还要放过一两颗彗星，让它们撞到地球上。（看到the gravitational shield，我们明白，原来木星那样的气态巨行星，是用引力，把本来路过的彗星吸引到自己身体上去。）为什么要偶尔放过一两颗彗星呢？因为内太阳系虽然有让水保持液态的温度条件（这个温度条件，外太阳系是没有的），但在内太阳系，构成生命的一些基本分子成分（building blocks）却无法occur。水就是构成生命的一种基本分子成分。水、碳、氮等构成生命的基本分子成分，都是只能先在小行星带以外遥远的地带（这里我们要推测“在小行星带以外”就是在外太阳系的彗星上）产生，然后随着彗星往地球方向撞（大部分被木星挡住，然后又不能全部被挡住，少数几颗躲过木星的阻挡），被送到地球上来。而且撞的猛烈程度还要恰到好处，不能太猛，撞得太猛了会把大气层和海洋毁掉。第四自然段：
Finally, Earth’s unique and massive satellite, the Moon, plays a crucial role in stabilizing the obliquity of Earth’s rotational axis, this obliquity creates the terrestrial seasonality so important to the evolution and diversity of life. Mars, in contrast, has a wildly oscillating tilt and chaotic seasonality, while Venus, rotating slowly backward, has virtually no seasonality at all. 白话版讲解： 地球上的生命要进化，并且能发展出那么多的物种，必须要陆地上有一年四季才行；要有一年四季，自转轴必须保持一定的倾斜角度；要保持这个角度，必须要有个独一无二的大卫星（月球），月球在这个事上起了至关重要的作用。火星因为没有自己的大卫星，自转轴的倾斜角度波动变化很厉害，不够稳定，导致火星上的季节变化一片混乱；金星因为没有自己的大卫星，自转的方向和其他行星相反且转得太慢，导致金星上几乎没有季节变化。GRE阅读题目解析
1. The passage is primarily concerned with A. enumerating conditions that may have been necessary for a particular development. B. outlining the conditions under which scientists may be able to predict certain events. C. explaining how a particular finding affected scientists understanding of a phenomenon. D. suggesting reasons why a particular outcome was more likely to occur than other possible outcomes. E. assessing the relative significance of factors that contributed to a particular occurrence. 本题答案：A 白话版讲解： 问文章主要是关于什么的？实际是问文章的修辞功能。 文章是列举了a particular development要发生所必须的四个前提条件。所以A选项正确。 B “概述了一些条件。没有这些条件，科学家就无法预测一些事情。”文章没有提到，所以B选项错误。 C“解释一个发现是怎样影响科学家们对一个现象的理解的”。文章开头讲了这个，但是整个文章primarily不是关于这个，后面主要是讲别的内容。所以C选项错误。 D“关于为什么一个后果会比其他后果更可能发生，说了好几个原因”，文章没有提到，所以D选项错误。 E“评估对某事发生起作用的几个factor的relative重要性。” relative的意思是“相关的；比较的，互比的”（considered in relation or in proportion to something else），“relative”重要性，就是说文章比较了一下哪个factor相对更重要一点，哪个factor相对不重要一点。但是文章并没有比较这个，所以E选项错误。 2. It can be inferred from the passage that the planetary scientists would be most likely to agree with which of the following statements concerning the development of complex life forms on Earth? A. It might have occurred earlier in Earth’s history if cometary impacts had been less frequent than they were. B. It could have occurred if Earth’s orbit were 1 percent larger than it is but not if Earth’s orbit were 5 percent smaller. C. It probably follows a pattern common on other terrestrial planets that occupy planetary systems containing gas giants. D. Its dependence on the effect that Jupiter’s gravitational shield has on Earth was difficult to recognize prior to 1995. E. It has been contingent on conditions elsewhere in Earth’s solar system as well as on conditions on Earth itself. 本题答案：E 白话版讲解： 问下列关于地球上发展出复杂生命的说法，文章中的行星科学家最可能会赞同哪一个？ A“如果彗星撞击地球的频率更低一些的话，复杂生命可以更早在地球上出现。”根据文章，撞击频率是不高不低刚刚好，所以A选项错误。 B“地球自转轨道（半径）大百分之一，仍然可以出现；小百分之五，就不可能出现。”根据文章第二自然段前半部分，应该是大百分之一也不行。所以B选项错误。 C“复杂生命在地球上的出现和发展，有一个pattern。凡是在包含气态巨行星的行星系统中的其他岩石行星，都遵循这个common pattern，地球也遵循这个pattern。”根据文章，地球独有一个月球，才能有生命，月球是地球独有的，所以C选项错误。 文章没有提到在1995年之前是不是很难认识到木星对地球的盾牌保护作用。所以D选项错误。 E“复杂生命在地球上的出现和发展，取决于太阳系地球外的一些条件，也取决于地球自身的一些条件。”首先请注意这个选项里的contingent的意思是“视情况而发生的”（dependent on or conditioned by something else），一般和介词on搭配。 例句：Payment is contingent on fulfillment of certain conditions. 满足一些条件，才付款。（付款不付款，取决于一些条件得不得到满足。） 而文章第三句话出现的“contingent events”中的contingent，意思是“偶然发生的”（happening by chance or unforeseen causes ）。有没有木星属于elsewhere的条件，地球上有没有液态水、有没有碳、氮等元素，属于地球本地的条件。所以E选项正确。 3. The author of the passage most likely mentions Mars oscillating tilt primarily in order to A. provide evidence for a proposition about the potential effects of cometary impacts. B. emphasize the absence from our solar system of normal planets. C. contrast the rotational axis of Mars with that of Venus. D. characterize the role of other planets in the solar system in earth’s development. E. emphasize the importance of the Moon to the development of life on Earth. 本题答案：E 白话版讲解： 问文章作者提到火星（自转轴的）倾斜角度波动变化很厉害，是为了……？ 段落中的一句话的修辞功能，往往为了整个段落的修辞功能服务的。文章第四自然段是为了讲地球有个大卫星（月球）这个precondition（同时也是contingent event）。所以在这一段后面提到火星和金星，都是在说火星、金星没卫星，就不行，要么没季节变化，要么季节乱变化。地球有月球，就行，一年四季很有规律适合生命出现。所以是在强调月球的重要性，E选项正确。 A“为一个关于彗星撞击可能产生的效果的观点（主张）提供证据。”如果选A，那应该是说火星被彗星撞出了多大的坑。 B“强调太阳系中没有normal planets”，不是在对比太阳系中没有什么，其他星系中有什么。所以B选项错误。 C“对比火星和金星的自转轴”。如果说是拿它们的自转轴跟地球对比，还算对。 D“描述太阳系中其他行星对地球发展所起的作用”。文章讲木星的部分（第二、第三自然段）算是讲这个的，但是这道题目问的是讲火星的部分，所以D选项错误。 4. The passage suggests each of the following about water on Earth EXCEPT: A. It was conveyed to Earth by comets. B. It appeared on Earth earlier than did carbon and nitrogen. C. Its existence in a liquid state is contingent on Earth’s orbital parameters. D. Much of it came from a part of the solar system where water cannot exist in a liquid state. E. It is unlikely that there would be much of it available to support life if the gravitational shield of the outer planets did not limit the frequency with which comets strike Earth. 本题答案：B 白话版讲解： 下列选项关于地球上的水的说法，只有一项是文章没有提到的，问是哪一个？ A“水是彗星运到地球上来的”符合文章第三自然段内容，所以A选项不选。 B“在地球上，水先出现，碳、氮后出现。”文章没有提到水和碳、氮哪个先出现哪个后出现。所以B选项正确。 C“水在地球上能不能以液态形式存在，取决于地球轨道的范围（大小）”符合文章第二自然段前半部分内容，所以C选项不选。 D“大部分地球上的水来自太阳系中水无法以液态形式存在的区域”，符合文章第三自然段内容，所以D选项不选。 E“如果外太阳系的行星不以重力盾来限制彗星撞地球的频率，地球上的水就不会有多少，就不足以支持生命出现。”符合文章第二自然段后半部分内容，所以E选项不选。文章作者
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