Official 41 Passage 2

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Climate of Venus

纠错

According to paragraph 5, what happens when temperatures rise above 374ºC?

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  • A
    Atmospheric pressure begins to decrease.
  • B
    Water vapor disappears from the atmosphere.
  • C
    Water evaporates regardless of atmospheric pressure.
  • D
    More energy is required to evaporate a given volume of water.
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正确答案: C

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  • Earth has abundant water in its oceans but very little carbon dioxide in its relatively thin atmosphere. By contrast, Venus is very dry and its thick atmosphere is mostly carbon dioxide. The original atmospheres of both Venus and Earth were derived at least in part from gases spewed forth, or outgassed, by volcanoes. The gases that emanate from present-day volcanoes on Earth, such as Mount Saint Helens, are predominantly water vapor, carbon dioxide, and sulfur dioxide. These gases should therefore have been important parts of the original atmospheres of both Venus and Earth. Much of the water on both planets is also thought to have come from impacts from comets, icy bodies formed in the outer solar system.



    In fact, water probably once dominated the Venusian atmosphere. Venus and Earth are similar in size and mass, so Venusian volcanoes may well have outgassed as much water vapor as on Earth, and both planets would have had about the same number of comets strike their surfaces. Studies of how stars evolve suggest that the early Sun was only about 70 percent as luminous as it is now, so the temperature in Venus' early atmosphere must have been quite a bit lower. Thus water vapor would have been able to liquefy and form oceans on Venus. But if water vapor and carbon dioxide were once so common in the atmospheres of both Earth and Venus, what became of Earth's carbon dioxide? And what happened to the water on Venus?



    The answer to the first question is that carbon dioxide is still found in abundance on Earth, but now, instead of being in the form of atmospheric carbon dioxide, it is either dissolved in the oceans or chemically bound into carbonate rocks, such as the limestone and marble that formed in the oceans. If Earth became as hot as Venus, much of its carbon dioxide would be boiled out of the oceans and baked out of the crust. Our planet would soon develop a thick, oppressive carbon dioxide atmosphere much like that of Venus.



    To answer the question about Venus' lack of water, we must return to the early history of the planet. Just as on present-day Earth, the oceans of Venus limited the amount of atmospheric carbon dioxide by dissolving it in the oceans and binding it up in carbonate rocks. But being closer to the Sun than Earth is, enough of the liquid water on Venus would have vaporized to create a thick cover of water vapor cIouds. Since water vapor is a greenhouse gas, this humid atmosphere, perhaps denser than Earth's present-day atmosphere, but far less dense than the atmosphere that envelops Venus today would have efficiently trapped heat from the Sun. At first, this would have had little effect on the oceans of Venus. Although the temperature would have climbed above 100° C, the boiling point of water at sea level on Earth, the added atmospheric pressure from water vapor would have kept the water in Venus' oceans in the liquid state.



    This hot and humid state of affairs may have persisted for several hundred million years. But as the Sun's energy output slowly increased over time, the temperature at the surface would eventually have risen above 374°C. Above this temperature, no matter what the atmospheric pressure, Venus' oceans would have begun to evaporate, and the added water vapor in the atmosphere would have increased the greenhouse effect.This would have made the temperature even higher and caused the oceans to evaporate faster, producing more water vapor. That, in turn, would have further intensified the greenhouse effect and made the temperature climb higher still.



    Once Venus' oceans disappeared, so did the mechanism for removing carbon dioxide from the atmosphere. With no oceans to dissolve it, outgassed carbon dioxide began to accumulate in the atmosphere, intensifying the greenhouse effect even more. Temperatures eventually became high enough to "bake out" any carbon dioxide that was trapped in carbonate rocks. This liberated carbon dioxide formed the thick atmosphere of present-day Venus. Over time, the rising temperatures would have leveled off, solar ultraviolet radiation having broken down atmospheric water vapor molecules into hydrogen and oxygen, With all the water vapor gone, the greenhouse effect would no longer have accelerated.


  • 地球在海洋中有丰富的水,但在稀薄的大气中含有极少的二氧化碳。相比之下,金星是非常干燥的,厚的大气层中大部分都是二氧化碳。金星和地球的大气层至少部分是起源于火山喷出或除去的气体。从现今地球上火山散发出来的气体,如圣海伦斯火山,主要是水蒸气、二氧化碳和二氧化硫。因此,这些气体应该是金星和地球原始环境中的重要部分。这两个行星上的大部分水也被认为是来自彗星影响,它是在太阳系外形成的冰体。

    事实上,水可能一度在金星的大气层中大量存在。金星和地球有着相似的大小和质量,所以金星火山很可能和地球火山排出一样多的水蒸汽,大约有相同数量的彗星撞击这两颗行星。恒星演化的研究表明,早期的太阳的亮度只有它现在的70%,所以早期金星的大气中的温度必定比现在低一点。因此,水蒸气将能够液化并在金星上形成海洋。但是,如果水蒸气和二氧化碳在地球和金星的大气中曾经如此普遍,地球的二氧化碳又变成了什么?金星上的水又发生了什么?

    第一个问题的答案是,在地球上的二氧化碳仍然存在,但现在,并不是以大气中的二氧化碳的形式存在,它不是溶解在海洋,就是化学性地存在于碳酸盐岩,如在海洋中形成的石灰石和大理石。如果地球变得像金星一样热,那么它的大部分二氧化碳就会从海洋中沸腾,并从地売中烘烤出来。我们的星球很快就会发展出一种厚厚的、沉重的二氧化碳大气层,就像金星一样。

    要回答关于金星缺乏水的问题,我们必须回归到地球的早期历史。正如在现今的地球上,金星的海洋通过将二氧化碳溶解在海洋中或者结合在碳酸盐岩中,限制了二氧化碳的数量。但比起地球,金星更接近太阳,金星上足够的液态水会蒸发创建厚的水蒸气云层。由于水蒸气是一种温室气体,这潮湿的气层(也许密度比现今地球的气层大,但比现在的金星要小得多)将有效地困住自太阳的热量。起初,这对金星的海洋影响不大。尽管温度会上升到100°C(地球海平面上的水的沸点),来自增加的水蒸气层的压力使金星的海洋保持液体的状态。

    这个炎热潮湿的天气可能已经持续了好几百年了。但随着时间的流逝,太阳的能量输出慢慢地增大,表面上的温度最终会上升到374°C以上的温度。在这个温度以上,无论大气压是多少,金星的海洋会开始黑发,大气中的水黑气会增加温室效应。这将使温度更高,使海洋蒸发得更快,产生更多的水蒸气。这反过来又会进一步加剧温室效应,使温度上升到更高。

    一旦金星的海洋消失了,那么从大气中清除二氧化碳的机制也消失了。没有海洋溶解二氧化碳,排出的二氧化碳开始积聚在大气中,进一步加剧温室效应。最终,温度变得足够高去“烤出”任何被困在碳酸盐岩中的二氧化碳。这释放了构成当今金星浓厚的大气层的二氧化碳。随着时间的推移,温度升高会呈现平稳状态,太阳紫外线辐射会将大气中的水蒸气分子分解成氢和氧。所有的水蒸气都消失了,温室效应将不再加速。
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    题型分类:事实信息题

    题干分析:关键词:374ºC

    选项分析:根据题干关键词,定位到线索句Above this temperature, no matter whatbegun to evaporate”。这句话提供给我们的信息是,若温度超过374摄氏度,那么无论气压强度如何,金星上的海洋都会开始蒸发。选项C是这句话的简化改写。

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