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OFFICIAL41 Why does the author mention "present-day volcanoes on Earth"?

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Climate Of Venus
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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 clouds. 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.

2.Why does the author mention "present-day volcanoes on Earth"?

你的答案:
正确答案:C
题目解析:
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【题目翻译】为什么作者提到“地球上的现今火山”? A:举例说明今天的金星和现在的地球之间的一个重要区别。 B:有助于解释为什么地球大气中仍然含有二氧化硫,而金星却没有。 C:表示一个有关金星和地球原始大气成分的信息来源。 D:考虑到地球的水供应不再主要来自撞击彗星这一事实。 【判定题型】:根据题干问法“why …… mention”,题目询问“xxxxxx”(文章引用内容)的修辞目的,可以判断本题为修辞目的题。 【关键词定位】根据关键词“present-day volcanoes”,定位到Passage 1 第5句,原句为“These gases should therefore have been important parts of the original atmospheres of both Venus and Earth.”。 【引用内容分析】作者提及"present-day volcanoes on Earth"是为了说明金星和地球原始的大气组成中的sources。 【选项分析】 A.不是要说明区别,是要说明大气成分。故错误。 B.不单单是二氧化硫,而是大气成分。故错误。 C.选项就是前文说的“water vapor, carbon dioxide, and sulfur dioxide”,所以C选项正确。 D.主题不对,与水供应无关。故错误。

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