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.
题干分析:关键词：Early Venus，Present-day Venus
A选项，关键词在于水汽，可定位到第二段。第二段开篇就告诉了我们“water probably once dominated the Venusian atmosphere”。因此A符合对早期金星的描述；
B选项，关键词是二氧化碳。通篇文章对于金星上二氧化碳的讨论只有第四段和第六段。前者描述金星早期状态，后者描述今日的状态。根据关键词，在第四段找到线索句“Just as on present-day Earth…carbonate rocks.”。由此我们得知，早期的金星上，二氧化碳不仅存在于大气中，也存在于海洋和碳酸盐岩中，故B不适于对早期金星的描述；但在第六段找到线索句“Once Venus’ oceans disappeared, so did…from the atmosphere.”以及“Temperatures eventually became high…in carbonate rocks”，可得知随着海洋消失，二氧化碳不会溶解于海中，而且由于温度足够高，任何二氧化碳都会被“蒸”出来，因此B选项符合对今日金星的描述。
D选项，关键词是大气的密度与温度，根据关键词定位到线索句“perhaps denser than Earth’s present-day…Venus today”以及“Studies of how stars evolve… a bit lower.”。由此可知，早期金星的密度和温度都相对低；今日的金星则是密度和温度都高，所以D选项表述本身错误；
F选项，关键词为surface water，据此定位到第五段。根据线索句“…the temperature at the surface would eventually have risen above 374°C. Above this temperature, no matter what…begun to evaporate”。因此符合对今日金星的描述；
选项G的关键词在于温度的稳定，据此找到线索句“Over time, the rising temperatures…hydrogen and oxygen”。这句话里，level off的表达意为“趋于平缓、稳定”，是在含义上对stable的同义改写，因此符合对今日金星的描述。