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.

地球的海洋中有丰富的水，但在稀薄大气中二氧化碳含量极少。相比之下，金星是非常干燥的，它厚厚的大气中的大多是二氧化碳。金星和地球的原始大气至少部分是来自火山喷出或排出的气体。来自现如今地球上的火山（例如圣海伦斯火山）的气体主要是水蒸气、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大气的重要部分。这两个行星上的大部分水也被认为来自在太阳系外形成的彗星和冰体的影响。 事实上，水蒸气可能曾经是金星大气层的主要成分。金星和地球的大小和质量都差不多，所以两个星球的火山排出的水蒸气可能是一样多的，而且撞击到两个星球的彗星数量也相当。恒星演化的研究表明，早期的太阳亮度只相当于现在的百分之七十，所以金星早期的大气中的温度肯定比现在要低一点。因此，水蒸气能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，地球的二氧化碳去了哪里？金星上的水蒸气又发生了什么？ 第一个问题的答案是，地球上的二氧化碳仍然很多，但现在不是以大气中的二氧化碳的形式出现，而是溶解在海洋里或通过化学作用进入碳酸盐岩中，比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，它大部分的二氧化碳就会被从海洋中沸腾出来，从地壳中烘烤出来，我们的星球很快就会形成一个厚重的二氧化碳大气层，就像金星的大气层一样。 要回答关于金星缺乏水的问题，我们必须回头看这个星球早期的历史。正如在现今的地球上一样，金星的海洋通过将二氧化碳溶解在海洋里、困在碳酸盐岩里来限制大气中二氧化碳的含量。但是金星比地球离太阳更近，很多液态水会蒸发，形成一层厚厚的水蒸气云。由于水蒸气是一种温室气体，这个潮湿的大气层——可能比地球现在的大气层厚，要比现在笼罩的金星的大气层薄得多——可以有效地留住来自太阳来的热量。起初，这对金星的海洋影响不大。尽管温度可能会上升到100°C，即地球海平面上的水的沸点，但水蒸气所增加的大气压力将使金星海洋中的水保持液态。 这种炎热潮湿的状态可能持续了好几百万年。但随着太阳能量输出渐渐增加，金星表面温度最终会上升到374°C以上。在此温度之上，无论大气压力有多大，金星的海洋都会开始蒸发，而大气中的水蒸气的增加将会加强温室效应。这会使温度更高，造成海洋蒸发更快，产生更多的水蒸气。反过来又会进一步加剧温室效应，使温度继续上升。 一旦金星的海洋消失，那么去除大气中二氧化碳的机制也会消失。没有了海洋溶解，那些排出来的二氧化碳开始积聚在大气中，进一步加剧温室效应。温度最终变得足以“烤出”任何困在碳酸盐岩中的二氧化碳。这些释放的二氧化碳形成了金星如今厚厚的大气层。随着时间的推移，温度上升会趋于平稳，太阳紫外线辐射会将大气中的水蒸气分子分解成氢和氧。所有水蒸气都消失了，温室效应也就不会加速了。