The Atmosphere of a Habitable Planet宜居行星的大气
Animal life as we know it requires an atmosphere with sufficient oxygen. In order for plants to carry out photosynthesis, there must be a sufficient amount of carbon in the atmosphere in the form of carbon dioxide. 我们所知的动物生命需要含有足够氧气的大气。为了进行光合作用，大气中必须存在足够的碳，以二氧化碳的形式存在。
For sustaining a stable habitable temperature, the mixture of greenhouse gases is critical. Loss of a suitable balance can lead to drastic heating or cooling of the planet. 为了维持稳定的宜居温度，温室气体的混合至关重要。失去适当的平衡可能导致地球出现剧烈的升温或降温。
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Index Habitable planet concepts Ward & Brownlee Ch 5 索引 可居住行星概念 Ward & Brownlee 第5章 | ||
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An Oxygen Atmosphere for a Habitable Planet适合居住的星球的氧气大气
Earth's oxygen content is 45% and 85% by volume according to Ward & Brownlee. This compares with about 0.1% in the Sun, the so-called "cosmic abundance". But having a lot of oxygen on a planet doesn't imply that there is an oxygen atmosphere. Oxygen tends to be bound in oxides so it would be quite possible to have almost no oxygen in the atmosphere. 根据Ward & Brownlee的数据，地球的氧含量按体积计算为45%和85%，这与太阳中的约0.1%（所谓的‘宇宙丰度’）相比要高得多。但一个行星上有大量氧并不意味着其大气中含有氧。氧倾向于与其他元素结合形成氧化物，因此大气中几乎不含氧的情况是完全可能的。
The models that have been constructed for the oxygenation of the Earth's atmosphere make a very long story. Ward and Brownlee's version of this story involves what they call the "oxygen revolution". In their model, the formation of continents at about 2.5 billion years ago provided increased habitat for vast quantities of stromatolites that released oxygen into the sea. Up until about 1.8 billion years ago, that oxygen was bound to the dissolved iron in the oceans, as evidenced by the deposits of banded iron. But around 1.8 billion years ago, the free iron was bound and oxygen began to be released into the atmosphere. Prokaryotic bacteria (cyanobacteria), using only water, carbon dioxide and energy from sunlight continued to release quantities of oxygen into the atmosphere. This revolution provided the basis for the oxygen metabolism of the eukaryotic cell machinery and became the basis for building higher animal life. 为了解释地球大气的氧化过程，已经构建了各种模型，这讲述了一个很长的故事。Ward和Brownlee对这个故事的版本涉及他们所说的“氧革命”。在他们的模型中，大约在25亿年前形成大陆提供了更多的栖息地，使大量层状叠层石生物得以生存，这些生物释放氧气进入海洋。直到大约18亿年前，这些氧气都与溶解在海洋中的铁结合，这从带状铁层沉积物中得到证实。但大约在18亿年前，自由铁被结合，氧气开始释放到大气中。原核细菌（蓝细菌）仅使用水、二氧化碳和来自阳光的能量继续向大气中释放大量氧气。这一革命为真核细胞机器的氧代谢奠定了基础，并成为构建更高动物生命的基础。
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Index Habitable planet concepts Ward & Brownlee Ch 5 索引 可居住行星概念 Ward & Brownlee 第5章 | ||
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The Amount of Carbon Needed for Life生命所需的碳量
The unique chemistry of carbon is essential for life as we know it. If the Earth did not have it, there would be no advanced life here. Yet too much carbon is also disastrous to life, so we should be appreciative that the Earth seems to have just about the right amount. Venus has about 100 times as much carbon and suffers from the runaway greenhouse effect that has produced a surface temperature of about 750 K compared to about 300K for the Earth. That is hot enough to melt lead! Mars hasn't been able to hold its atmosphere and has 1/100 the carbon of the Earth. Carbonaceous meteorites have up to 4% carbon, but the Earth is only 0.05% carbon. 碳的独特化学性质对于生命而言至关重要。如果地球没有这种化学性质，这里就不会有高级生命。然而，过多的碳对生命也是灾难性的，因此我们应当感激地球似乎恰好拥有适量的碳。金星的碳含量大约是地球的100倍，它遭受着 runaway greenhouse effect 的影响，导致地表温度约为750 K，比地球的约300 K高得多。这温度足以熔化铅！火星无法保持其大气层，其碳含量仅为地球的1/100。碳质陨石中的碳含量最高可达4%，但地球的碳含量仅为0.05%。
With the large variations in carbon content in our neighbordhood of the solar system, the Earth seems to be in the narrow window that provides enough carbon for life in general and and photosynthesis in particular. Yet it does not have so much as to trigger a runaway greenhouse effect. 由于我们太阳系附近碳含量的显著变化，地球似乎处于一个狭窄的窗口内，这个窗口提供了生命所需的足够碳，尤其是光合作用所需的碳。然而，它并不足以引发温室效应的恶性循环。
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Balancing Greenhouse Gases平衡温室气体
The unique chemistry of carbon is essential for life as we know it. If the Earth did not have it, there would be no advanced life here. Yet too much carbon is also disastrous to life, so we should be appreciative that the Earth seems to have just about the right amount. Venus has about 100 times as much carbon and suffers from the runaway greenhouse effect that has produces a surface temperature of about 750 K compared to about 300K for the Earth. That is hot enough to melt lead! Mars hasn't been able to hold its atmosphere and has 1/100 the carbon of the Earth. Carbonaceous meteorites have up to 4% carbon, but the Earth is only 0.05% carbon. 碳的独特化学性质对于生命而言至关重要。如果地球没有这种化学性质，这里就不会有高级生命。然而，过多的碳对生命也是灾难性的，因此我们应当感激地球似乎恰好拥有适量的碳。金星的碳含量大约是地球的100倍，它遭受着 runaway greenhouse effect 的影响，导致地表温度约为750 K，比地球的约300 K高得多。这足以熔化铅！火星无法保持其大气层，其碳含量仅为地球的1/100。碳质陨石中的碳含量最高可达4%，但地球的碳含量仅为0.05%。
With the large variations in carbon content in our neighbordhood of the solar system, the Earth seems to be in the narrow window that provides enough carbon for life in general and and photosynthesis in particular. Yet it does not have so much as to trigger a runaway greenhouse effect. 由于我们太阳系附近碳含量的显著变化，地球似乎处于一个狭窄的窗口内，这个窗口提供了生命所需的足够碳，尤其是光合作用所需的碳。然而，它并不足以引发温室效应的恶性循环。
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