如果你可以从地球上看到外星球，你便没有必要去寻找证据。你只需要监测大气中的氧气。



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选材／校对／翻译：hqj5715
If you could see an alien planet from Earth, you wouldn't necessarily need to look for cities. You could just check for oxygen in the atmosphere. Oxygen in the atmosphere? What would that prove? Well, if there are aliens, they're undoubtedly different from what we're used to on Earth. But one thing we know is that oxygen is the same everywhere. So? So, oxygen gas is highly reactive. That means it combines very rapidly with other elements, such as carbon and hydrogen. Now when the Earth was formed, any free oxygen must have quickly combined with other elements to form less reactive compounds. None would have been left over in the atmosphere. No! Then where did it come from? The oxygen that we find in our atmosphere now has only been around since plants first evolved. Plants give off oxygen as a by-product, just as you and I give off carbon dioxide. Without life on Earth, there'd be no oxygen in the atmosphere. Aha! So if we find oxygen around some alien planet… That’s a good place to look for life.
如果你可以从地球上看到外星球，你便没有必要去寻找证据。你只需要监测大气中的氧气。 大气中的氧气？氧气能够证明什么？如果有外星人的话，毫无疑问他们和我们地球上的习惯不一样。但是一件事据我们所知在任何地方都是一样的，那便是氧气。 氧气具有很高的活性。这表示其可以快速的与其它的成分结合，比如碳元素和氢元素。当地球形成的时候，任何自由氧一定会快速的和其他元素结合以便形成活性低得化合物。大气中不会剩下任何的氧气。 氧气从何而来呢？ 在我们大气中所发现的氧气仅仅存在于地球刚开始形成的周围。星球以副产品的形式释放氧气，就像你我释放二氧化碳一样。 没有生命在地球上的话，大气中便不会存在氧气。因此，如若我们发现外星球周围存在氧气……那个地方将很可能存在生命。