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We know that Jupiter has an atmosphere made up mainly of hydrogen and helium.
我们知道木星的大气层主要由氢和氦组成。
Europa, a moon of Jupiter, has a very thin oxygen atmosphere,
木卫二是木星的一颗卫星，它有一层非常稀薄的氧气大气层，
and HD 209458 b, a Jupiter-sized exoplanet orbiting the star HD 209458 which is 154 lightyears away,
HD 209458 b是一颗与木星大小差不多的系外行星，围绕着154光年远的HD 209458恒星运行，
has an atmosphere that contains hydrogen, carbon, oxygen, sodium, carbon dioxide, methane, and even water vapor.
它的大气层中含有氢、碳、氧、钠、二氧化碳、甲烷，甚至还有水蒸气。
All this even though we haven’t visited any of these places to directly sample the air... but we don’t need to.
我们知道所有这些，即使我们没有直接去这些地方取样… 但我们并不需要。
We can study the air on other planets, moons, and exoplanets just by looking at them.
我们可以通过观察其他行星、卫星和系外行星来研究这些气体组成。
In particular, by looking at light that bounces off or passes through their atmospheres,
特别是，通过观察从它们的大气中反射或穿过的光，
because when you shine light on a gas, the molecules absorb and scatter different frequencies of that light in different amounts.
因为当你把光照射到一种气体上时，这些分子会以不同的数量吸收和散射不同频率的光。
If we then split the transmitted or scattered light apart into its constituent colors using a prism or diffraction grating,
如果我们用棱镜或衍射光栅将透射或散射的光分解成其组成颜色，
we can see a molecule’s light-absorption fingerprint, or its light-emission fingerprint.
我们就可以看到一个分子的光吸收指纹，或它的光发射指纹。
This is hydrogen. This is nitrogen.
这是氢气。这是氮气。
Oxygen. Methane. Carbon dioxide. Water.
氧气。甲烷。二氧化碳。水。
So when we look at the sunlight bouncing off of the atmospheres of planets and notice spikes of certain heights in certain frequencies,
因此，当我们观察阳光从行星的大气层反射回来，注意到特定频率的特定高度的峰值时，
we can carefully match those to the known fingerprints of gas molecules,
我们可以仔细地将这些与已知的气体分子指纹进行匹配，
and learn not just what gases make up the air, but even their relative abundances!
不仅了解到哪些气体构成了空气，甚至还了解到它们的相对丰度!
In fact, we don’t even need to be able to see a planet at all to learn about its atmosphere –
事实上，我们甚至不需要通过观察一颗行星来了解它的大气——
many exoplanets have been discovered because they pass in front of their parent star, which we see as a dip in the overall intensity of the star’s light.
许多系外行星已经被发现了，因为它们会从自己的母恒星前面经过，我们能看到恒星光的整体强度下降。
But if an exoplanet has an atmosphere, the gas molecules in its atmosphere will block some frequencies an extra amount, according to their molecular fingerprints,
但如果一颗系外行星有大气层，根据它们的分子指纹，其大气中的气体分子会额外阻挡一些频率，
and we can do the same gas-matching process as before.
我们就可以像以前一样进行气体匹配过程。
And that’s how we know what’s in the atmosphere of HD 209458 b!.
这就是我们如何知道HD 209458 b!的大气层中有什么的过程。
Of course, in practice it’s pretty darn challenging to use molecular fingerprints to study exoplanet atmospheres,
当然，在实践中，利用分子指纹来研究系外行星的大气是相当具有挑战性的，
because air is thin so the fingerprints are super faint and we need big sensitive telescopes and spectrometers;
因为空气很薄，所以指纹非常微弱，我们需要大型灵敏的望远镜和光谱仪;
and because atmospheres are complicated and their fingerprints can be ambiguous or hard to match;
而且因为大气很复杂，它们的指纹可能模糊不清或难以匹配;
and because different parts of a single star emit different amounts of different colors of light,
因为一颗恒星的不同部分会发出不同数量不同颜色的光，
so a planet’s effect on the star’s spectrum will depend on which part the planet passes in front of.
所以一颗行星对恒星光谱的影响取决于它从恒星前面经过的部分。
But all of these difficulties can be dealt with by clever astronomers,
但所有这些困难都可以由聪明的天文学家来解决，
and thus we have been able to figure out what the air is like on planets hundreds of light years away that we can’t even see.
因此我们就能够弄清楚几百光年之外我们甚至看不见的行星上的空气是什么样子的。
