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We know that Jupiter has an atmosphere made up mainly of hydrogen and helium.
我们都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,
木星的卫星木卫二的氧气大气层非常稀薄，
and HD 209458 b, a Jupiter-sized exoplanet orbiting the star HD 209458 which is 154 lightyears away,
HD 209458 b是一颗木星大小的系外行星，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.
该行星上的大气层中含有氢、碳、氧、钠、二氧化碳、甲烷甚至水蒸气。
All this even though we haven't visited any of these places to directly sample the air... but we don't need to.
虽然我们都没有直接去过这些地方采集样本，但是我们不需要去。
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它们上边的空气。
In particular, by looking at light that bounces off or passes through their atmospheres,
尤其是通过观察反射或穿过它们大气层的光，
because when you shine light on a gas, the molecules absorb and scatter different frequencies of that light in different amounts.
因为当你把光照射在气体上时，分子会吸收并散射不同数量的光的不同频率。
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把透射光或散射光分解成组成光的颜色，
we can see a molecule's light-absorption fingerprint, or its light-emission fingerprint.
我们可以看到分子的光吸收指纹或是它的光发射指纹。
This is hydrogen. This is nitrogen. Oxygen. Methane. Carbon dioxide. Water. Sulfuric acid.
这是氢气。这是氮气。氧气。甲烷。二氧化碳。水。硫酸。
So when we look at the sunlight bouncing off of the atmospheres of planets and notice spikes of certain heights in certain frequencies,
所以，当我们观察阳光反射到行星的大气层，并注意到在某些频率的高度峰值时，
we can carefully match those to the known fingerprints of gas molecules, and learn not just what gases make up the air,
我们可以仔细地将这些指纹与已知的气体分子指纹进行比对，我们不仅可以了解上边空气的气体构成情况，
but even their relative abundances! In fact, we don't even need to be able to see a planet at all to learn about its atmosphere
还可以了解它们的元素丰度！实际上，我们甚至不需要看到这颗行星就能了解它的大气层
– many exoplanets have been discovered because they pass in front of their parent star,
——很多系外行星已经被发现，因为它们从它们的母星前面经过，
which we see as a dip in the overall intensity of the star's light.
我们看到的是恒星光的整体强度的下降。
But if an exoplanet has an atmosphere, the gas molecules in its atmosphere will block some frequencies an extra amount,
但如果系外行星上有大气层，那么大气层中的气体分子会根据它们的分子指纹
according to their molecular fingerprints, and we can do the same gas-matching process as before.
会额外地阻碍某些频率，并且我们可以进行和前边一样的气体匹配。
And that's how we know what's in the atmosphere of HD 209458 b!.
我们就是通过这种方法了解HD 209458 b上的大气层的。
Of course, in practice it's pretty darn challenging to use molecular fingerprints to study exoplanet atmospheres,
当然，在实践中使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;
因为空气稀薄，所以指纹非常模糊，我们需要灵敏的大型望远镜和光谱仪；
and because atmospheres are complicated and their fingerprints can be ambiguous or hard to match;
并且因为大气层很复杂，它们的指纹可能会很模糊或难以匹配；
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.
所以行星对恒星光谱的影响将取决于行星所经过的位置。
But all of these difficulties can be dealt with by clever astronomers,
但所有的这些难题都会被聪明的天文学家解决，
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，我们已经能够弄清楚几百光年之外的行星上的空气是什么样子的，哪怕我们不看见它们。
