ElI5: how do they know what other planets are made off, how can the be so sure? Jupiter could be made of a completely unknown gas?
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I have nothing to add beyond what the others have said, but I find this topic particularly interesting because I actually performed this analysis as a NASA scientist years ago for Saturn. We used IR data collected from the Cassini probe and compared the spectra to simulations using forward solutions to the radiative transfer equation.
Basically, using some assumptions of the basic atmosphere composition (hydrogen, helium, and methane), you can match the ambient IR intensity (there are long portions of the spectrum which are noise-free). From there, you’ll see extraneous bumps and dips in the spectrum which are caused by absorption and emission of IR radiation due to molecular vibrations. We then look up the wavelengths of those perturbations in a spectral database (HITRAN is the main one), and we can figure out what gas is causing it based on experimental measurements. If you then run your radiative transfer equations backwards, you can solve for the amount of that gas needed to produce the observed blip. If you’re lucky, you can find multiple isolated signatures of the same gas to verify your solution.
We quantified measurements for ammonia, carbon dioxide, a special carbon dioxide isotope, and water. As far as I’m aware, no one has made a more precise measurement of Saturn’s water composition than I have (or at least no one has published it).
I haven’t thought about this work in a long time. I almost miss the academic life… too bad it didn’t pay well.
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