just curious, what's the rub? the evidence has been all over the skies since, at least, the early 90's. it's easier now than ever to prove it to yourself. i've even found youtube videos that show how to DIY a spectroscope with your own smartphone camera. you can measure the light spectrum yourself and see what is in the atmosphere. the spectrum produced by our sun is a known thing, and has been studied at least as far back as Newton (mid to late 1600's). the continuous spectra is produced by the sun. the emission spectra, is the spectrum that is produced when light is absorbed by the electron shell of a substance in space, and then emitted again by that substance. this can be used to identify atmospheric constituents. this is how astronomers can discover the makeup of clouds of nebulae and atmospheres of distant planets.

i can't say definitively why this is being done, only that it is.