This is a drawing which illustrates the composition of Neptune's atmosphere relative to possibilities for other planets.
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A Look at what Neptune's Atmosphere is Made of
The atmosphere of Neptune is a lot like that of Uranus, and unlike that of Saturn and Jupiter. On Jupiter and Saturn, the atmosphere is mostly made of the simple molecules hydrogen and helium. The atmosphere of Neptune contains much more of a complex molecule called methane.
Neptune's atmosphere originally formed from complex molecules and ice, rather than the simple molecules out of which Jupiter and Saturn formed.
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