This is a drawing which illustrates the composition of Neptune's atmosphere relative to other possibilities for planets.
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A Look at what Neptune's Atmosphere is Made of
The atmosphere of Neptune is very similar to that of Uranus, and unlike that of Saturn and Jupiter. On Jupiter and Saturn, the atmosphere is mostly composed of the simple molecules hydrogen and helium. The atmosphere of Neptune contains more of the complex molecules such as methane gas, ethane gas, acetylene, and diacetylene. These constituents collectively are known as "hydrocarbons".
Hydrocarbons form hazes of smog high in the upper atmosphere. There are clouds of methane ice low in the atmosphere.
Neptune's atmosphere originally formed from complex molecules and ice, rather than the simple molecules of which Jupiter and Saturn formed.
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