This is a drawing which illustrates the composition of Neptune's atmosphere relative to possibilities for other planets.
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An Overview of Neptune's Atmospheric Composition
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 a greater percentage of the more 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 troposphere.
One of the reasons these molecules are the primary constituents of the atmosphere of Neptune is that Neptune evolved from the more complex hydrocarbons, and ices, rather than solely from hydrogen and helium.
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