Atmospheres of the Outer Giants

Compare to Jupiter: similar atmospheric compositions (but not identical), but very different appearances. Why?

Farther from Sun:

Less UV light hits the outer planets, meaning fewer complex chemical reactions are produced. The colors of Jupiter come from complex hydrocarbons in the atmosphere.
Colder Planets:
White ammonia clouds form at cold temperatures. Saturn's colder temperatures mean that it contains more ammonia clouds than Jupiter. But at very low temperatures (like on U & N), ammonia can freeze out of the atmosphere.

Whitish methane haze also forms more easily. At Neptunian temperatures, methane clouds can form as well.
 

Different chemical compositions:
More methane, less ammonia on Uranus and Neptune. Methane haze and clouds appear bluish, and fewer white ammonia clouds exist.
Reality check: why do we talk about methane (CH4) and ammonia (NH3) so much in the outer planets, instead of molecular oxygen and nitrogen (O2 and N2) or carbon dioxide (CO2)?
 
 

Finally, what about the higher temperature of Neptune? Think about things:

  • Where would excess heat come from?
  • Why on Neptune, but not on Uranus?
  • How does this help explain the cloud patterns on Neptune?
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