Atmopheres of Uranus and Neptune
The masses of Uranus and Neptune are 5 and 6 percent, respectively, that of Jupiter, while their mean densities are about equal to or larger than Jupiter's. This strongly suggests that, whereas Jupiter and Saturn are made primarily of hydrogen and helium, Uranus and Neptune contain greater percentages of oxygen, nitrogen, carbon, silicon, and iron. That is, these two planets are not likely to have solar compositions.
In the 1930s spectroscopic studies revealed methane in the atmospheres of Uranus and Neptune as in those of Jupiter and Saturn. Since then, hydrogen has been identified, helium has been inferred indirectly, and some other hydrogen-containing molecules are also known. Greatly distant from the sun, Uranus and Neptune are very cold, and thus a number of molecular combinations are probably frozen into a crystal or liquid-drop form.
Although alike in many respects, evidence shows that the atmospheres of Uranus and Neptu ne are not highly similar to each other or to those of Jupiter and Saturn. The atmosphere of Uranus appears to be cold and clear to great depths: In the upper atmosphere there rarely appear to be any haze and no observed clouds; but in the lower atmosphere, it probably becomes misty or hazy. Finally, the greenish cast to Uranus is thought to be due to the methane molecules in its atmosphere.
By contrast, the atmosphere of Neptune possesses a variable haze of unknown chemical composition. At times nearly half the planet's atmosphere is hazed over. This haze can dissipate and reform in a matter of weeks or even a few days. The haze is partly responsible for trapping solar radiation in the atmosphere so that Neptune's upper atmosphere is warmer than that of Uranus.
Important for Uranus is the fact that its axis of rotation lies almost in its orbital plane, causing regions near the poles to remain alternately in sunlight or darkness for periods approaching 42 years. What effect such a phenomenon has on the overall structure of the atmosphere and how much of a difference it produces between Uranus and Neptune is not known.
