Neptune

The Voyager 2 images show that there were many more prominent features in Neptune's atmosphere in 1989 than were seen in Uranus's atmosphere in 1986: a Great Dark Spot at 20° South Latitude, a Small Dark Spot at 55° South Latitude which was encompassed by a circumpolar dark band from 45° South to 70° South Latitudes, another dark band from 6° North to 25° North Latitudes, and several elongated white clouds. One of these rapidly-moving white clouds was given the humorous nickname Scooter. Evolution of the cloud features in Neptune's atmosphere was observed by Voyager 2, suggesting that the atmosphere was quite dynamic. Except for their bluish color, Voyager 2 images of Neptune seem more like those of Jupiter than Uranus.

The temperature at the aerosol layer in Neptune's atmosphere is about -346°F (-210°C), which is close to the temperature at the main cloud level in Uranus's atmosphere, and the effective temperatures of the atmospheres of both Uranus and Neptune were found to be close to this temperature. One would expect Neptune's visible troposphere and lower stratosphere to be about 59°F (15°C) colder than those of Uranus because of Neptune's greater distance form the Sun (30.1 a.u. vs. 19.2 a.u.); instead, the temperatures of these parts of the atmospheres of both planets are found to be about the same. Neptune's atmosphere seems to be considerably warmer than it would be if it received all or nearly all its heating from sunlight, as seems to be the case for Uranus. This is another indication that Neptune has a powerful internal heat source, unlike Uranus, which has at most a weak internal heat source (compatible with radioactivity in its interior) or none at all. Voyager 2 infrared observations confirmed this; the emission to insolation ratio was found to be 2.6 from them instead of the value 2.4 found earlier. This internal heat source may be what drives the active features in Neptune's visible atmosphere, which are much less noticeable in Uranus's atmosphere.

Neptune's atmosphere was found to be similar to that of Uranus in that it seems to have little temperature change with latitude. This probably indicates enormous heat capacities for both atmospheres. Also Neptune has a hot (about 900°F [482°C]) ionosphere and an exosphere that consists mainly of a hydrogen thermal corona; both these atmospheric components seem similar to those of Uranus. However, Neptune's stronger gravity and slightly colder stratosphere cause much lower particle densities in Neptune's upper atmosphere than are found at the same heights above the cloud layers in Uranus's atmosphere.

There is evidence of methane-hydrocarbon recycling in Neptune's atmosphere, and the methane there is likely broken down by sunlight. After breakdown, resultant hydrocarbons sink into Neptune's atmosphere. The hydrocarbons likely decomponse on their trip downward and release carbon which recombines with methane at depth in the upper atmosphere.

Neptune's atmosphere has distinctive visible structures, including cloud bands, spots (similar to spots that are storms on Jupiter and Saturn), and methane ice clouds. Cloud bands circle the planet at given latitudes. Spots are stormy areas that have counter-clockwise rotation as a result of shear between cloud bands. The Great Dark Spot occurs at about 20 degrees south latitude and the Lesser Dark Spot at about 50 degrees south. Methane ice clouds are white and appear approximately 31 mi (50 km) above dark spots, but do not rotate with the spots.