Neptune’s atmosphere, like those of the other gas giants, is composed almost entirely of hydrogen and helium. However, it also contains smaller amounts of methane, ammonia and water ice, has extremely frigid temperatures because of its great distance from the Sun, and is continually whipped by extraordinarily high winds and storms.
Neptune is the eighth most distant planet from the Sun and also the most distant of the gas giants (which include Jupiter, Saturn and Uranus), about 30 times as far from the Sun as Earth is. Like the other gas giants, its atmosphere is made up of the same gases from which the Sun itself formed – mostly hydrogen (about 80 percent) and helium (about 19 percent). According to the Case Western Reserve University astronomy program, unlike rocky planets like Earth, gas giants like Neptune are made up of a very small rocky core, an upper atmosphere made up of gases, but middle layers that form a sort of “ocean” or “soup,” which at low altitude is essentially liquid but gradually thins with altitude until it is eventually gaseous.
In addition, like Uranus, Neptune formed far enough from the Sun, and hence in cold enough conditions, that smaller trace amounts of water, methane and ammonia can also be found in the upper atmosphere in the form of ice. Case Western says that it is the unusually high degree of methane in the atmosphere which gives Neptune its characteristic blue colour. (Uranus’s atmospheric composition is slightly different, which is why it is a much paler blue.)
The upper atmosphere – the bright blue layer which is the only part of Neptune that has ever been imaged by a human spacecraft – is extremely cold, only a few dozen degrees above absolute zero, and also exceptionally low pressure. Under these conditions, the Voyager 2 space probe confirmed that Neptune has some of the most dynamic weather systems in the solar system, with winds whipping through the atmosphere at speeds well over 1,000 miles per hour. The harshest storms take the form of gales approaching what would be supersonic speeds – twice the speed of sound – here on Earth.
Neither NASA nor any other space agency has ever sent a space probe specifically to study Neptune, and there are limits to what scientists can learn from telescopes and the pictures taken by Voyager 2 as it sped past in 1989. Voyager 2’s photographs indicate that the largest feature of Neptune’s atmosphere was a massive storm, dubbed the “Great Dark Spot” by astronomers (after the more famous Great Red Spot on Jupiter).
Initially, they thought that it might be a permanent storm, like the Great Red Spot, but, according to physicists at the University of Pennsylvania, when they trained the Hubble Space Telescope on its location during the 1990s, it could not be seen. Astronomers now believe that large “dark” storms come and go in Neptune’s atmosphere on a regular basis. From a human perspective, these storms can be overwhelmingly large – the Great Dark Spot was roughly the size of the Earth.