Neptune is the eighth planet from the sun and has some of the most violent weather in the Solar System. Neptune’s atmosphere has a lot in common with the atmospheres of all of the large gas planets in the Solar System, but it has a number of unique features, as well. Like the other large planets, Neptune’s atmosphere is largely composed of helium and hydrogen. It also has trace amounts of methane and water.
Unlike the gas giants, Saturn and Jupiter, Neptune’s atmosphere has large amounts of ice, making it more similar in that way to Uranus. Neptune and Jupiter are farther from the sun and colder, allowing more ice to form. Neptune also has unique concentrations of methane. The blue color that the planet is noted for is actually caused by the methane in the upper parts of the atmosphere. The entire spectrum of light from the sun hits the planet, but the methane absorbs all of the light at the red end of the spectrum. This allows the blue light to bounce back, making the planet blue. Neptune and Uranus have similar compositions, but Neptune appears brighter and scientists are unsure why.
Distribution of atmospheric particles on Neptune is interesting. It has higher concentrations of methane, ethane and ethyne at its equator. Sometimes the concentrations are up to 100 times higher than at the poles.
Also like the other gas giants, Neptune has no distinct firm surface. The “surface” of the planet is considered to be where the pressure is equal to sea level on Earth. Immediately above the surface is the troposphere. Then there are the stratosphere, thermosphere, and the very outer layer is the exosphere.
The type of clouds found on Neptune varies with the region of the atmosphere. High-altitude clouds on Neptune have been photographed casting shadows on lower cloud decks up to 56 kilometers (35 miles) below. The high-altitude clouds are condensed and frozen methane and only occur at specific pressures found in the exosphere. Closer to the surface, clouds of water form, similar to clouds on Earth. There are also clouds composed of hydrogen sulfide, ammonium sulfide, and ammonia. At high altitudes Neptune also has a smog-like haze, which is made up of hydrocarbons very similar to the smog on Earth.
The fastest-moving winds in the Solar System were measured on Neptune, reaching speeds of 2,400 kilometers per hour (1,500 miles per hour). Similar to other planets, there are distinct bands of storms that can grow exceptionally large and remain so for long periods of time. Like on Earth, they encircle the planet. Neptune has a Great Dark Spot, similar to the Great Red Spot on Jupiter. Both are large storm systems. The Great Dark Spot was discovered by Voyager II. It is surrounded by white cirrus clouds of methane and is about the size of Earth.
The weather on Neptune is driven in large part by heating from within the core of the planet. The overall temperature of the planet is similar to that of Uranus, despite its being half as far from the sun, making it a bit of an anomaly. It may have been the result of a collision early in the formation of the planet.