Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth–Sun distance.
Neptune’s more varied weather when compared to Uranus is believed to be due in part to its higher internal heat. Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight, the two planets’ surface temperatures are roughly equal. The upper regions of Neptune’s troposphere reach a low temperature of −221.4 °C (51.8 K). At a depth where the atmospheric pressure equals 1 bar (100 kPa), the temperature is −201.15 °C (72.00 K). Deeper inside the layers of gas, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; while Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet’s core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause.
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s vivid azure differs from Uranus’s milder cyan. Since Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s colour.
Neptune’s atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa). The thermosphere gradually transitions to the exosphere.
In contrast to the relatively featureless atmosphere of Uranus, Neptune’s atmosphere is notable for its active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet’s southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet’s centre are approximately 5,400 K (5,000 °C).
For reasons that remain obscure, the planet’s thermosphere is at an anomalously high temperature of about 750 K. The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet’s magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.
Wikipedia is still pretty good for some stuff we’ll be needing.