Great Dark Spot

The Great Dark Spot (also known as GDS-89) was one of a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. GDS-89 ((G)reat (D)ark (S)pot - 19(89)) was the first Great Dark Spot on Neptune to be observed in 1989 by NASA's Voyager 2 spaceprobe. Like Jupiter's spot, Great Dark Spots are anticyclonic storms. However, their interiors are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be shorter, forming and dissipating once every few years or so. Based on observations taken with Voyager 2 and since then with the Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Almost all that is known about Neptune is based on the research carried out by Voyager 2.

The dark, elliptically-shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi) of GDS-89 was about the same size as Earth, and was similar in general appearance to Jupiter's Great Red Spot. Around it, winds were measured blowing up to 2,400 kilometers (1,500 mi) an hour, the fastest in the Solar System. The Great Dark Spot is thought to represent a hole in the methane cloud deck of Neptune. The spot was observed at different times with different sizes and shapes.

The Great Dark Spot generated large white clouds at or just below the tropopause layer similar to high-altitude cirrus clouds found on Earth. Unlike the clouds on Earth, however, which are composed of crystals of ice, Neptune's cirrus clouds are made up of crystals of frozen methane. And while cirrus clouds usually form and then disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.

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