Great Dark Spot
The Great Dark Spot (also known as GDS-89, for Great Dark Spot, 1989) was one of a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. In 1989, GDS-89 was the first Great Dark Spot on Neptune to be observed 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.
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 the edges of the storm, winds were measured at up to 2,100 kilometers per hour (1,300 mph), the fastest recorded in the Solar System. The Great Dark Spot is thought to be 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 water 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.
Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter upper cloud deck features. As they are stable features that can persist for several months, they are thought to be vortex structures.
When the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it has either been covered up or vanished. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms.
However, in 2016, an almost identical spot emerged in Neptune's northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years. It is unknown whether this spot is still present on the planet, as observations using the Hubble telescope are limited. More recently, in 2018, a newer main dark spot and smaller dark spot were identified and studied. In August 2020, the new Great Dark Spot suddenly stopped its southward motion and reversed direction, contrary to projections that the storm would continue to the equator, where it would have met its likely demise. Around the same time, a smaller "Dark Spot Jr." was found near the larger storm, before disappearing later on. This led astronomers to believe that the storm's reversal of motion may have been related to the birth of the smaller storm.
- Dragon Storm (astronomy)
- Extraterrestrial cyclone
- Great Red Spot, a similar storm on Jupiter
- Great White Spot, a similar storm on Saturn
- Oval BA
- Small Dark Spot
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