The Great Dark Spot of Neptune

The Great Dark Spot as seen from Voyager 2

The Great Dark Spot (GDS-89) was a dark spot on Neptune similar in appearance to Jupiter's Great Red Spot. It was detected in 1989 by NASA's Voyager 2 probe. Although it appeared similar to Jupiter's spot, which is an anticyclonic storm, it is believed that the Great Dark Spot was a relatively cloud-free region.

Characteristics:

The spot was about the same size as Earth, and was very similar in appearance to Jupiter's Great Red Spot. At first it was thought to be a storm, like the Great Red Spot, but closer observation revealed it to be a dark, elliptically-shaped depression in Neptune. Around the Great Dark Spot, winds were measured blowing up to 2,400 kilometers (1,500 miles) an hour, the fastest in our Solar System. The Great Dark Spot is thought to be a hole in the methane cloud deck of Neptune, similar to the holes in the ozone layer on Earth. In many images of Neptune, the spot has been observed at different sizes and shapes.The Great Dark Spot generated large white clouds similar to high-altitude cirrus clouds found on Earth. Unlike cirrus clouds, 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 dissolve within a period of a few hours, the clouds in the Great Dark Spot had not dissolved after 36 hours, or two rotations of the planet.

Disappearance:

When the spot was to be photographed again in 1994 by the Hubble Space Telescope, the spot had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished. However, 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.

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Special Note:

An Anticyclonic storm is a weather storm where winds around the storm flow contrary to the direction dictated by the Coriolis effect about a region of low pressure. In the northern hemisphere, anticyclonic storms involve clockwise wind flow; in the southern hemisphere, they involve anticlockwise (also called counterclockwise) wind flow.Anticyclonic storms usually form around high-pressure systems. These do not "contradict" the Coriolis effect; it predicts such anticyclonic flow about high-pressure regions. Anticyclonic storms, as high-pressure systems, usually accompany cold weather and are frequently a factor in large snowstorms. Jupiter's Great Red Spot is a well-known extraterrestrial example of an anticyclonic system.Anticyclonic tornadoes often occur; while tornadoes' vortices are low-pressure regions, this occurs because tornadoes occur on a small enough scale such that the Coriolis effect is negligible.