Great Dark Spot: Difference between revisions

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Mission	Spacecraft	Instrument	Producer	Addition Date	Primary Data Set
Voyager	Voyager 2	Imaging Science Subsystem - Narrow Angle	P34672	1999-08-30	Voyager EDRs

General

In 1989 NASA's Voyager II probe detected a spot while in orbit around Neptune. The spot was relatively the same size as Earth itself, and was very similiar looking to Jupiter's Great Red Spot. At first it was thought to be a storm, like the Great Red Spot, but closer observation revealed that it is more like a dark depression in Neptune. The spot has an elliptical shape according to the pictures taken by the Voyager II. 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.

Attributes

The Great Dark Spot has a very dynamic weather system, generating massive, white clouds similar to high-altitude cirrus clouds on Earth. Unlike cirrus clouds on Earth however, which are composed of crystals of water ice, Neptune's cirrus clouds are made up of crystals of frozen methane. Our clouds usually form and then dissolve within a period of a few hours to ten or more hours. Pictures were taken by the Voyager II, which observed two rotations of Neptune, that took about 36 hours, and revealed that the clouds had not dissolved, but remained in the same general location as the Great Dark Spot. The pinwheel (spiral) structure of both the dark boundary and the white cirrus suggest a storm system rotating counterclockwise.

Disappearance

When the spot was to be photographed again in 1994, the spot had disappeared completely, leaving astronomers to believe that it had been covered up, or it had vanished. It could no longer be seen through the Hubble Space Telescope (HST)in the same location. However, an almost identicle spot had emerged in the planets northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained in vision for several years. It seems to have stayed at 35degrees north from 1994 to at least 1996, suggesting that its [[zonal drift rate]] may be some what constant and that it's position may be highly predictable. A model (based on our expierence) of its motion can be constructed by the archive of images from the HST in combination with existing ground based images. These images are sensitive to the bright "companion" clouds of the NGDS. By combining sparadic ground based observations of the companion clouds during 1993, 1994, 1996, 1997, and 1998, with HST observations, scientists expect to provide new information on NGDS "dynamics and companion lifetimes and a prediction equation for NGDS future positions."