36. Great Dark Spot
This is the final face-on view of the Great Dark Spot obtained as Voyager 2 neared Neptune. The Great Dark Spot is comparable in size to the diameter of Earth. The image was taken only 45 hours before closest approach and it shows features as small as 50 kilometers (30 miles) across. The feathery white clouds that overlie the boundary of the spot are composed of methane ice crystals. The methane clouds consistently accompanied the Great Dark Spot in the Voyager images, although the detailed shape of the individual clouds was variable from one rotation of Neptune to the next. The indistinct pinwheel (spiral) structure of both the dark boundary and the white cirrus clouds suggest a storm system rotating counterclockwise, like the Great Red Spot of Jupiter. Confirmation of the direction of rotation of the Great Dark Spot must await the tracking of features within the spot itself, features that are very subdued in appearance and difficult to identify. In 1995, Hubble Space Telescope (HST) images showed that the great Dark Spot had disappeared, while a smaller, new, dark spot was present in the northern hemisphere.
Voyager 2 image (Press Release P-34672).