Neptune’s Great Dark Spot, like Jupiter’s Great Red Spot and Saturn’s Great White Spot, indicates an atmospheric storm.
It is an anticyclone, a cyclone rotating in a counterclockwise direction, about the size of Earth that travels eastward about 8° south of the equator at a pace of 100 feet (30 m) per second.
The Great Dark Spot is an anti-cyclonic storm system spanning 13,000×6,600 km, and was discovered by NASA’s Voyager 2 spacecraft in 1989.
About 42° south, Scooter, a spot with a bright center, rotates much more quickly.
Still farther south, at 55°, is another dark spot.
Over these spots rise high, white wisps of methane ice crystals similar to Earth’s cirrus clouds.
Dark spots may dissipate and disappear when they migrate too close to the equator or possibly through some other unknown mechanism.