The Great Dark Spots

Neptune's First Great Dark Spot
Image Credit:

Voyager Project, JPL, and NASA

The Great Dark Spot is actually a series of storms that were first discovered by Voyager 2 in 1989. They are anticyclonic (weather systems with high pressure eyes) much like the Great Red Spot on Jupiter. However, unlike the Great Red Spot, these storms are generally cloud-free and only last a few months to few years.

The winds associated with the Great Dark Spots are the fastest known in the solar system, topping out at 2400 kilometers per hour (about 1500 miles per hour). They are thought to be holes in the methane cloud deck, occuring in the troposphere at lower altitudes than the clouds. The first spot to be discovered varied in size and shape as it was viewed from Earth and there had been a plan to photograph the storm with Hubble space telescope in 1994. By the time, Hubble was in position to take an image, the storm had dissipated. However, the spots do reappear on Neptune, and a new storm appeared in the northern hemisphere of Neptune. Unlike the Great Red Spot which is a single storm, the Great Dark Spots is a series of storms with similar appearances and properties to each other.

One of the prevailing theories of what happens to the storms is that as the storms migrate towards the equator of Neptune, the storms break up and disappear. Also, clouds usually appear outside the storms so may indicate that a storm has just dissipated or may appear soon.

The storms themselves are relatively stable because they are vortexes, but again, don't last as long as the Great Red Spot.