<P> The Great Dark Spot generated large white clouds at or just below the tropopause layer similar to high - altitude cirrus clouds found on Earth . Unlike the clouds on Earth, 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 disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet . </P> <P> Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter upper cloud deck features . As they are stable features that can persist for several months, they are thought to be vortex structures . </P> <P> When the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it has either been covered up or vanished . The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature . Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms . </P> <P> However, in 2016 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 . It is unknown whether this spot is still present on the planet, as observations using the Hubble telescope are limited . </P>

What happened to the great dark spot on neptune