The Planet Neptune is a cold gas giant, in eighth place away from the Sun. It is 17 times bigger then the Earth and 30 times further from the sun. It consists of a rocky core covered by ice, with a very deep atmosphere comprising molecular hydrogen and clouds of methane at a temperature of minus 220 degrees Centigrade. Pink poles are thought to be frozen methane and there is a central “cantaloupe” green area of unknown composition. There are also a number of dark spots which appear and disappear of over the years.
Very little is known about Neptune as it is barely visible from Earth. The best photos come from a Voyager 2 bypass in 1989. There are more recent images from the Infrared Telescope Facility in Hawaii and from the orbiting Hubble Space Telescope, but these are not as clear as the ones taken when Voyager 2 passed within 5000 kilometres of the planet after a journey lasting over twelve years.
At that time a Great Dark Spot was visible. Like the Great Red Spot on Jupiter, a much hotter planet, the Dark Spot on Neptune is thought to be a vortex structure, an anticyclone, tearing an elliptical atmospheric hole in the ozone above a methane cloud layer over an area bigger than Europe and Asia combined. Storm winds have been estimated at 2400 kilometres per hour, higher than any winds recorded on the other planets of the solar system. The hole has been estimated once to have been as large as Earth, but is not stable. In fact, the Dark Spots on Neptune change shape and come and go every few years. They are only visible about half of the time. The centres are usually cloud free, although the storms generate peripheral cirrus-type clouds of methane crystals which last for several days.
It is not known if a hole ceases to exist or is simply covered during the time when it is invisible, although, sometimes, persistent “scars” are left in the cloud deck. This may be a sign of dissipation of the storm, or the storm may travel beneath the cloud layer until it gets too close to the Equator. The mechanisms are not understood.
Currently, the great dark Spot is not visible, although a new dark spot, the Northern Great Dark Spot, similar in size and shape, was photographed by the Hubble telescope in 1994 and is still visible.
There is also a smaller spot, known as The Wizard’s Eye, near the South pole. This is about one-third of the size of the Great Dark Spot and has a persistent central white cloud which suggested its name.
The weather pattern on Neptune is complicated by an unusual magnetic field which is not centrally aligned. There are also volcanoes, not of red-hot lava, but of nitrogen gas, heated above its liquid state and spewing out dark clouds, possibly of carbon, from beneath the surface of the frozen world. The latest successor to the Hubble Space telescope, the colossal Web Space Telescope will work on infra-red frequencies with micro shutters. It is hoped that the clearer images it can produce will resolve some of these mysteries.