With the most detailed views seen since the Voyager 2 spacecraft flew past Nepune in 1989, HST images have revealed how Neptune's appearance can change dramatically in only a few weeks. Bright clouds and dark spots appear and disappear regularly.
In June 1994, HST images showed that the Great Dark Spot seen by Voyager 2 in Neptunes's southern hemisphere had disappeared. By October-November, when these images were taken, a new dark spot had developed in the northern hemisphere, accompanied by bright high-altitude clouds made of frozen methane crystals. Gas flowing up over the spot cools as it rises and the crystals freeze out as the temperature falls.
Neptune is one of the gas giant planets with a diameter almost four times Earth's. Hydrogen and helium are its main constituents, but the visible atmosphere also contains methane and other gases. Methane, which absorbs red light and reflects blue light, contributes to Neptune's characteristic blue-green color. The pink features in these images are high-altitude clouds of methane ice crystals. Though they appear white in visible light, they are shown in pink here because they were imaged in the near infrared.
Earth's weather systems are driven by heat energy received from the Sun. The rapidly changing weather on Neptune, however, must be stirred up in a different way. Neptune has its own internal source of heat and radiates twice as much energy as it receives from the distant Sun. This means that Neptune's clouds are being warmed from below. Small variations in the temperature range between the top and bottom of the clouds could trigger the rapid large-scale changes witnessed by the HST.
Neptune was about 4.5 billion kilometers (2.8 billion miles) from Earth when these images were taken.
Camera: WFPC2
Technical Information: Picture reconstructed from images taken
through filters at visible and near-infrared wavelengths.
Credit: H. Hammel (Massachusetts Institute of Technology), and
NASA
