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Neptune. Neptune is a gas giant, composed mostly of hydrogen and helium with some methane. Seen at centre is the Great Dark Spot, thought to be a hole in the methane cloud deck of Neptune, accompanied by bright, white, high-altitude clouds. This image was reconstructed from two images (one through a green filter and one through an orange filter), taken by the narrow-angle camera aboard the Voyager 2 spacecraft in August 1989. (KEYSTONE/SCIENCE PHOTO LIBRARY/JPL/NASA/SCIENCE PHOTO LIBRARY)