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Which gases make up most of Neptuneās atmosphere?
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Neptune
A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)āabout the diameter of Earthāin its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.
Neptuneās outer atmosphere is composed predominantly of hydrogen and helium. Together, these two gases account for nearly 98 percent of the atmospheric molecules near the one-bar pressure level.12
Most of the remaining atmospheric composition consists of methane gas. While hydrogen and helium are nearly invisible, methane strongly absorbs red light, which gives the planet its characteristic bluish appearance.12