Dark brown spot on Jupiter, photographed by Voyager 1, 1979.
© National Aeronautics & Space Administration / Science & Society
This brown oval is characteristic of the atmosphere of Jupiter. Such features are believed to be openings in the planet's upper cloud cover, and usually last for one to two years. Jupiter's atmosphere is made up of 90 % hydrogen and almost 10 % helium, together with traces of other gases, including methane and ammonia. Immensely strong winds occur, and the storm clouds exhibit colours which are thought to be due to chemical reactions in the atmosphere. Some of Jupiter's atmospheric features are very long lived; the famous Great Red Spot has been in existence for at least 300 years. The two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 flew past Jupiter at a distance of 278,000 kilometres in March 1979.