Taken by Voyager 1, this picture shows Io, the innermost of Jupiter's four main moons, with the planet's turbulent atmosphere behind it. NASA launched two Voyager spacecraft in 1977 to explore the planets in the outer solar system. Voyager 1 flew past Jupiter in March 1979 before going on to Saturn in 1980. The largest planet in the solar system, Jupiter is a type of planet known as a gas giant, with an atmosphere composed mainly of hydrogen and helium. The atmosphere contains a number of large storms, the most famous of which, the Great Red Spot, has been in existence for at least 300 years. Io's proximity to Jupiter exposes it to immense tidal forces which cause the moon to be highly volcanically active.
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