Io, one of the moons of Jupiter, 1979.
© National Aeronautics & Space Administration / Science & Society
This picture was taken by one of the two Voyager spacecraft launched in 1977 to explore the planets in the outer solar system. Both flew past Jupiter in 1979. Io is the innermost and third largest of Jupiter's four main moons, which were first observed by Galileo. It is one of the most volcanically active bodies in the Solar System - Voyager 1 observed 8 volcanic plumes on Io between March and July 1979. The volcanic activity is believed to be due to immense tidal effects caused by Io's proximity to Jupiter and another moon, Europa, heating and melting Io's interior.