Sodium cloud around Io, one of the moons of Jupiter, 1977.
© National Aeronautics & Space Administration / Science & Society
This image, obtained at the Table Mountain Observatory in the United States on 19th February 1977, shows the halo of sodium atoms surrounding Io. This phenomenon was discovered in 1973 using spectroscopic measurements. 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 - the Voyager 1 spacecraft 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. The volcanoes are the source of the sodium cloud.