Jupiter has at least sixteen moons. The four main ones — Callisto, Ganymede, Io and Europa — were discovered by Galileo in 1610 and are called the Galilean satellites.
Ganymede, the biggest of Jupiter's moons, is even larger than the planet Mercury.
Heavily-cratered Callisto is almost as large. Colourwise, it is Jupiter's darkest moon.
The spacecraft, Voyager, sent back pictures of volcanoes erupting on Io. The volcanoes spew out sulphur and the entire surface of the moon is covered with the substance, giving the moon an extraordinary colour that is a mixture of black, red and yellow.
Europa is about the size of our own moon. Formerly it was thought that Europa was a frozen lump of ice. Now pictures sent back by the spacecraft, Galileo, suggest that the ice may be only a thick crust and that there might be water underneath.
Where there is liquid water, there is possibility of life. So scientists are excited about this new discovery about Europa.