This composite of the Galilean Satellites shows images of the moons taken by the Galileo spacecraft. Details of their surfaces are given in the lower two rows of the composite, including features produced through volcanism, ice, and cratering. Click on image for full size version (60K JPG) Image from: NASA

These moons were discovered by Galileo Galilei in 1610. Their discovery by Galileo provided the key piece of evidence for Galileo's proof that the Earth was not the center of the Universe. Although Galileo initially thought they were stars, through continued observations over a couple of weeks, he realized that the objects he had observed remained in the vicinity of Jupiter. He was finally able to show that these objects were orbiting Jupiter, thus proving that not all objects in the heavens orbited the Earth.

Interestingly, Galileo named these natural satellites of Jupiter the "Medicean satellites" , after the famous Medici family of Renaissance Italy. The colorful names we now use for these satellites can be attributed to Simon Marius (who claimed to have observed the satellites before Galileo in 1609, but did not publish his findings). Marius attributed the suggestion of these names to a suggestion from Johannes Kepler in 1613.