As various space missions examine their surfaces, Io, Europa, Ganymede, and Callisto are beginning to give up their secrets.
THE DISCOVERY of the four great moons of Jupiter is usually attributed to Italian astronomer Galileo Galilei, who published The Starry Messenger, describing the results of observations he made at Padua on Jan. 10, 1610. Galileo had climbed to the roof of his house, aimed his telescope at the giant planet Jupiter, and spotted three bright fuzzy balls dancing ...