Transcript: Jupiter's Callisto is the third largest moon in the solar system behind Jupiter's Ganymede and Saturn's Titan. It is similar is size to the planet Mercury. Callisto is the outermost of the four Galilean satellites, and it has an orbital period around Jupiter of nearly seventeen days. Galileo used observations of Callisto especially to show that Kepler's laws applied to moons orbiting a planet as well as to planets orbiting the Sun. Callisto has a dark and bright surface, the dark regions representing soils that have been blasted by craters and the bright areas representing geologically younger regions of frozen water ice.