NASA reports that Callisto is the third largest satellite in the solar system and roughly the size of Mercury. Pictured here in color, NASA points out that its many markings show a turbulent history of collisions with space objects. In fact, Callisto is known to be the most heavily cratered object in our solar system. And while Callisto is uniformly cratered, it is not uniformly colored. Experts believe the different colors come from ice and ice erosion. It is the darkest of Jupiter's four largest moons, known as the Galilean satellites. But it is still twice as bright as our moon.