Jupiter IV - Calisto - NASA reports that Callisto is the third largest satellite in the solar system and roughly the size of Mercury. 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 colours come from ice and ice erosion.