Figure 7-30 Saturn’s Diverse Moons (a–c) These Voyager 1, Voyager 2, and Cassini images show the variety of surface features seen on three of Saturn’s seven spherical moons. They are not shown to scale (refer to the diameters given below each image). (d) In comparison, this Cassini image shows the nonspherical moon Phoebe, almost as dark as coal, carrying many craters, landslides, grooves, and ridges. Phoebe is barely held in orbit by Saturn. Astronomers believe that it was captured after wandering in from beyond the orbit of Neptune. Two particularly intriguing moons are (e) Iapetus and (f) Hyperion. The ridge running along the equator of Iapetus is believed to have developed as the moon formed. Apparently Iapetus cooled so rapidly that the ridge did not have time to settle away. Perhaps the most bizarre object photographed in the solar system, ice-covered Hyperion, shows innumerable impact craters. These features are different from craters seen in other objects in that the crater walls here have not filled in the bottom of the craters. This moon’s low gravity and the pull of nearby Titan may explain this unusual phenomenon.