When it comes to Saturn's moons, Phoebe tends to be overshadowed by its siblings. Titan's size earns her the title of Saturn's biggest moon (and the second-biggest moon in our solar system), while Enceladus boasts those attention-getting fountains of water and ice pouring from its south pole. But new data from the Cassini mission shows that Phoebe might be more interesting than we thought, with a different origin and more planet-like qualities than Saturn's other moons.
NASA's Cassini-Huygens mission to Saturn has been collecting information about and images of Saturn's icy moons. And they started with little Phoebe, with a diameter of 140 miles, a fifteenth the diameter of Earth's own Moon. Unlike Saturn's other moons, which formed from the dust and gas surrounding the planet, Phoebe took shape out past the orbit of Neptune, in the Kuiper Belt. Within the first 3 million years of the solar system's formation, Phoebe had condensed into a dense, hot, spherical body with a rocky center. Phoebe is 40 percent denser than the average moon in Saturn's inner orbit, closer to the density of Pluto, which also lives in the Kuiper Belt. Many objects the size of Phoebe start out with irregular shapes, until collisions with other objects gradually bang them into spheres. But when NASA researchers used the Cassini images to model the history of Phoebe's craters, they concluded that the moon originally had a nearly spherical shape.
Phoebe formed quickly, and because it formed so early in the history of the solar system, it could have contained heat-producing radioactive materials, which kept it hot and possibly let it house liquid water. Cassini's fly-by also detected bright streaks, which are thought to be traces of ice, on Phoebe's surface.
Although the object remained warm for tens of millions of years and actively evolved for some time, it eventually froze up. Because it had formed so quickly, Phoebe could have continued to evolve into a planet. But since its development stalled, it's only the remains of a potential planet: a planetisimal.
A few hundred million years after it cooled off, Phoebe left its home in the Kuiper Belt and began drifting towards the inner solar system. As the it neared Saturn, the planet's gravitational field snagged Phoebe and it became one of Saturn's 60 moons. But its planetisimal status means that among those many moons, despite Titan's size and Enceladus's fountains, Phoebe remains unique.