Saturn's oddball Phoebe was a wannabe planet
Sen—Saturn's moon Phoebe is not particularly special at first glance. A cratered world just a fifteenth the size of our own Moon, its original claim to fame was that it was the first of Saturn's satellites to be discovered using photography.
That find, in 1899, made Phoebe Saturn's ninth moon, a number that stuck for a considerable time until the tenth, Janus, was spotted in 1966. Dozens more have been found since, bringing the retinue to 60, largely thanks to visits by space probes, including the latest, Cassini.
A particular oddity about Phoebe, the largest of Saturn's irregularly-shaped moons with a diameter of about 220 km (132 miles), was that it orbited the planet in the opposite direction to its main companions.
NASA's Cassini has also given us our first good close-up views of Phoebe, revealing its spectacularly stark terrain. But data from the probe's observations since it arrived at Saturn in 2004 also tells us something remarkable about its history.
Apparently Phoebe was one of the early building blocks of the solar system, or planetesimals, and formed much further out in the Solar System in the Kuiper Belt of icy bodies that lies beyond Neptune.
NASA scientists have learned about Phoebe from information gathered by the spacecraft's instruments and a computer model of the moon's chemistry, geophysics and geology.
The data shows that Phoebe was formed within the first three million years since the Solar System was born 4.5 billion years ago. Phoebe was spherical in shape and hot early in its history, and has denser rock-rich material concentrated near its centre. Its average density is similar to that of former planet Pluto, the most famous object in the Kuiper Belt.
After remaining a warm body for tens of millions of years, allowing it to hold liquid water, Phoebe froze up. Then, for some reason it was sent drifting into the inner Solar System where it was ultimately captured by Saturn's gravity.
Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory at Pasadena, California, said: "Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out.
"Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of giant planets and their moons."
Another Cassini scientist, Jonathan Lunine of Cornell University, New York, said: "By combining Cassini data with modeling techniques previously applied to other solar system bodies, we've been able to go back in time and clarify why Phoebe is so different from the rest of the Saturn system."