Data from NASA's Cassini mission and a computer model of the moon's chemistry, geophysics and geology show Phoebe is a so-called planetesimal, or remnant planetary building block, the National Aeronautics and Space Administration reported Thursday.
"Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out," Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said.
"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 planets and their moons."
The Cassini images suggest Phoebe originated in the Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit.
Astronomers estimate Phoebe formed within the first 3 million years of the beginning of the solar system 4.5 billion years ago and may have been porous but appears to have collapsed on itself as it warmed.
Phoebe was spherical and hot early in its history, researchers said, and has denser rock-rich material near its center.
Phoebe likely was captured by Saturn's gravity when it got too close to the giant planet, they 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 it is so different from the rest of the Saturn system," said Jonathan Lunine, a Cassini team member at Cornell University.