New data from the Cassini spacecraft show that the moons are porous and have only about half the density of water ice.
So while each of these moons may have a dense chunk of debris at its core, half or more of its present size is due to gradually growing layers of small particles that float within Saturn's rings.
This clumping together of particles likely occurred in several stages, the last of which may have resulted in the puzzling equatorial ridges that give Pan and Atlas their flying saucer shapes.
Those ridges harbor a second major clue to the origins of Pan and Atlas.
If the moons had been born inside rings, Pan and Atlas could not have grown any thicker than the rings themselves.
This means that Pan and Atlas were likely created outside the rings—for instance by the demolition of a bigger moon. Only after joining the rings' orbits did the satellites grow to their present sizes.
Matt Tiscareno, a planetary scientist at Cornell University in Ithaca, New York, called the research a good, plausible model that might have much wider implications.
"By looking at how these moons interact with the disk [ring], we can then draw conclusions about the disks around other stars and even our own early solar system," said Tiscareno, who was not involved in the study.
"This massive seed"—the chunk that forms the core of Pan or Atlas—"embedded in a gap in a disk, [looks] very much like the models of solar system formation."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES