Saturn has a number of middle size moons, icy bodies that are dwarfed by the ringed planet's massive moon Titan and whose existence has puzzled astronomers.
Titan, which is larger than the planet Mercury, is a giant compared to all of Saturn's other moons.
Saturn is thought to have once had a family of major satellites comparable to the four large moons of Jupiter, but while Jupiter has dozens of small satellites in addition to its large moons, it has no middle-sized moons.
Erik Asphaug, professor of Earth and planetary sciences at the University of California, Santa Cruz, says cosmic collisions around Saturn could explain why the two planetary moon systems are so different.
"We think that the giant planets got their satellites kind of like the sun got its planets, growing like miniature solar systems and ending with a stage of final collisions," Asphaug said in a university release Thursday. "In our model for the Saturn system, we propose that Titan grew in a couple of giant impacts, each one combining the masses of the colliding bodies, while shedding a small family of middle-sized moons."
Earth is thought to have undergone a similar kind of giant impact, astronomers say, in which it gained the most recent 10 percent of its additional mass but also spun off the moon.
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