Galileo first spotted Saturn’s rings 400 years ago. But since then, scientists have been stumped about how they got there. Because the rings are almost pure water ice—and material in the outer solar system is generally an ice-rock mix. But a new analysis in the journal Nature may have solved the mystery. [Robin Canup, "Origin of Saturn's Rings and Inner Moons by Mass Removal from a Lost Titan-Sized Satellite"]

Today, Saturn's only massive moon is Titan. But Saturn’s fellow gas giant planet Jupiter has four big moons. So Saturn might once have had more—one of which could have had a rocky core surrounded by a shell of water ice. That moon would have interacted with a disk of gas surrounding Saturn at the time, dragging its orbit closer and closer.

As the moon spiraled in, tidal forces would have flexed its icy shell, stripping off chunks to build rings a thousand times more massive than Saturn has today. Eventually, ice boulders in the rings would have smashed into each other, spreading out the rings, and causing the outer edge to spawn icy moons—the small ones we find orbiting Saturn today.

As for that ancient moon’s rocky core? Saturn probably swallowed it up. Leaving Titan unique. And leaving scientists with a puzzle they finally may have solved.

—Christopher Intagliata

[The above text is an exact transcript of this podcast.]

[Scientific American is part of Nature Publishing Group.]