Saturn's rings are the remains of an exploded moon

Saturn's rings are among the most iconic sights in the solar system, but where did they come from? Long ago, an icy moon was ripped apart by Saturn's gravity, creating rings once a 100 times bigger than they are now.

4.5 billion years ago, as the solar system was still in its primordial stages, Saturn was likely home to a number of large moons. These days, Titan is its only really big moon, as its lost siblings were likely pulled into Saturn by its gravity and destroyed. Most of the moons would have exploded within Saturn's giant gaseous atmosphere, leaving no trace of their former existence. But the last moon to be pulled apart would have left a remarkable memorial behind: Saturn's ring system.

Saturn's rings - not to mention its inner moons, such as Tethys, Enceladus and Mimas - are unusually high in water. The rings are 90 to 95 percent ice, and that makes them hard to explain. One of the leading theories for the rings was that a comet was crashed into Saturn, and its remains formed the rings. But a comet is made of rock as well as ice, and the absence of much rock in the rings makes that theory difficult to accept.

An icy moon, on the other hand, would explain just about everything. As the moon spiraled in towards Saturn, the planet's intense gravity would have actually ripped off the icy outer layers of the moon, leaving its rocky core exposed and ready to plunge into Saturn, never to be seen again. The ice would then break apart and form Saturn's ring system.

But as huge and remarkable as Saturn's rings are, an explosion of that size would create much, much bigger rings. A Titan-sized moon would probably create a ring system anywhere between 10 and 100 times bigger than what we see today. It's hard to imagine just how awe-inspiring it would be to look upon Saturn all those billions of years ago with a sight like that waiting for us.

Over the eons, the rings would have steadily shrank, as parts of the ice fell back into Saturn and others drifted out into further orbits, where they would have clumped together and began to form new moons. Saturn's inner moons all have masses and compositions that fit in with such an explanation very nicely, which is a good boost for the veracity of this new theory.

This model, the brainchild of researcher Robin Canup, will be put to the test in 2017, when the Cassini space probe is due to complete its mission by flying close over the rings of Saturn. That will allow astronomers to get a much better idea of the age and mass of the rings, and by extension hopefully provide some good supporting evidence for where Saturn's rings came from.

As Canup observes, there's a certain poetry to the idea that Saturn's rings were born from the sacrifice of an exploding moon 4.5 billion years ago:

"I think it's pretty neat to realize that the ring system, which is so famous, is probably the last surviving remnant of a lost satellite."

[Nature via]