On the origin of Saturn’s rings


One of the outstanding mysteries of our Solar System is how Saturn’s rings formed.

We know they rings are made of water ice with very few contaminants. We know they are different to the rings around Jupiter, Neptune and Uranus which are much smaller and probably the result of the surface erosion of nearby moonlets.

But Saturn’s spectacular rings are different. They are far more massive, probably several times the mass of the Saturnian moon Mimas. So how did they get there?

There are three main theories, says Julien Salmon from the Université Paris Diderot in France and a couple of mates.

The first is that the rings are leftovers from the primordial cloud and never formed into a moon around Saturn. That seems unlikely say the researchers, because the rings have a different chemical composition to other Saturnian satellites which must have formed from the same cloud.

The next idea is that the rings formed when a comet collided with and destroyed an ancient Saturnian moon.

The final theory is that the rings formed when Saturn’s gravity captured one or more comets and tidal forces broke the comets apart.

These last two are much more difficult to tease apart because we know that about 4 billion years ago, the solar system was filled with comets which bombarded the planets and their moons. This period, known as the Late Heavy Bombardment, could have caused either scenario.

But a detailed analysis by Salmon and co cause them to lean towards the theory that a comet must have collided with an existing moon. Here’s why: if passing comets could be captured and torn apart by tidal forces, then all four gas giants ought to have Saturn-like rings. And Saturn’s ring system ought to be the smallest of the lot because of the planet’s low density and mass compared to Jupiter and its distance from the main body of comets compared to Uranus and Neptune.

So Saturn’s rings must have been formed by a collision between a comet and moon, say Salmon and buddies. And it turns out that only  Saturn (and possibly Jupiter) could have had a moon at the relatively close distance that the rings have formed. (At that distance, moons around the other gas giants would not have been stable because of tidal forces.)

So that settles it: Saturn’s rings formed about 4 billions years ago when  a number of comets smashed apart one of its moons.

Well, not quite. There are still a number of important outstanding details. For instance,  moons and comets are known to contain relatively high fractions of silicates. And yet the rings contain very little silicates. Nobody has adequately explained where these silicates have gone.

And then there is the annoying evidence that the rings may be much younger than 4 billion years old because we can see some of them darkening at a rate which cannot have been going on for too long without turning the rings black.

Salmon and co say that on balance, the late heavy bombardment is your best bet if you wnat to plump for a mechanism that created the rings.

But there’s no need to be hasty– there’s more mileage in this mystery yet.

Ref: arxiv.org/abs/0809.5073: Did Saturn’s Rings Form During The Late Heavy Bombardment?

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