What created Saturn’s magnificent rings?

October 30, 2023   Issue #636

Subscribers should put a ring on it

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

Where did Saturn’s rings come from?

For years astronomers have assumed they’re the product of the breakup of a large body, perhaps a moon, and the easiest way to do that is a collision. But if two moons collided to form the rings, what were they like? How big were they, and how fast were they moving? When did it happen, and what happened after the collision?

Newly published research from a team of planetary scientists may have the answers. Using computer models that calculate the outcome of collision physics, they simulated the impact of two moons orbiting Saturn, keeping tabs on the debris to see what happens to it [link to journal paper]. This new simulation used far higher resolution than previous ones, meaning it could keep closer track on each individual particle blasted away from the collision site to see what they all were doing in more detail.

The scientists suppose that some hundred million years ago or so, Saturn had two more moons than it does now, roughly about the same size as the moons and distance from Saturn as Rhea and Dione are now (1,000 – 1,500 km wide, and a third to half a million kilometers away from Saturn). Due to the complicated gravitational interactions of all of Saturn’s moon, their orbits can slowly change: the shape of the orbit can become more elliptical, for example, or it can slowly grow or shrink.

They ran over 200 different simulations, changing the parameters a bit each time to get slightly different results, to see what best fit what’s seen today (especially with the amazing observations made by the Cassini spacecraft, which provided key details). In this case they find that the constant tweaking and pulling from other moons caused the orbits of these two purported moons to elongate, becoming elliptical. Eventually, they got so distorted that their paths crossed each other’s the orbits. That makes a collision nearly inevitable.

And so they did. Assuming the moons had rocky cores covered by a thick mantle of ice (which is what we think many of Saturn’s icy moons are like), they found that a range of different orbital configurations, collision speeds, and impact angles (head-on, or a grazing collision, for example) could create a huge expanding cloud of debris after the two moons whacked into each other.

Here’s a visualization based on one such collision scenario, with the impact sending out a huge wave of debris:

The results had a range of outcomes, too. In some cases, like head-on collisions, much of the moons were totally disrupted. For grazing impacts, the outer ice mantle was torn off to create debris clouds, with the rocky cores still somewhat intact; that’s interesting because they would still be on those dangerous elliptical orbits, meaning a second impact was again inevitable. In some case, though, chunks that survived the impact could be as large as Saturn’s inner moon Mimas (the Death Star moon), which is 400 km wide.

What of the rings? The impact simulations showed that more than 10^19 (10,000,000,000,000,000,000, or ten million trillion) kilograms of disrupted material — about the same mass we see in Saturn’s rings now — could be flung into a close orbit around Saturn. That’s important, because the giant planet’s tides can rip apart big chunks if they get too close (a distance called the Roche limit), preventing a new moon from forming from the debris. Any time two big chunks tried to coalesce, Saturn would pull them apart again. In many cases the debris was in the form of nearly pure water ice, which is in fact what the rings are made of.

None of this proves this is what happened to create Saturn’s rings, but it does show that according to the physics of collisions as we understand it, this is a viable path to create Saturn’s gorgeous rings. That’s pretty cool.

We also are now pretty sure the rings are young, only a hundred million years old or so. Remember, Saturn formed 4.56 billion years ago, so the rings are very recent! And they may only last a few hundred million years before they either fall into Saturn or dissipate due to small meteoroid impacts, too. And all this still doesn’t necessarily tell us how the rings around the other giant planets (Jupiter, Uranus, and Neptune) formed either; they have different compositions and structures, so perhaps the collisions that created them were very different than what happened around Saturn.

What gets me is that we can figure this out at all! Questions that were extremely difficult to understand not all that long ago are now being answered, or at least getting understood far better, and I love it. We can gaze out into the heavens and stare agog at the beauty of structures like Saturn’s immense rings, and also know why they’re there at all.

That’s one of the big reasons I love science in general and astronomy in particular. You wanna know our place in the Universe? We got that.

Et alia

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