Space News Deep Dive: How Saturn Got Its Rings (And Many Other Things)

You know that rush you get when you experience something improbably lovely? Like when you try a new dish and the flavors meld in such a way that all your tastebuds tingle at once? Or when a piece of music hits just the right notes and sends shivers up your spine? Or when you see a sunset and the colors are so incredible that you can somehow feel them in your ribcage? 

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That happened to me this week, and me being me you can probably guess it came from a space news story. Specifically from a new study that set out to examine specific models for how Saturn’s rings could have formed. Instead it wound up finding a way to pinball all over the Saturn system, look at any number of oddities among the planet, rings and moons, and tie all of them back to a single precipitating event that set a whole string of cosmic dominos falling. And it’s all due to an absolutely magnificent bit of gravitational dancing. 

If you read this week’s newsletter you got a brief outline of what this study shows because I only have so much space to devote to a single story there. But I love the Saturn system. I love weird gravity shenanigans. And this story definitely deserves some deeper coverage. So join me in falling down this Saturnian gravitational rabbit hole and find out how a single impact may be responsible for the glorious planet and moon system we know and love today!

Putting a Ring on It

The thing that started this whole study happening is Saturn’s famously fabulous ring system. It’s a young feature by solar system standards, estimated at a few hundred million years at most, and the dominant theory has long been that it used to be a moon. This would have been a sizeable icy moon, nicknamed Chrysalis, that was gravitationally destroyed by Saturn, with its rocky core plunging into the planet while the icy outer layers were shredded to make the ring particles.

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It’s a solid theory, but when astronomers recently tried to model this interaction precisely using simulations, they encountered a problem. In most of the simulations, Chrysalis didn’t hit Saturn, it hit the mega moon Titan instead. And many of the simulations also had the odd moon Hyperion, which orbits near Titan and is famous for its spongy-looking surface and its oddly non-spherical shape for an object its size, getting flung out of the system.

In fact, Hyperion proved to be a major piece of the puzzle. It’s in resonance with Titan (there is a going to be a lot of talk about resonance with this story, fair warning), but only has been for a few hundred million years at most. That’s also about the same age as the rings, and also happens to be about the time Saturn appears to have been pulled out of what is called a spin-orbital resonance (where the number of times it spins in a single orbit is a whole number, unlike say Earth, which spins 365.25 times per orbit), as well as out of a possible orbital resonance with outer solar system objects.

What if, the research team wondered, Hyperion isn’t a survivor of whatever event formed the rings? What if it’s one of the results of that event? Once they started working with that theory as a base, things began to fall into place.

A Crazy Sequence of Events

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Here’s what the team believes happened. First, you have to go back over 400 million years, at which point Saturn’s inner regions looked very different than they do now. There were no rings and the moons we see there were not the moons we see now. Two of those moons were large. The paper outlining this study refers to them as Proto-Hyperion and Proto-Titan. A major change from the old theory is that Proto-Hyperion is a lot more massive than Chrysalis was theorized to be, about four times as much. And Proto-Titan is smaller than modern day Titan.

About 400 million years ago Proto-Hyperion shifted into an orbit that put it into a 2:1 resonance with Proto-Titan. This means that for every one time Proto-Titan orbited Saturn, Proto-Hyperion orbited twice. As I’ve written before, resonance is a powerful thing in space. When things are in resonance with each other, it gives them far more power over one another than they otherwise possibly could, and the lower the resonance the greater the power. And 2:1 is the strongest resonance there is.

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The amped up influence of Proto-Titan on Proto-Hyperion sets the latter moving inward fairly quickly. The wandering of this large moon has an effect on a moon much farther out, the walnut-shaped Iapetus. As Proto-Hyperion drifted inward, it managed to yank Iapetus out of its nice flat orbit in line with Saturn’s equator and send it farther and farther above that equatorial line into a highly tilted orbit.

Eventually the inevitable happens. Having first gravitationally embraced each other via resonance, Proto-Hyperion’s wanderings bring it into a much more physical interaction with Proto-Titan. They hit each other. And that changes everything. 

System Scramble

The impact all but destroys Proto-Hyperion. A great deal of it falls onto the surface of Proto-Titan, while the rest is a conglomeration of debris. That debris eventually coalesces into a single body, the modern moon Hyperion. 

Meanwhile Proto-Titan is reeling from the hit. The heat from the impact and the rain of Proto-Hyperion debris onto its surface completely melts the terrain. The ground itself cracks open, causing gases within the moon’s crust to leak out. Proto-Titan’s mass grows as it assimilates the material from Proto-Titan, and it holds on to those leaked gasses, transforming itself from an airless husk to a world surrounded by a thick atmosphere. It becomes recognizable as the modern Titan.

Like billiard balls, the hit knocks Titan for a loop, sending it into a wider and more elliptical orbit around Saturn than it previously had. This causes its gravitational tug on Saturn to vary widely at different points in its orbit. Saturn is a lot more massive than Titan, but the constant little tugs the moon has on the planet build up. Eventually the planet, which had been in a spin-orbit resonance, is pulled out of that stabilizing pattern. Reeling slightly from this destabilization, it begins to tilt.

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As you can imagine, if the suddenly varying pull of Titan’s gravity thanks to its new orbit can have an effect on the planet itself, it plays absolute merry hell with the inner moons. The effects take time to build up, but by about 100 million years or so ago a sizeable inner moon (Proto-Rhea) got caught in a 4:1 resonance with the shifted Titan, throwing its own orbit out of whack and putting it onto a collision course with another inner moon (Proto-Dione). They hit.

The two moons shred each other, sending whatever other inner moons existed at the time into chaos. There were collisions and orbit shifts aplenty. Moons were destroyed. New moons were created from the debris, including the modern moons Rhea, Dione, Mimas, and Enceladus. And at least a solid hunk of what used to be Proto-Rhea and Proto-Dione spread into a shimmering layer of ice particles that settled itself around the planet’s equator—the rings of Saturn.

Still Dancing to Gravity’s Tune

This isn’t quite the end of the story. The repercussions of this series of events are still playing out. For instance, the gravitational relationship between Titan and Saturn means Titan wants to be back in a circular orbit, and it’s working on getting back to one. Which means it’s still shifting the way it interacts with things around it. 

About 50 million years ago it moved into a 5:1 resonance with Iapetus, which threw Iapetus into an even more tilted orbit. Today Iapetus’s orbit is inclined by over 15 degrees to Saturn’s equator, an unheard of angle for such a large, old moon. And not too long after its formation, Titan caught the newly reborn Hyperion in a 4:3 resonance and has kept it there ever since—where Titan wanders, so does Hyperion.

Saturn, meanwhile, currently has an axial tilt of 26.7 degrees, a far greater tilt than that of Jupiter (3.13 degrees), which is the most similar world to Saturn in our solar system. This means Saturn has more intense seasons than our own planet, and it also happens to allow us to see the rings in stunning detail from Earth when Saturn is experiencing summer and winter.

Not to mention the small matter of the ever-evolving ring system and that entire cadre of inner moons. Can’t forget those.

Enter the Dragonfly

Having just expounded rather breathlessly on this study for a large number of words, I should probably ground myself long enough to point something out. As abso-freaking-lutely amazing and heart-breakingly elegant as this new theory is with the perfectly poetic way it delivers to us our picturesque Saturn system via a cosmic butterfly effect, I need to point out that it is just a theory. It’s based on models which look good, but none of this has been definitely proven. Yet.

Because, you see, we’re going to Titan! Under construction as I write is Dragonfly, an octocopter drone that will spend its space mission flying through the cloudy skies of Titan. But it’s also going to spend a lot of its time actually on the surface—the same surface that this study posits was completely remade only 400 million years ago. Dragonfly should be able to test that. 

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Of course that mission doesn’t launch until 2028, and it’s a long trip out to Saturn. Dragonfly is due to touch down on Titan in 2034. Of course, that also means I have plenty of time in the meantime to savor this new theory. A butterfly flaps its wings and weeks later a storm happens. Two moons enter resonance and an entire planet-moon system is changed forever.

Sometimes the universe doesn’t show its beauty only through lovely telescope images (though we all know I love a pretty space picture). Sometimes it’s more subtle, hidden in the dance of gravity and the inevitability of physics. But Sweet Carl Sagan when it’s there it’s incredible!