Saturn

Colliding Moons Might Have Created Saturn’s Rings

If we could wind the clock back billions of years, we’d see our Solar System the way it used to be. Planetesimals and other rocky bodies were constantly colliding with each other, and new objects would coalesce out of the debris. Asteroids rained down on the planets and their moons. The gas giants were migrating and contributing to the chaos by destroying gravitational relationships and creating new ones. Even moons and moonlets would’ve been part of the cascade of collisions and impacts.

When nature crams enough objects into a small enough space, it breeds collisions. A new study says that’s what happened at Saturn and created the planet’s dramatic rings.

The research is “A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons,” and it’s published in The Astrophysical Journal.” The lead author is Luis Todorow, a Research Fellow at the School of Physics and Astronomy at the University of Glasgow.

Saturn’s rings are so iconic that even schoolchildren can identify them. Astronomers have puzzled over them for a long time, trying to figure out how they formed and when. We know they’re mostly made of ice, but a consensus for their formation has been hard to reach.

This study, conducted by NASA and its partners, says a collision between two icy moons is responsible, and the debris is still circling the planet.

We don’t have to wind the clock back too far to find the impact the research identifies. It occurred only a few hundred million years ago, maybe even more recently than that. The research team says that it was triggered by “resonant instabilities in a previous satellite system.”

The research is based on detailed simulations of Saturn and its system of moons (it has 146 confirmed satellites) and rings.

NASA’s Cassini mission laid the groundwork for this research. The spacecraft spent more than ten years in the Saturn system. One of its main discoveries was that the gas giant’s rings and moons are not very old in astronomical terms. The larger ones are probably old, and their cratered surfaces are a clue to their ages. But some of the planet’s smaller moons are likely much younger.

An annotated picture of Saturn’s many moons captured by the Cassini spacecraft. Image Credit: By Kevin Gill from Los Angeles, CA, United States – Saturn – September 9, 2007 – Annotated, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=131463918

A moon’s distance from its planet plays a role in this. The gravitational struggle between a planet and its moon tends to drive moons away. Earth’s Moon is receding a tiny yet measurable amount each year. Some research shows that if the moons nearest to Saturn’s rings were old, they would’ve been pushed away by now. Since they’re still there, they must be young.

But it’s not that cut and dry because the smaller inner moons also have cratered surfaces.

Saturn’s moon Mimas is covered in craters, including the dramatic Herschel crater that gives the moon its “Death Star” nickname. But it’s close to Saturn. What’s going on? Image credit: NASA/JPL/SSI

So Saturn is still mysterious.

Adding to the intrigue is our fascination with icy moons. Saturn’s moon Enceladus, as well as other moons like Jupiter’s Europa, contain vast oceans underneath icy shells. They’re prime targets in the search for life, so their histories have elevated importance. If two of them collided to form Saturn’s rings, what does it all mean?

“There’s so much we still don’t know about the Saturn system, including its moons that host environments that might be suitable for life,” said Jacob Kegerreis, a research scientist at NASA’s Ames Research Center and one of the paper’s co-authors. “So, it’s exciting to use big simulations like these to explore in detail how they could have evolved.”

There’s abundant research into Saturn’s rings. One study in 2022 proposed that there used to be an additional moon between Iapetus and Titan. The moon’s presence helped the Saturn system form a resonance with Neptune, and that drove Saturn’s obliquity. As the system became more destabilized, the moon grazed Saturn, the planet’s powerful gravity tore it to pieces, and the debris formed the icy rings while also kicking Saturn out of the resonance. This evidence supports a young age for Saturn’s rings, perhaps only 100 million years old.

This new research is in the same vein, but instead of a single moon getting torn apart by Saturn, two moons experience a high-speed impact that destroys them both.

An artist’s conception of two bodies smacking into each other. A collision like this could’ve formed Saturn’s rings. Credit: NASA/JPL-Caltech

The researchers performed simulations with the powerful Distributed Research using the Advanced Computing (DiRAC) supercomputing facility at Durham University’s Institute of Computational Cosmology in the UK. It’s dedicated to particle physics, astronomy, and cosmology. The team used the powerful computer to model collisions between precursor moons in the Saturn system.

The Roche Limit governs a critical part of the relationship between a planet and its moons. It’s the minimum distance a moon can approach its planet without being torn apart by the planet’s gravity. Saturn’s rings are inside the Roche Limit, and beyond that limit, planets can form from debris. So debris beyond the Roche Limit wouldn’t last long because the material would likely coalesce into new moons.

That’s basically what happened, according to this research. An ancient collision between two moons created a shower of debris inside Saturn’s Roche Limit. The massive planet’s powerful gravity prevented the debris from forming a new moon, so the debris formed into rings. The team performed almost 200 simulated collisions, each one with different masses, velocities, and angles of impact. In a wide range of scenarios, material settled into rings around Saturn, inside its Roche Limit.

“This scenario naturally leads to ice-rich rings,” said Vincent Eke, Associate Professor in the Department of Physics/Institute for Computational Cosmology at Durham University and a co-author on the paper. “When the icy progenitor moons smash into one another, the rock in the cores of the colliding bodies is dispersed less widely than the overlying ice.”

This is a strong point of the study. Icy moons still have rocky cores, and other scenarios can’t explain why there would be almost no rock in Saturn’s rings. The simulations show that only a negligible amount of rock from the collisions finds its way inside the Roche Limit, which matches the icy nature of Saturn’s rings.

This would’ve been a messy process that played out over time. The study shows there would’ve been a lot of debris from the collision and that it would’ve impacted other moons, which may have led to collisional cascades.

“Furthermore, more than a Mimas mass of material—and even more than an Enceladus mass in some cases—is placed onto crossing orbits with present-day Mimas, Enceladus, and Tethys (and Titan), facilitating the possibility of a collisional cascade to further distribute material across the system,” the paper states.

It would’ve taken a long time for things to settle down. But what caused it?

Everything in the Universe is in motion, and every object exerts a gravitational force on other objects. In our Solar System, the Sun’s mass dominates. So even though Saturn is almost 1.5 billion km (932 million mi) away from the Sun, the star’s gravity still affects things.

The Sun’s gravitational input at that distance is small, but it can build up in orbital resonances. Eventually, things can become destabilized, and Moons are driven from their circular orbits into elongated and tilted orbits. Saturn is rich in moons, so it’s only a matter of time until their orbits cross, and that causes a high-speed impact and the resulting cloud of mostly icy debris.

Saturn’s moon Rhea has something to tell us about this collision scenario. It’s Saturn’s second-largest moon, and its orbit is significant. It’s just beyond the point where these orbital resonances would affect it. Since moons tend to drift away from their planets, Rhea should’ve crossed this threshold of resonance eviction recently. That would’ve messed with its orbit, but its orbit is circular and flat. This supports the moon’s recent formation.

Saturn’s moon Rhea, as imaged by the Cassini-Huygens space probe. Credit: NASA/JPL-Caltech

But if Rhea formed recently, it clouds some of our thinking about Saturn’s icy moon Enceladus and its potential for life. How old is Enceladus? Did it form only a few hundred million years ago, maybe even more recently? If so, that’s not enough time for life to appear, as far as we understand it.

Saturn and its rings and moons are a fascinating system. There are so many factors at work that scientists struggle to come up with definitive explanations. The Cassini mission showed us that the rings are likely much younger than thought, somewhere between 10 million and 100 million years old. These simulations support that idea, though they’re not conclusive.

“We conclude that the impact of two destabilized icy moons is a promising scenario for the recent formation or rejuvenation of Saturn’s rings and reaccretion of mid-sized moons,” the researchers say in their conclusion.

But more research is needed before we can rule out other scenarios.

“Future work on the long-term evolution of the orbit-crossing debris, combined with further and more detailed modelling of collisions between both icy moons and smaller fragments, will help to constrain the implications of this scenario for Saturn’s rings, its moons, their craters, and other surface environments,” they write.

Evan Gough

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