Saturn might have torn apart a moon to make its rings

Saturn is a world of surprises. Flybys of Voyager 1 and 2 and later the Cassini mission opened our collective eyes to the intricate details of its rings and atmosphere. They also gave us a personal, up-close look at these amazing moons. But, one thing they didn’t show us was Chrysalis, Saturn’s proposed moon. It’s because it doesn’t exist. Well, actually, it’s there, but in the form of these dazzling rings.

This is the conclusion reached by planetary scientists at MIT after creating a model of the Saturn system. The model included its tilt, mass, and connection to the distant planet Neptune. When they added the existence of a missing moon, it helped solve some Saturnian mysteries.

The rings provide clues

When you look at Saturn through a telescope, the rings immediately catch your eye. They also tell you that the planet is rotating at an angle of 26.7 degrees from the plane of the solar system. Why so inclined? The most plausible explanation says that Saturn and Neptune were once precisely locked together by gravitational interactions. They caused Saturn’s tilt to precess at the same rate as Neptune’s orbit. Then something happened. Today, this inclination is slightly different from that of the past. So why is this?

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The rings themselves are another mystery. They are NOT as old as the planet. Some studies suggest they could be between 10 and 160 million years old. For reference, this means that they could have formed before the appearance of the first hominids or as early as the time of the dinosaurs. This makes them quite young in the history of the solar system. But what happened to create them?

To answer these questions, a team led by Jack Wisdom of MIT created a computer simulation that modeled Saturn’s evolutionary past.

the rings of saturn
A 2007 artist’s impression of the aggregates of icy particles that form the “solid” parts of Saturn’s rings. They could be what’s left of a moon called Chrysalis. Credit: NASA/JPL/University of Colorado

Looking for a hidden clue

The idea of ​​a moon affecting Saturn’s tilt and breaking apart to form rings requires some pretty complex computer modeling. One piece of information that the model needs is what is called the “moment of inertia”. It is a characteristic that explains how the mass of a planet is distributed inside. This distribution affects the tilt of the planet’s axis. The inclination changes if the material is distributed more towards the core or closer to the surface. Think of Saturn as a spinning top with the poles sticking out from the top and the bottom from the top. The precession (or wobble) of the top changes depending on where the mass inside it is.

However, Saturn’s moment of inertia was not well known and the team needed to determine its exact value. To get it, they used some of the last sightings taken by Cassini in his “Grande Finale”. It was then that the spacecraft made an extremely close approach to accurately map the gravitational field of the entire planet. It can be used to determine the distribution of mass inside the planet.

Using this data, the team modeled Saturn’s interior and identified a mass distribution that matched the gravitational field observed by Cassini. What they found told them that Saturn is currently close to, but just out of resonance with Neptune. The planets may have been in sync hundreds of millions of years ago, but they are no longer. Something happened to change that.

“We went looking for ways to get Saturn out of Neptune’s resonance,” Wisdom said. “A missing satellite could therefore explain two long-standing mysteries: Saturn’s current tilt and the age of its rings, which were previously estimated to be around 100 million years old, far younger than the planet itself.

Simulate, Simulate, Simulate

The next step was to run simulations that changed the orbital dynamics of Saturn and its current 83 moons over time. This allowed the team to see if any natural instabilities among existing satellites might have influenced the planet’s tilt. Turns out none of them did. So the team added a new moon to the mathematical equations that describe a planet’s precession (how a planet’s axis of rotation changes over time). The idea was to put the moon in and then take it out, to see if that would affect Saturn’s precession. They performed simulations to determine the mass of the satellite, as well as its orbital radius and orbital dynamics. And, eventually, they figured out that Saturn’s current tilt is the result of resonance with Neptune. The loss of the satellite, Chrysalis, which was roughly the size of Iapetus, allowed it to escape resonance.

However, Saturn’s loss was also its gain. Between 100 and 200 million years ago, Chrysalis entered a chaotic orbital zone. He had a number of close encounters with Iapetus and Titan. Eventually it came too close to Saturn and brushed past the planet. This tore the satellite to pieces, leaving chunks circling the planet like a debris-strewn ring.

If this modeling holds up to scrutiny and follow-up modeling, then it will solve two mysteries about Saturn. And, what we see today as we gaze at the planet through our telescopes may well be the remnants of that tiny moon that had such an outsized influence on its more massive planetary companion.

For more information

Saturn’s rings and tilt could be the product of a missing ancient moon
The loss of a satellite could explain Saturn’s obliquity and young rings

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