Saturn's rings were once a moon ripped apart by bizarre forces

Saturn’s rings are not only the planet's most iconic features, but also rank among its most enigmatic aspects. Scientists aren’t quite sure how the rings formed in the first place. The rings are likely relatively new features, forming in the past 100 million years or so. But despite being recent additions in the scale of cosmic time, it’s an enduring mystery how they got there. Now, a new study suggests their creation story may be rooted in destruction.

What’s new — In a new study published Thursday in the journal Science, researchers suggest the destruction of a former moon of Saturn, dubbed Chrysalis, might help explain the mysterious origins of the planet’s rings. The moon’s ruin could also help solve a number of other scientific unknowns about the planet.

For example, Saturn's obliquity — the angle at which its axis of spin is tilted compared with its orbit around the Sun — is much greater than expected. As gas giant worlds such as Saturn formed from the protoplanetary disk of gas and dust around the newborn sun, researchers expect the spin of the resulting planets to mostly line up with the direction in which they orbit the star. But Saturn's obliquity is about 26.7 degrees, much greater than the expected 2 to 5 degrees, study lead author Jack Wisdom, a planetary dynamicist at MIT, tells Inverse.

Wisdom and his team’s research suggests Saturn’s former moon, Chrysalis, may answer both mysteries. They theorize that some 100 to 200 million years ago, Chrysalis met a grisly end, coming too close to its host planet and being ripped apart in the process. The scattered debris became the planet’s young rings. Meanwhile, the loss of the moon would also change the gravitational pulls acting on Saturn, altering the planet’s moment of inertia, or how mass is distributed inside the planet.

"It is exciting to find a scenario that explains a number of things that were not previously thought to be related," Wisdom says.

Digging into the details — Astronomers have long suspected Saturn’s strange obliquity results from gravitational interactions between Saturn and its fellow outer Solar System planet Neptune. Saturn's tilt precesses — that is, shifts over time, much like how a spinning top leans back and forth before it falls over — at nearly the same rate as the orbit of Neptune.

This link is due to a phenomenon known as orbital resonance. When a pair of bodies orbit a common center — for instance, Saturn and Neptune, around the Sun — if the rate at which they orbit that middle body, or some other aspect of their orbits, can be expressed as the ratio of a pair of whole numbers, they are in orbital resonance. For example, two planets, both orbiting a parent star, are said to be in a 2:1 resonance when one of those worlds takes about twice as long to orbit the star as the other planet.

When two bodies are in orbital resonance with one another, they exert a regular cyclical gravitational influence on their partners. Imagine pushing a child on a swing — each repeated nudge will have a cumulative effect over time and the child goes higher. In the case of Saturn and Neptune, the manner in which specific aspects of their orbits align may lead those planets to have a certain orbital effect on their companions.

But observations from NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, added a new wrinkle to the problem. Scientists found that Titan, the largest of Saturn's 83 known moons, was moving away from the planet at a faster rate than expected, roughly 11 centimeters per year.

Researchers speculated that Titan's fast migration and gravitational pull interacted with Saturn in a manner that helped Neptune tilt Saturn. But this explanation depended on a major unknown—Saturn's moment of inertia, which is how mass is distributed in the planet's interior.

How they did it — Wisdom and his team combined gravitational data from Cassini with a model of Saturn's interior to deduce the planet's moment of inertia. Surprisingly, their findings suggested that although Saturn may once have been in sync with Neptune, it was no longer.

To see how Saturn might have escaped its link with Neptune, the researchers developed computer simulations to evolve the movements of Saturn and its moons backward in time to see if any natural orbital instabilities might have influenced Saturn's tilt. They found nothing.

The scientists next examined what might have occurred if Saturn lost a moon. They ran computer simulations to estimate the properties of this moon, such as its mass and the distance at which it orbited Saturn.

The researchers suggest the gravitational pull of a hypothetical satellite, Chrysalis, may have helped keep Saturn's tilt in sync with Neptune. Their model estimates Chrysalis was roughly the size of Iapetus, Saturn's third-largest moon.

Wisdom and his colleagues theorize that sometime between 100 million and 200 million years ago, Chrysalis entered a chaotic orbital zone. It experienced a number of close encounters with Titan and Iapetus "and eventually has a grazing encounter with Saturn, breaking it apart and forming the rings," Wisdom says.

The loss of Chrysalis would then have let Saturn escape Neptune's grasp. The researchers say the unstable orbit of Chrysalis may also help explain why Titan's orbit is more oval-shaped instead of circular.

Why it matters — Understanding how Saturn’s iconic rings formed has been a problem plaguing space scientists for decades. After all, to understand how other planetary systems form, discovering the processes governing our own Solar System is key. In theorizing the lost moon may be the culprit, Wisdom presents a novel explanation for the rings and another quirk of the gas giant in a neat package.

“Previous simulations of the formation of the rings from a disrupted cometary body, similar to the mass we assume for Chrysalis, indicate that it provides enough mass to produce the present-day rings,” the researchers write in their paper.

"I have known about the 100-million-year-age-of-the-rings problem since I was a graduate student 40 years ago. So it is exciting to find an explanation. We also explain naturally why Titan's orbit is more out-of-round than expected. It fits together,” Wisdom says.