Saturn’s Rings Came From a Two-Moon Collision About 100 Million Years Ago, Study Says

Of the solar system’s planets, Saturn piques the human imagination with its signature rings and impressive moon count of 274. But compelling new research reignites theories of an ancient collision shaping Saturn’s environment as we know it today—especially Titan, its biggest moon.

The study, accepted for publication in the Planetary Science Journal, addresses a well-known mystery surrounding the unusually young age of Saturn’s rings as well as the oddity of Titan’s orbit. Researchers led by the SETI Institute consider the possibility that Titan was born from a two-moon collision, the impact of which subsequently led to the creation of Saturn’s younger rings. The paper is currently available as a preprint on arXiv.

Cassini’s outstanding questions

Humanity’s first close-up of Saturn, the sixth planet from the Sun, came from NASA’s Pioneer 11 spacecraft in 1979. Voyagers 1 and 2 then made their respective flybys a couple of years later.

But it was Cassini that really brought Saturn into clearer focus. The spacecraft’s 13-year mission collected valuable data about Saturn, its rings, and its moons for Earthbound scientists to pick apart.

However, some of the data Cassini sent back challenged some long-held beliefs for astronomers. For instance, several of Saturn’s many moons had odd, lopsided orbits that didn’t quite match the equations. Saturn’s rings were also a lot younger than expected.

In addition, the planet’s internal mass was more concentrated at the center than astronomers believed, suggesting knowledge gaps in the scientific consensus surrounding Saturn’s orbital behavior.

A daring what-if

In 2022, one team of astronomers proposed that these discrepancies could make more sense if Saturn had lost a moon around 100 million years ago, which is when Saturn’s younger rings presumably formed. The latest study tests this hypothesis, using computer simulations to check whether an extra moon could fly close enough to Saturn to form rings.

Of course, the effect of such a collision would have to be consistent with the distribution and characteristics of Saturn’s moons today, the team noted in the paper. Accordingly, what clued the researchers into a good starting point was a consistent anomaly in their simulations.

“Hyperion, the smallest among Saturn’s major moons, provided us the most important clue about the history of the system,” Matija Ćuk, the study’s lead author and a researcher at the SETI Institute, said in a statement.

Specifically, the addition of an unstable extra moon kept driving Hyperion—a moon we know is real—out of existence, which let the researchers know something was up. The team also noted that Hyperion’s orbit was locked with Titan’s, but the orbital lock of the two was also likely around a few hundred years old.

Not one, but two

The team finally arrived at one possible scenario. What if there were two earlier moons, not one? If a so-called “Proto-Titan” merged with a smaller “Proto-Hyperion,” it would explain the general lack of impact craters on the moon. If a smaller object tampered with Titan’s orbit pre-merger, it also made sense that Titan would have an eccentric orbit, the researchers added.

Then the fragments near the Titan merger could have come together to form Hyperion—a lopsided, lumpy moon whose appearance perhaps befits such a wild, unusual origin story.

As for Saturn’s rings, the researchers were surprised to find that, more often than expected, Titan’s eccentric orbit destabilizes the planet’s inner moons. This would destabilize the orbits of smaller moons, forcing them into extreme routes that ended in massive collisions, forming rings.

All that said, the team is now counting on NASA’s Dragonfly, an upcoming mission that will reach Titan in 2034, to delve deeper into the mystery. Since the new research primarily focuses on simulations, fresher data from Dragonfly should allow them to put the hypothesis to the test, they said.