Saturn’s rings have captivated astronomers since Galileo first glimpsed them in the early 17th century.* While the 13-year Cassini Saturn mission answered many questions about the iconic planetary feature, such as the ring’s age, it raised new ones about the planetary system.
Namely: Why are Saturn’s rings so young? Why do so many of Saturn’s 274 moons have lopsided orbits? And if Saturn’s mass is so centrally located, why does its axis have a wobble?
In 2022, astronomers from MIT and the University of California, Berkeley proposed an elegant answer to all three questions: Saturn once had a moon that was ejected after a brush with Titan and broke up to form the rings.
To test this hypothesis, researchers from the SETI Institute led by Matija Ćuk performed a series of computer simulations to determine if this moon could come near enough to Saturn to form its rings. While not published yet, their findings have been accepted for publication in the Planetary Science Journal.
The researchers noticed that Hyperion, another of Saturn’s moons, tended to disappear when the hypothetical moon in their simulations became unstable. And yet it still exists, so what gives?
Read more: “When the Earth Had Two Moons”
“Hyperion, the smallest among Saturn’s major moons provided us the most important clue about the history of the system,” said Ćuk in a statement.
Hyperion is an oblong, potato-shaped moon with a shaggy, chaotic orbit, but one that’s locked to Titan. The Titan-Hyperion lock, the researchers say, happened only a few hundred million years ago—around the same time the extra hypothetical moon disappeared.
And so, the researchers proposed an alternative scenario to fit the facts. “Perhaps Hyperion did not survive this upheaval but resulted from it,” Ćuk explained. “If the extra moon merged with Titan, it would likely produce fragments near Titan’s orbit. That is exactly where Hyperion would have formed.”
In this scenario, a small “proto-Hyperion” and a large “proto-Titan” merged to form the moon we know as Titan while the debris became Hyperion. This would explain some odd things about Titan, like its eccentric orbit and the lack of impact craters on its surface, which would have formed in the event of a collision.
But what about the rings?
If Titan formed from a merger, the researchers found, its eccentric orbit could destabilize smaller moons closer to Saturn, sending them coursing on cosmic demolition derby. If so, some of the debris from the collisions would have scattered inward to form the rings.
While astronomers are still parsing data from Cassini, we’ll get more information about Saturn in the coming decade. Launching in 2028, NASA’s Dragonfly mission is scheduled to reach Titan in 2034, when a nuclear-powered octocopter will touch down on the moon’s surface—something Galileo never could have dreamed of. 
*As originally published, this article misstated the century in which this occurred. It has been corrected above.
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Lead image: NASA/JPL-Caltech
