Titan, Saturn’s largest moon, with the giant planet behind it in a view from the Cassini-Huygens spacecraft
ZUMA Press, Inc./Alamy
The story of Saturn, its rings and moons, may have started with its largest moon, Titan. A collision between an early proto-Titan and a smaller object about 400 million years ago could have set into motion the series of events that formed Saturn’s iconic rings and altered both the planet’s wobble and the orbits of its moons.
The Saturn system is awash in mysteries. Its rings seem to be younger than expected, the planet’s wobble isn’t tied to the motion of Neptune as simulations have suggested it ought to be, and its small moon Iapetus has a strangely tilted orbit. Titan itself has strangely few craters and an oval, or eccentric, orbit.
A huge collision that created the Titan we see today could explain all of these elements. “This is sort of a grand unified theory that covers all of the major problems,” says Matija Ćuk at the SETI Institute in California, who led the research team behind this work. “We had some idea about each of them, but this might be how they relate in one story that can be tested.”
It starts with a hypothesised extra moon called Chrysalis in the outer reaches of the system, which was proposed in 2022 to explain how Saturn’s wobble got decoupled from Neptune. The idea was that Chrysalis got tossed towards Saturn and broke up to form the rings, destabilising Saturn’s wobble and Iapetus’s orbit in the process. However, Ćuk and his colleagues noticed that in simulations, the most likely outcome was that Chrysalis would collide with Titan.
That’s a problem, says Ćuk. “If there was a collision with Titan, it could not have become the rings.” So he and his team went about calculating what would happen if Chrysalis did smash into Titan. They found that such a collision about 400 million years ago would erase Titan’s craters and push its then-circular orbit to become elliptical, as well as creating a shower of debris. The smaller moon Hyperion could be a piece of that debris, which would explain why it is so much younger than Saturn’s other moons.
Then, over time, Titan’s changing orbit would have destabilised the small inner moons and sent them careening into one another, grinding each other down into the tiny particles that now make up Saturn’s rings. “It all starts from Titan and then trickles down to a second catastrophe in the inner system,” says Ćuk.
“If a collision with Titan 1.0 can explain many other things about the Saturn system, then I think that would really centre Titan as being pivotal to how we see the system today,” says Sarah Hörst at Johns Hopkins University in Maryland. “I appreciate the elegance of how many Saturn system problems it would solve at once.”
Evidence that could prove or rule out this scenario isn’t too far off. NASA’s Dragonfly mission, which is slated to launch in 2028 and arrive at Titan in 2034, will get a close look at Titan’s surface, which should help determine whether it did merge with Chrysalis. If so, we may finally understand some of the many oddities of Saturn.
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