Titan, Saturn’s largest moon, with the giant planet behind it as seen 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 in motion the series of events that formed Saturn’s iconic rings and changed both the planet’s wobble and the orbits of its moons.
The Saturn system is awash in mystery. The rings appear to be younger than expected, the planet’s wobble is not tied to the motion of Neptune as simulations have suggested it should be, and its small moon Iapetus has a strangely tilted orbit. Titan itself has curiously few craters and an oval, or eccentric, orbit.
A massive collision that created the Titan we see today could explain all of these elements. “This is a kind of grand, unified theory that covers all the big problems,” says Matija Ćuk of the SETI Institute in California, who led the research team behind this work. “We had an idea about each of them, but this could be how they relate in a story that could be tested.”
It starts with a putative extra moon called Chrysalis in the outer reaches of the system, which was proposed in 2022 to explain how Saturn’s wobble became uncoupled from Neptune. The idea was that Chrysalis was thrown at Saturn and broke up to form the rings, destabilizing Saturn’s wobble and Iapetus’ orbit in the process. However, Ćuk and his colleagues noticed that in simulations, the most likely outcome was that Chrysalis would collide with Titan.
It is a problem, says Ćuk. “If there was a collision with Titan, it couldn’t have been the rings.” So he and his team calculated what would happen if Chrysalis crushed Titan. They found that such a collision about 400 million years ago would have erased Titan’s craters and pushed its circular orbit into an elliptical one, as well as creating a shower of debris. The smaller moon Hyperion may be part 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 destabilized the tiny inner moons and sent them crashing into each other, grinding each other into the tiny particles that now make up Saturn’s rings. “It all starts from Titan and then trickles down to another catastrophe in the inner system,” says Ćuk.
“If a collision with Titan 1.0 can explain a lot of other things about the Saturn system, then I think it will really center Titan as central to how we see the system today,” says Sarah Hörst of Johns Hopkins University in Maryland. “I appreciate the elegance of how many Saturn system problems it would solve at once.”
Evidence that can prove or rule out this scenario is not too far away. NASA’s Dragonfly mission, which is slated to launch in 2028 and arrive at Titan in 2034, will take a closer look at Titan’s surface, which should help determine whether it merged with Chrysalis. If so, we can finally understand some of the many oddities of Saturn.
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