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Lost moon may have spawned Saturn’s rings

The rings of Saturn are lovely to look at but a colossal headache to explain. Now, planetary scientists have come up with a new theory about their origin. About 160 million years ago, they say, an icy moon was ripped apart when its orbit brought it too close to the planet. The lost moon, which they call Chrysalis, may also help explain the evolution of Saturn’s oddly tilted axis of rotation.

Other researchers are intrigued, but not convinced. The finding “opens a new window about the origin of the rings of Saturn,” says Maryame El Moutamid, a planetary dynamicist at Cornell University who wrote a companion piece explaining its significance. “I think it’s a good explanation, but I will stay agnostic about it.” Luke Dones, a planetary dynamicist at the Southwest Research Institute, says the study is “clever” but also has some reservations about it. “I’m not sure how you would test this idea,” he says.

Scientists have long debated whether Saturn’s rings are billions of years old—as ancient as the Solar System itself—or perhaps just 100 million years old or so. Data from NASA’s Cassini spacecraft, which ended in 2017 after threading the gap between the rings and Saturn’s cloud tops, made a strong case for the youthful view. The spacecraft found the rings were both less massive than previously thought and exposed to a stronger than expected shower of sooty dust from the Solar System, which would tend to darken them over time. Together, those findings suggest the relatively pristine water-ice rings must be young, says Jack Wisdom, a planetary dynamicist at the Massachusetts Institute of Technology (MIT). But explaining what created the rings in the first place remained a challenge—especially so late in the history of the Solar System, when the chance of a large comet grazing the planet or shattering a moon would be low.

Initially, Wisdom and his colleagues were focused on another saturnian oddity: the surprisingly big 27° tilt of its spin axis. (A planet with no tilt would have no seasons and rotate around an axis perfectly perpendicular to the plane in which it orbits the Sun.) When Saturn coalesced out of a swirling disk of gas and dust soon after the Solar System was born, it ought to have ended up with a small tilt, just like Jupiter, the other gas giant in our Solar System. Even though the Solar System was chock full of protoplanets at the time, it would take an impact with something as big as Uranus to knock Saturn so far over, Wisdom says. “You can’t tilt Saturn over that much with the size of impactors that are believed to have been present.”

Instead, Wisdom and his colleagues believe Saturn acquired its tilt because of a peculiar synchronicity: the precession of Saturn’s spin axis—the way it wobbles like a top with a particular rhythm—is suspiciously in tune with a precession in Neptune’s orbit. If Saturn and Neptune were trapped in this resonance, Saturn’s tilt would be “kind of vulnerable to other forces that could cause it to change,” says Rola Dbouk, an MIT graduate student in planetary science. In 2020, Cassini scientists discovered what the study team thinks is that external stimulus: Titan, Saturn’s largest moon, is migrating away from Saturn by 11 centimeters a year. In a study published today in Science, Dbouk, Wisdom, and colleagues show how Titan’s migration, in combination with the Saturn-Neptune resonance, could have ratcheted up Saturn’s tilt over the course of 1 billion years.

The work also yielded a potential explanation for the origin of Saturn’s rings. Using Cassini’s measurements of Saturn’s gravitational fields to model the planet’s interior structure, the researchers refined calculations for the wobble of Saturn’s spin axis and found it is no longer in sync with Neptune. “Something kicked it out of the resonance,” Dbouk says. They first ruled out the possibility that chaotic changes in the orbits of some of the largest of Saturn’s dozens of moons could be responsible. But when they added another moon to the mix, things got interesting.

In simulations, the researchers included an object about the size of Iapetus, Saturn’s third largest moon, orbiting about 43 Saturn radii out—between the orbits of Titan and Iapetus. They found this moon could have provided the necessary nudge to the resonance if it were suddenly knocked from its orbit because of chaotic interactions with its neighbors about 160 million years ago. The team ran 390 total simulations of this event. In some, the lost moon collided with Titan or Iapetus; in others it was ejected from the Saturn system altogether. But in 17 of the simulations, the moon grazed Saturn—where it could have been shredded by gravitational forces, eventually becoming the iconic rings. “We thought that 17 was enough for us to believe this could have happened,” Dbouk says.

For El Moutamid, it’s still a “series of unlikely events.” She hopes other researchers will model the dynamics of the possible event in more detail. Regardless, she is a fan of the unofficial name the team gave the hypothetical moon, with its evocation of a shimmering butterfly—the rings—hatching from a hard cocoon. “That’s a beautiful name,” she says.

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