A missing moon could have given Saturn its rings — and tilt

The idea could explain some things about the planet once thought to be unrelated

Saturn from an angle of roughly 45 degrees from the plane defined by its rings. The sunlight casts a shadow of the planet across its rings.

The origins of Saturn’s iconic rings are a mystery. So is the planet’s tilted orbit. A single missing moon could explain both.

Space Science Institute, JPL-Caltech/NASA

A single, doomed moon could clear up a couple of mysteries about Saturn.

The suspected missing moon is named Chrysalis. If it existed, it could have helped tilt Saturn over. That, in turn, could have thrown the moon’s orbit into chaos. This might have led to the moon getting shredded by Saturn’s gravity. And such lunar debris could have formed the iconic rings that encircle Saturn today.

Jack Wisdom and his colleagues suggest this idea in the Sept. 15 Science. Wisdom is a planetary scientist at the Massachusetts Institute of Technology in Cambridge.

“We like [the idea] because it’s a scenario that explains two or three different things that were previously not thought to be related,” Wisdom says. “The rings are related to the tilt. Who would ever have guessed that?”


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Two mysteries, one explanation

The age of Saturn’s rings is a longstanding mystery. The rings appear surprisingly young — a mere 150 million years old or so. Saturn itself is more than 4 billion years old. So if the dinosaurs had telescopes, they might have seen a ringless Saturn. 

Another mysterious feature of the gas giant is its nearly 27-degree tilt relative to its orbit around the sun. That tilt is too large to have formed when Saturn did. It’s also too big to be from collisions knocking the planet over.

Planetary scientists have long suspected Saturn’s tilt is related to Neptune. The reason: a coincidence in the timing in how the two planets move. Saturn’s axis of rotation wobbles like a spinning top. Neptune’s entire orbit around the sun wobbles like a struggling hula hoop. The rhythm of those two wobbles is almost the same. This phenomenon is known as resonance.

Scientists thought that gravity from Saturn’s moons — especially its largest, Titan — helped the planetary wobbles match up. But some features of Saturn’s interior were not known well enough to prove that the timing of the two was linked.

Wisdom was part of a team that reviewed precise data on Saturn’s gravity. Those data had been supplied by NASA’s Cassini spacecraft. This space probe plunged into Saturn in 2017 after having orbited the gas giant for 13 years. Those gravity data revealed details of the planet’s internal structure.

Specifically, Wisdom’s team found Saturn’s “moment of inertia.” That value relates to how much force would be needed to tip the planet over. The moment of inertia was close to, but not exactly, what it would be if Saturn’s spin were in perfect resonance with Neptune’s orbit. That suggests something else must have helped Neptune nudge Saturn over.

Explains Wisdom, “That’s where this [moon] Chrysalis came in.”

The team realized that another smallish moon would have helped Titan bring Saturn and Neptune into resonance by adding its own gravitational tugs. Titan drifted away from Saturn until its orbit synced up with that of Chrysalis. The extra gravitational kicks from the larger moon (Titan) would have sent the smaller moon (Chrysalis) on a chaotic dance. Eventually, Chrysalis would have swooped so close to Saturn that it grazed the giant planet’s cloud tops. At this point, Saturn would have ripped the moon apart. Over time, the moon’s pieces slowly ground into bits, making up the planet’s rings.

Plausible, but not probable

Computer models show that the scenario works. But it doesn’t work all the time.

Only 17 out of 390 simulated scenarios ended with Chrysalis breaking apart to create the rings. But this scenario being unlikely doesn’t mean it’s wrong. Massive, dramatic rings like Saturn’s are rare, too.

The name Chrysalis came from the moon’s hypothesized spectacular ending. “A chrysalis is a cocoon of a butterfly,” Wisdom says. “The satellite Chrysalis was dormant for 4.5 billion years, presumably. Then suddenly the rings of Saturn emerged from it.”

The story hangs together, says Larry Esposito. This planetary scientist at the University of Colorado Boulder was not involved in the new work. But he’s not entirely convinced by the Chrysalis idea.

“I think it’s all plausible. But maybe not so likely,” he says. “If Sherlock Holmes is solving a case, even the improbable explanation may be the right one. But I don’t think we’re there yet.”

Lisa Grossman is the astronomy writer at Science News. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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