Saturn's gravitational pull shredded an ancient moon, creating its iconic rings and unusual tilt, new research suggests

Scientists propose a lost moon of Saturn, which they call Chrysalis, pulled on the planet until it ripped apart, forming rings and contributing to Saturn’s tilt.
Scientists propose an ancient moon of Saturn, which they call Chrysalis, pulled on the planet until it ripped apart, forming rings and contributing to Saturn's tilt.NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
  • New models suggest Saturn's gravity shredded a moon, Chrysalis, about 160 million years ago.

  • The ancient moon could explain two long-standing mysteries: Saturn's iconic rings and dramatic tilt.

  • Researchers think Chrysalis was probably about the size of Iapetus, Saturn's third-largest moon.

Scientists say a single moon could clear up two cosmic mysteries about Saturn.

When Galileo Galilei first peered at Saturn in 1610, the astronomer noted that the planet had what appeared to be "ears." They turned out to be Saturn's iconic rings. How and when these rings formed have puzzled astronomers ever since.

Another Saturn mystery is its dramatic 27-degree tilt to one side. According to researchers, that tilt is too large to have formed when the gas giant did or to have been the result of collisions knocking the planet over. In comparison, Earth's tilt oscillates between 22.1 and 24.5 degrees.

In a study published Thursday in the journal Science, researchers ran a series of simulations that suggest Saturn's rings and its unusual tilt might have formed 160 million years ago, when one of its icy moons destabilized and fell into a chaotic orbit around the planet. Eventually the moon — which researchers dubbed Chrysalis — got too close to the gas giant and was ripped apart.

The models are based on data from the final stage of NASA's Cassini mission, which spent 13 years orbiting Saturn and its moons before plunging into the planet's atmosphere in 2017.

On July 29, 2011, Cassini captured five of Saturn’s moons in a single frame.
Cassini captured five of Saturn's moons in a single frame, on July 29, 2011.NASA/JPL-Caltech/Space Science Institute

Today, the giant's planetary system hosts 83 moons. Researchers think Chrysalis was probably about the size of Iapetus, Saturn's third-largest moon.

Researchers said about 99% of Chrysalis' remains plunged into Saturn's atmosphere, while the remaining 1% stayed in orbit, leaving a debris-strewn ring in its wake that formed the planet's iconic large rings.

"Just like a butterfly's chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged," Jack Wisdom, lead author and professor of planetary sciences at MIT, said in a statement.

Saturn’s rings display their subtle colors in this view captured on Aug. 22, 2009, by NASA’s Cassini spacecraft.
Saturn's rings display their subtle colors in this view captured by NASA's Cassini spacecraft, on August 22, 2009.NASA/JPL-Caltech/Space Science Institute

Planetary scientists have long suspected Saturn's tilt may have come from gravitational interactions with Neptune. To glean information about the planet's tilt, researchers used simulations to calculate Saturn's moment of inertia, which relates to how much force was needed to tip the planet on its side. They found that while Saturn may have once been gravitationally in sync with Neptune, something changed about 160 million years ago, which removed Saturn from Neptune's influence.

"Then we went hunting for ways of getting Saturn out of Neptune's resonance," Wisdom said. Resonance occurs when two celestial bodies keep realigning after a certain number of orbits. They theorized that an ancient moon, Chrysalis, could have kept Saturn under Neptune's influence until it disintegrated, allowing Saturn to move just out of resonance with Neptune.

Wisdom stressed that more data will be necessary to see if the theory holds up. "It's a pretty good story, but like any other result, it will have to be examined by others," Wisdom said. He added that the small moon appears to have acted as a butterfly in the chrysalis phase, with its rings emerging once it was ripped apart by Saturn's gravity.

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