Saturn's rings could have been formed by a lost moon that crashed into the planet 160 million years ago

Saturn’s rings could have been formed by a lost moon that crashed into the planet 160 million years ago

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Saturn’s rings and tilt could be the product of a missing ancient moon, a new study suggests, and a “grazing encounter” may have ripped the moon apart to form Saturn’s rings.

jennifer chu | MIT News

Spinning around the planet’s equator, Saturn’s rings are a dead giveaway that the planet is spinning at a tilt. The belted giant rotates at an angle of 26.7 degrees relative to the plane in which it orbits the sun. Astronomers have long suspected that this tilt stems from gravitational interactions with its neighbor Neptune, since Saturn’s tilt proceeds, like a top, at about the same rate as Neptune’s orbit.

But a new modeling study by astronomers at MIT and elsewhere has found that while the two planets may have once been in sync, Saturn has since escaped Neptune’s pull. What was responsible for this planetary realignment? The team has a meticulously tested hypothesis: a missing moon.

In a study appearing this week in Science, the team proposes that Saturn, which today hosts 83 moons, once hosted at least one more, an additional satellite they call Chrysalis. Along with its siblings, the researchers suggest, Chrysalis orbited Saturn for several billion years, tugging and tugging on the planet in a way that kept its tilt, or “skewness,” in resonance with Neptune.

But about 160 million years ago, the team estimates, Chrysalis became unstable and got too close to its planet in a grazing encounter that separated the moon. The loss of the moon was enough to knock Saturn out of Neptune’s clutches and leave it at its current tilt.

What’s more, the researchers surmise, while most of Chrysalis’s shattered body may have collided with Saturn, a fraction of its fragments may have remained suspended in orbit, eventually breaking off into small chunks of ice to form the planet’s characteristic rings.

The missing satellite, therefore, could explain two long-standing mysteries: Saturn’s current tilt and the age of its rings, previously estimated to be around 100 million years, much younger than the planet itself.

“Like a butterfly’s chrysalis, this satellite was inactive for a long time and suddenly it became active and the rings came into being,” says Jack Wisdom, a professor of planetary sciences at MIT and lead author of the new study.

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Study co-authors include Rola Dbouk of MIT, Burkhard Militzer of the University of California, Berkeley, William Hubbard of the University of Arizona, Francis Nimmo and Brynna Downey of the University of California, Santa Cruz, and Richard French of Wellesley College.

a moment of progress

In the early 2000s, scientists proposed the idea that Saturn’s tilted axis is the result of the planet getting caught in a resonance, or gravitational partnership, with Neptune. But observations taken by NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017, put a new spin on the problem. Scientists found that Titan, Saturn’s largest satellite, was moving away from Saturn at a faster rate than expected, at a rate of about 11 centimeters per year. Titan’s rapid migration and gravitational pull led scientists to conclude that the moon was likely responsible for tilting and keeping Saturn in resonance with Neptune.

But this explanation depends on a big unknown: Saturn’s moment of inertia, which is how mass is distributed inside the planet. Saturn’s tilt could behave differently, depending on whether the matter is more concentrated in its core or towards the surface.

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“To move the problem forward, we had to determine Saturn’s moment of inertia,” says Wisdom.

the missing item

In their new study, Wisdom and colleagues sought to pin down Saturn’s moment of inertia using some of the latest observations taken by Cassini on its “Grand Finale,” a phase of the mission during which the spacecraft made an extremely close approach to accurately map the gravitational field around the entire planet. The gravitational field can be used to determine the distribution of mass on the planet.

Wisdom and his colleagues modeled the interior of Saturn and identified a mass distribution that matched the gravitational field that Cassini observed. Surprisingly, they found that this newly identified moment of inertia placed Saturn close to, but just out of, resonance with Neptune. The planets may have once been in sync, but they are no longer.

“Then we looked at ways to get Saturn out of Neptune’s resonance,” says Wisdom.

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The team first ran simulations to evolve the orbital dynamics of Saturn and its moons back in time, to see if any natural instability among existing satellites might have influenced the planet’s tilt. This search was empty.

So the researchers reexamined the mathematical equations that describe a planet’s precession, which is how a planet’s axis of rotation changes over time. One term in this equation has contributions from all satellites. The team reasoned that if one satellite were removed from this sum, it could affect the precession of the planet.

The question was, how massive would that satellite have to be and what dynamics would it have to undergo to bring Saturn out of Neptune’s resonance?

Wisdom and his colleagues ran simulations to determine a satellite’s properties, such as its mass and orbital radius, and the orbital dynamics that would be required to bring Saturn out of resonance.

They conclude that Saturn’s current tilt is the result of resonance with Neptune and that the loss of the satellite Chrysalis, which was about the size of Iapetus, Saturn’s third largest moon, allowed it to escape the resonance.

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Sometime between 200 and 100 million years ago, Chrysalis entered a chaotic orbital zone, experienced a series of close encounters with Iapetus and Titan, and finally got too close to Saturn, in a grazing encounter that shattered the satellite, leaving a small fraction to encircle the planet as a debris-filled ring.

They found that the loss of Chrysalis explains Saturn’s precession and current tilt, as well as the late formation of its rings.

“It’s a pretty good story, but like any other outcome, it will have to be scrutinized by others,” Wisdom says in an article reprinted with permission from MIT News. “But it seems that this lost satellite was just a chrysalis, waiting for its instability.”

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