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How a lost moon could've knocked Saturn into its present-day tilt

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An ancient moon which was torn apart after it spun too close to Saturn may be the cause of the planet’s tilted rings, according to new research.


Saturn’s lean has always been clear through its rings, which spin around the planet at a 26.7-degree angle compared to the planet’s orbit around the Sun.


While this was long thought to be connected to the gravitational force of Saturn’s neighbour, Neptune, due to how closely the spin of Saturn aligns with the pattern of Neptune’s orbit, astronomers now believe that connection between the two planets has since been broken.


But if Saturn isn’t tilting to pull toward Neptune, what is the reason behind its current tilt? And could it be connected to the relatively recent formation of Saturn’s rings, which have previously been estimated to be only 100 million years old?


Astronomers believe they have found an explanation that could answer a number of these unexplained Saturn anomalies: an extra moon which died so the rings could form.


In a new study published Thursday in the journal Science, authors have dubbed this moon “Chrysalis.”


If Chrysalis existed as moon number 84, it would’ve assisted in keeping Saturn in line with Neptune for several billion years, the study suggests.


Then, around 160 million years ago, according to researchers’ computer modelling, Chrysalis’s orbit became unstable and it grazed the planet itself — a catastrophic event which would’ve pulled the moon apart, and would also explain how Saturn was pulled from its pattern with Neptune to acquire its current tilt.


The shattered pieces of Chrysalis which didn’t fall to Saturn were then flung into orbit around it, eventually crumbling into smaller icy pieces to make up the planet’s rings.


“Just like a butterfly’s chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged,” Jack Wisdom, professor of planetary sciences at Massachusetts Institute of Technology and lead author of the new study, said in a press release.


This theory patches a number of holes in previous explanations for Saturn’s orbit, rings and tilt, researchers say.


It was first suggested in the 2000s that Neptune and Saturn were bound in a gravitational association, but when NASA’s Cassini flew out to visit the planet from 2004 to 2017, its observations brought new complications.


Cassini’s observations of Titan, Saturn’s largest moon, led to the theory that this large moon was actually responsible for Saturn’s tilt, forcing it into alignment with Neptune. However, this only made sense if the gas giant’s mass was distributed in a particular way — since the planet’s composition makes it difficult for us to tell if its mass is concentrated more towards the core or not, the planet’s moment of inertia is hard to pinpoint.


Wisdom and his colleagues set out to see if Cassini’s final observations — gathered in the last moments of its existence as the spacecraft plunged towards the surface of Saturn — could shed light on the issue.


These final observations made it possible to create a gravitational field of Saturn that allowed researchers to model the way mass is distributed across the planet.


They found that the moment of inertia they’d been searching for meant that Saturn was actually slightly out of alignment with Neptune. The planets were no longer in sync.


“Then we went hunting for ways of getting Saturn out of Neptune’s resonance,” Wisdom said.


After modelling numerous scenarios, the team discovered that the math balanced out if a new moon was added and then subtracted in a cataclysmic event.


They theorize that Chrysalis’ orbit became chaotic between 100-200 million years ago, and that after it had some near misses with some of the other large moons such as Titan, it grazed by Saturn itself, travelling too close to survive the encounter.


Chyrsalis would’ve had to be about the size of Iapetus, Saturn’s third-largest moon, to explain how its destruction and loss could’ve pulled Saturn out of resonance with Neptune.


“It’s a pretty good story, but like any other result, it will have to be examined by others,” Wisdom says. “But it seems that this lost satellite was just a chrysalis, waiting to have its instability.” 



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