Saturn's rings could have come from a destroyed moon named Chrysalis

Saturn may have got both its tilt and its rings from a moon that got ripped apart. Simulations using data from the Cassini spacecraft shows that an additional moon between Titan and Iapetus, destroyed between 100 million and 200 million years ago, could explain both of these long-standing mysteries.

Saturn’s axis is tilted by about 27 degrees with respect to the plane of its orbit, and that tilt slowly changes over time in a phenomenon called precession, like a spinning top wobbling on a table. The rate of this precession is almost exactly the same as the rate of precession of Neptune’s orbit, so astronomers thought the two could be coupled together, called a resonance. If that is the case, it would be possible that the movement of Titan – Saturn’s biggest moon – in combination with the resonance, could have dragged Saturn onto its side.

However, when Jack Wisdom at the Massachusetts Institute of Technology and his colleagues examined the data from Cassini, they found Saturn was just barely outside this resonance, possibly indicating that it was relatively recently pushed out of it. They also found that had it previously been in the resonance, based on Titan’s current movement it would have a tilt of about 36 degrees by now.

They claim that both of these findings can by explained by an extra moon that once orbited the planet and then got destroyed or thrown out of the system, violently pushing Saturn out of resonance and pushing it back towards upright. “When you have an event like that, the whole system would have shaken and tilted Saturn back up,” says Maryame El Moutamid at Cornell University in New York, who was not involved in this work.

If the extra moon was destroyed, the debris left behind could have later become Saturn’s rings, leading the researchers to name this wrecked moon Chrysalis after the form a caterpillar takes as it transforms into a butterfly. “The butterfly is long dormant in this chrysalis phase and then it unveils itself and flaps its wings,” says Wisdom. “Similarly, this was just a small moon made of ice and then the rings suddenly emerged when it was ripped apart.”

If this event happened between 100 million and 200 million years ago, it would explain Saturn’s current tilt, its proximity to the resonance with Neptune and the origin of Saturn’s rings, all in one fell swoop. When the researchers performed 390 simulations of the Saturn system including Chrysalis, 17 of them approximately matched the system we see now, rings and all.

As elegant as this solution seems, it will be difficult to prove, says El Moutamid. “It is hard to validate one unlikely event, and here there are two unlikely events that had to happen at the same time,” she says. “But the physics is not wrong – the simulations show a small likelihood, but it’s not negligible.” It might take another spacecraft, orbiting extremely close to Saturn so it can take detailed observations, to know for sure whether Chrysalis did exist.

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