"Lost" moon could explain several Saturn mysteries, say astronomers
Saturn is home to several solar system mysteries. Using data from Cassini, astronomers now suggest a simple answer to a few questions – a lost moon once orbited the planet before being torn to shreds.
Saturn’s most striking feature is its rings, made mostly of small chunks of ice. It was long thought that these rings were leftover material from the formation of Saturn itself some 4.5 billion years ago, but recent studies suggest they’re much younger – between 10 and 100 million years old. If that’s the case, they could have formed from an icy comet or moon that wandered too close.
The planet also rotates tilted at a 27-degree angle, relative to the plane it follows as it orbits the Sun. This was long believed to be caused by the gravitational influence of the nearby Neptune, but closer inspection has revealed that Saturn is no longer in-step with its neighbor.
The astronomers on the new study have now proposed a story that explains both mysteries: Saturn used to have another moon. This hypothetical moon, which the team named Chrysalis, would have been roughly the size of the existing moon Iapetus, which spans about 1,470 km (913 miles) wide. Chrysalis, if it existed, would have orbited Saturn alongside the 83 current moons for billions of years.
But gravitational interactions with other large moons, like Titan and Iapetus, gradually made the orbit of Chrysalis more chaotic. About 160 million years ago, the doomed moon swung too close to Saturn, where it was ripped apart by the intense gravity. Most of the debris would have rained down on the planet, but some became stuck swirling around Saturn, forming the famous rings.
This look might actually be a relatively short phase in the planet’s life – other studies suggest the rings will probably vanish in the next 100 million years or so. By then, Mars may become the solar system’s new ring-bearer.
So what of Saturn’s tilt? The team says the gravitational influence of Chrysalis would have kept its tilt in resonance with Neptune, so the loss of the moon would have allowed the planet to drift to its current angle.
The researchers came up with this explanation using data gathered by Cassini during its final sweep, before it plunged into Saturn’s atmosphere in 2017. The probe measured the gravitational field of the planet, which allowed the team to model the distribution of mass in its interior. From this, they were able to calculate that Saturn was only just out of sync with Neptune, suggesting it had been until recently.
Next, the astronomers investigated what it would take to nudge the planet out of this dance. They simulated the history of the orbits of Saturn’s 83 moons, and found that it did work – but only if an 84th, rather large moon was added.
As neat an explanation as Chrysalis may be, the team acknowledges that it remains a hypothesis until further evidence can be found.
“It’s a pretty good story, but like any other result, it will have to be examined by others,” said jack Wisdom, lead author of the study. “But it seems that this lost satellite was just a chrysalis, waiting to have its instability.”
The research was published in the journal Science.