Ellen Phiddian
Cosmos science journalist
What caused Saturn’s rings and tilt? It might have been a moon destroyed over 100 million years ago, according to new research.
Saturn’s rings are currently thought to be around 100 million years old, much newer than the planet itself.
Astronomers don’t have a well-established reason for the rings. Nor to they have one for the planet’s odd axial tilt.
It was thought that the Saturn’s tilt may have come from gravitational interactions with Neptune, or possibly Titan, Saturn’s largest moon.
But a new paper in Science has uncovered a key value – Saturn’s moment of inertia – and used this to propose a different theory.
The moment of inertia relates to how mass is distributed in a planet’s interior. “To make progress on the problem, we had to determine the moment of inertia of Saturn,” says lead author Professor Jack Wisdom, a researcher in planetary sciences at Massachusetts Institute of Technology, US.
The researchers used data from the Cassini spacecraft to refine the moment of inertia.
They found that while Saturn may once have been in sync, gravitationally, with Neptune, it wasn’t quite anymore.
“Then we went hunting for ways of getting Saturn out of Neptune’s resonance,” says Wisdom.
After a stream of simulations, they came up with a new theory: a former satellite which kept Saturn under Neptune’s influence, but then broke up and allowed it to escape.
They’ve named this former moon Chrysalis, and they think that it was probably about the size of Iapetus, Saturn’s third-largest moon.
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“Just like a butterfly’s chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged,” says Wisdom.
Wisdom and colleagues theorise that at some point between 200 million and 100 million years ago, Chrysalis destabilised and fell into a chaotic orbit around Saturn. Eventually it got too close to the gas giant and was ripped apart.
The violent destruction of Chrysalis caused Saturn’s rings to form, and also explains why Saturn is no longer quite in resonance with Neptune – at least, according to the researchers’ theory.
“It’s a pretty good story, but like any other result, it will have to be examined by others,” says Wisdom.
“But it seems that this lost satellite was just a chrysalis, waiting to have its instability.”