The story of how Saturn's strange moons formed

Images captured by the Cassini spacecraft in 2017 revealed oddly-shaped inner moons of Saturn, like Pan and Atlas. Now, astrophysicists have discovered how these strange Saturnian satellites came to be.

Saturnian moons Atlas (left) and Pan (right) look rather like cosmic ravioli! A new study has attempted to explain the reason behind their odd shapes. Image Credit: NASA/JPL-Caltech/Space Science Institute
Published: April 23, 2018 at 12:00 pm
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Saturn’s inner moons Pan and Atlas have unusual shapes with bulbous centres and thin ridges, leading some to compare them to ravioli or flying saucers.


The question as to how such moons could form captured the attention of astrophysicists at the University of Bern in Switzerland, who have now come up with an explanation.

Pan and Atlas first came to the public’s attention through images captured by the Cassini spacecraft in April 2017.

These images revealed small moons, just 30km in diameter, with a distinct ‘ravioli’ shape.

Martin Jutzi and Adrien Leleu deduced that these odd shapes could not be the result of simple accretion of material over time, and decided to use computer simulations to explore the possibility that they formed from smaller moons colliding.

Saturn has 95 per cent more mass than Earth, yet its inner moons orbit at a distance of less than half that between Earth and the Moon.

Saturn's moon Prometheus, as seen by the Cassini spacecraft on 6 December 2015. The shape of this elongated moon was also explained in the study.Credit: NASA/JPL-Caltech/Space Science Institute
Saturn's moon Prometheus, as seen by the Cassini spacecraft on 6 December 2015. The shape of this elongated moon was also explained in the study. Credit: NASA/JPL-Caltech/Space Science Institute

The tidal forces at play around Saturn are so strong they can rip bodies apart, ruling out the formation of Pan and Atlas through accretion.

Turning to a ‘pyramidal regime’ of collisions between smaller moons as an explanation, Jutzi and Leleu used computer simulations to show how these processes could produce exactly the shapes seen in the Cassini images.

Near head-on mergers of moons create flattened objects with large equatorial ridges, just like Pan and Atlas.

Slightly more oblique angles of collision resulted in the elongated shapes of the moon Prometheus, a 90km long body also imaged by Cassini.

"If the impact angle is bigger than ten degrees, the resulting shapes are not stable anymore," says Adrien Leleu.

"That is why Saturn’s small moons look very different to comets that often have bilobed shapes," explains Martin Jutzi.

What’s more, the small inner moons are thought to originate from a thin disc in Saturn’s equatorial plane.

Since Saturn isn’t a perfect sphere, this makes it difficult for any object to leave the plane, so near head-on collisions are not uncommon.


"Saturn is almost a toy system to study these processes," says Martin Rubin, co-author of the study.


Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Staff Writer. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.


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