Computer simulations suggest reason for strange moon shapes

Recent computer simulations show how the inner moons of Saturn might have formed during collisions of smaller moonlets, giving them shapes similar to ravioli. The top three images are from NASA’s Cassini mission showing the moons Pan, Atlas and Prometheus, while the bottom three are from the computer simulations. Image: NASA/JPL-Caltech/Space Science Institute/University of Bern

The small, inner moons of Saturn have intriguing shapes resembling in some cases ravioli and spaetzle. Just how they came to resemble giant pieces of pasta has been a mystery, but new computer simulations show they likely formed when smaller moons collided in the extreme tidal environment close to the ringed planet.

It all started when Martin Rubin, an astrophysicist at the University of Bern, saw closeup pictures of Saturn’s moons Atlas and Pan that were taken by the Cassini orbiter in 2017. They featured large ridges and central bulges that gave them the appearance of huge flying saucers.

Or pasta.

Wondering how the bodies might have formed, Rubin asked his colleague Martin Jutzi to apply computer modelling to the problem that he used earlier studying comets.

It turned out to be more difficult than expected. The inner moons orbit at half the distance from Saturn as the Moon orbits Earth, in a region where enormous tidal forces would have prevented moons from forming in the usual fashion through the gradual accretion of material around a single core.

Instead, the simulations favoured an alternative model known as the pyramidal regime that indicates the moons were formed in a series of mergers of smaller bodies, in a relatively narrow range of impact angles and velocities. Jutzi and co-worker Adrien Leleu showed such collisions would generate the same shapes that were seen by Cassini.

The research indicated impact velocities of a few tens of meters per second (less than 100 mph). But they only found ravioli-type shapes at low impact angles.

“If the impact angle is bigger than 10 degrees, the resulting shapes are not stable anymore,” said Leleu. In other words, bi-lobe duck-shaped objects would be ripped apart by Saturn’s tides.

As it turns out, near head-on collisions are not all that rare in Saturn’s ring plane and impact angles tend to get lower in subsequent collisions.

“A significant fraction of such merging collisions take place either at the first encounter or after one (or) two hit-and-run events,” the team concludes in a paper published Monday in Nature Astronomy.

While Jutzi and Leleu focused on Saturn’s inner moons, the research also indicates a possible explanation for the shape of the planet’s third largest moon, Iapetus, which features a distinctive equatorial ridge.

This artist’s impression shows how Saturn’s moon Atlas might have appeared after the collision of two similar-size moonlets but before tidal forces finished its reorientation. Image: A. Verdier