Saturn’s ridged moons may have gotten their weird shapes from a moonlet demolition derby. The small inner moons have a range of strange shapes: Pan and Atlas are disks with bulging middles like ravioli, Prometheus is elongated like a cigar, and others look like misshapen potatoes. They may have formed when pairs of smaller moonlets crashed into one another and merged.
To figure out whether mergers are a plausible explanation for Saturn’s menagerie of moons, at the University of Bern and his colleagues performed thousands of computer simulations of different-sized moonlets encountering one another at different velocities and angles.
“They just go splat as they hit each other,” says at Johns Hopkins University, who was not involved in the research. Depending on the angle of the collision, the resulting moons end up with different shapes.
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Going splat
When the moonlets hit each other head-on at fairly low speeds, they tended to form flattened ravioli shapes like Pan and Atlas. When they struck more of a glancing blow before merging, they ended up elongated like Prometheus.
Depending on the range of angles the researchers selected to simulate, 20 to 50 per cent of the collisions resulted in ravioli or cigar shapes – which makes sense because about half of Saturn’s small moons have those shapes.
The simulations showed that a moonlet smashup could also account for the ridge around the edge of Iapetus, a larger walnut-shaped moon.
“We know these sorts of collisions were likely pretty common in the early portions of solar system formation when there was a lot of material flying around,” says , also at Johns Hopkins University. “The fact that this new model can form both the ravioli moons and the oblong ones shows considerable promise.”
In the past, the odd ridges around Pan and Atlas have been explained as particles from Saturn’s rings that fell onto their surfaces. The Cassini spacecraft, which burned up in Saturn’s atmosphere in September 2017, only took a few images of each of these small moons, and Stickle says that we’ll have to go back and take more pictures before we have enough information to be sure how they formed.
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Nature Astronomy
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