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In its final look around the Saturn system, the Cassini spacecraft captured one last image of the propellers nestled in the outermost part of the main rings. These beautiful patterns show up when tiny moonlets disturb the material of the rings. But they’re not just nice to look at – they also offer clues to how planets form.

In these images, “the moon is as small as a pixel, so we don’t really see it”, says at the SETI Institute. “But we see the disturbance.”

Cassini’s multi-year imaging of the propellers nestled in Saturn’s outermost rings is the first time people have tracked the orbit of objects embedded in a disc rather than moving in free space. “This has strong parallels to what happens when solar systems form,” Tiscareno says. “The moonlet is forming embedded in a disc.”

Ů��С��Ƶs are interested in both how a disc can affect an object, such as a moonlet around Saturn or a protoplanet in an alien system, and how the object affects the disc. “We can’t go back in time and see our solar system when it was forming, but Saturn can give us a window into some of these processes,” says Tiscareno.

Jewels in Saturn’s crown

Ever since researchers realised that these propeller-shaped patterns were a window onto planet formation, Cassini has regularly checked on both the large propellers of the outer ring and the swarms of smaller ones in the A-ring, the outermost of Saturn’s bright rings. The most recent observation before today was last week.

The larger propellers are about the size of several city blocks, and the smaller ones a soccer pitch. There are so many propellers close together in the smaller swarms that researchers can’t track individual objects between images.

“We don’t know how long-lived the swarming objects in the propeller belts are. We don’t know whether their orbits are changing in important ways,” says Tiscareno. “We just can’t track the same object time after time.”

But the larger propellers are distinct enough that scientists can easily pick them out weeks, months or even years after they were first spotted. By regularly updating the orbits of these unusual objects based on data from Cassini’s passes through Saturn’s rings, scientists have identified two types of motion.

Much of the time, the propeller’s shape is caused by a gradual interaction of the disc’s gravity with the moon, but sometimes a catastrophic event can cause it, says Tiscareno. For example, a collision can disturb the moonlet’s orbit, or the moon can grow too large and shed part of its mass.

“They are as big as they can get given their current location,” he says. When a moonlet accretes more material, it grows larger and starts protruding out, only to be sheared off by the gravitational pull of Saturn’s ring material.

Cassini’s final image of the propellers doesn’t reveal anything unexpected or new, but it does complete a several-year project tracking these unusual objects. “It’s the last one, and that’s very poignant,” says Tiscareno.

Read more: Cassini’s Grand Finale: The spacecraft that unveiled Saturn