The twist in Saturn’s rings

A propeller-shaped disturbance in Saturn's rings

A propeller-shaped structure formed by the influence of an unseen moon on the particles in Saturn's rings, is brightly illuminated by sunlight in this image obtained by NASA’s Cassini spacecraft.

  • Dozens of unseen moons within Saturn’s rings
  • Cause surrounding ring material to twist and warp
  • Twists look like giant propellers

Scientists using NASA’s Cassini spacecraft—currently orbiting through the Saturnian system—are stalking a new class of moons that create distinctive “propeller-shaped” gaps in Saturn’s rings.

Scientists first discovered double-armed propeller features in 2006 in an area now known as the “propeller belts” in the middle of Saturn’s outermost dense ring, the A ring.

The gaps in the rings are created by a new class of moonlets—smaller than known moons, but larger than the particles in the rings—that could clear the space immediately around them.

Those moonlets, estimated to number in the millions, are not large enough to have cleared out their entire path around Saturn, as do the moons Pan and Daphnis.

The new research, led by Matthew Tiscareno, a Cassini imaging team associate based at Cornell University, reveals a new cohort of larger and rarer moons in another part of the A ring farther out from Saturn.

A propeller-shaped disturbance in Saturn's rings

A "propeller" is brightly illuminated on the sunlit side of Saturn's rings, where the ring has been disturbed and ring material boosted above the ringplane.

With “propellers” as much as hundreds of times as large as those previously described, the scientists have been able to track them for as long as four years.

The features are up to several thousand kilometres long and several kilometres wide. The gravitational effect of the moons embedded in the ring appears to kick up ring material as high as 500 metres above and below the flat ring plane, which is well beyond the typical ring thickness of about 10 metres.

Dozens of giant “propellers”

Cassini is too far away to see the moons amid the swirling ring material around them, but scientists estimate that they are about a kilometre in diameter because of the size of the propellers.

Tiscareno and colleagues estimate that there are dozens of these giant propellers; 11 of them were imaged multiple times between 2005 and 2009.

One of them, nicknamed Bleriot after the famous aviator Louis Bleriot, has been a veritable Forrest Gump, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation over this time.

“Scientists have never tracked disc-embedded objects anywhere in the universe before now,” Tiscareno said. “All the moons and planets we knew about before orbit in empty space.”

“In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects,” added Tiscareno. “With this new discovery, we can now track disc-embedded moons individually over many years.”

“Propellers give us unexpected insight into the larger objects in the rings,” said Linda Spilker, Cassini project scientist based at NASA’s Jet Propulsion Laboratory. “Over the next seven years, Cassini will have the opportunity to watch the evolution of these objects and to figure out why their orbits are changing.”

A short movie of one of Saturn's propellers

A short movie of one of Saturn's propellers

A glimpse into the Solar System’s past

The observations also mark the first time scientists have been able to track the orbits of individual objects in a “debris disc”, which is what the rings are—billions upon billions of chunks of ice of all sizes, encircling the planet.

This gives scientists an opportunity to “time-travel” back into the history of our Solar System, when the planets were forming within a much bigger version of Saturn’s rings, circling the youthful Sun.

“Observing the motions of these disc-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disc of material surrounding the early Sun,” said Carolyn Porco, Cassini imaging team lead based at the Space Science Institute.

“It allows us a glimpse into how the Solar System ended up looking the way it does.” And, by extension, what by might be happening in other star systems.

Adapted from information issued by NASA / JPL / SSI.

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