NASA's Cassini spacecraft has discovered a new class of moons in the rings of Saturn that create distinctive propeller-shaped gaps in ring material.
It's the first time scientists have been able to track the orbits of individual objects in a debris disk.
"Observing the motions of these disk-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disk of material surrounding the early sun," said Carolyn Porco, leader of the Cassini imaging team.
"It allows us a glimpse into how the solar system ended up looking the way it does."
In 2006, a class of 'moonlets' was discovered to be clearing propeller-shaped areas in Saturn's dense outer A ring. The moonlets, which were estimated to number in the millions, were not large enough to clear out their entire path around Saturn.
The new observations show a group of larger moons - about a kilometer in diameter - in another part of the A ring farther out from Saturn, creating propellers hundreds of times bigger.
The propeller features are up to several thousand kilometers long and several kilometers wide. The moons embedded in the ring appear to kick up ring material half a kilometer above and below the ring plane - well beyond the typical ring thickness of about 10 meters.
The team estimates that there are dozens of these giant propellers - and several have been imaged multiple times. One has been 'a veritable Forrest Gump', says the team, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation.
"Scientists have never tracked disk-embedded objects anywhere in the universe before now," said Matthew Tiscareno, a Cassini imaging team associate. "All the moons and planets we knew about before orbit in empty space."
Over the last four years, the giant propellers have shifted their orbits, but scientists are not yet sure why. Their path may be upset by other smaller ring particles, but the gravitational attraction of large moons outside the rings may also be a factor.
Scientists will continue monitoring the moons to see if the disk itself is driving the changes, similar to the interactions that occur in young solar systems. If it is, Tiscareno said, this would be the first time such a measurement has been made directly.
"Propellers give us unexpected insight into the larger objects in the rings," said Linda Spilker, Cassini project scientist. "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."