Posted on behalf of Richard A. Lovett
Long, skinny propeller-shaped holes in Saturn’s rings are helping scientists understand the early history of the Solar System. The features, shaped like aeroplane propellers, are several kilometers wide and thousands of kilometers long. They are formed by the gravity of tiny moons, only a kilometer in diameter, which deflect the orbits of nearby ring particles, leaving dark gaps.
Some of them have been unofficially named after famous aviators. “Flight enthusiasts will recognize Bleriot, Earhart, Santos-Dumont, and others,” says Carolyn Porco, head of the imaging team for the Cassini spacecraft, which spotted the first one in 2005. “Eleven have been tracked, but we estimate that there are probably dozens.”
The propellers lie in the outer section of Saturn’s A Ring, the outermost of the planet’s dense rings. They are important because they mark the locations of the moonlets that create them, even though the moonlets themselves are too small to be seen. This allowed the scientists to track them over the course of four years, seeing how their motion changes in the rings’ kaleidoscopically shifting gravity.
“Scientists have never tracked disk-embedded objects anywhere in the universe before,” says Matthew Tiscareno of Cornell University in Ithaca. He and Porco are coauthors of a paper that appeared in Astrophysical Journal Letters on 8 July. “All the moons and planets we knew about before orbit in empty space,” Tiscareno adds.
Propeller features had previously been found in another section of Saturn’s A ring, but these were up to 100 times smaller, Porco says. They were also much more numerous, making it impossible to track the movement of individual ones.
The motion of the moonlets creating the propellers is important to astrophysicists seeking a better understanding of the early history of the Solar System, when the growing planets were embedded in a debris disk much like Saturn’s rings.
Collisions and gravitational interactions cause particles in the debris disk to accumulate into larger clumps that continue interacting with each other until eventually a solar system results. The details of this process have been very hard to model, however.
In the four years that the scientists have been watching them (in one case, with more than 100 snapshots), the giant propellers have shifted orbits intriguingly. Scientists aren’t sure what’s causing this: possibly collisions with other ring particles, or possibly the gravitational forces of distant, larger moons, outside the rings. The team will continue monitoring them, Tiscareno said, trying to determine more precisely how moonlets and ring particles interact – providing data for scientists attempting to model the formation of new solar systems.
“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,” says Porco. “It allows us a glimpse into how the Solar System ended up looking the way it does.”