Saturn’s rings take seven siblings to stay in place

Several moons keep the A ring from falling apart.
By | Published: October 20, 2017 | Last updated on May 18, 2023
The neat “grooves” in Saturn’s A ring are actually density waves caused by the seven moons that keep it in place.
Cassini Imaging Team, SSI, JPL, ESA, NASA
New data from the Cassini spacecraft suggests that Saturn’s outermost “main” A ring is held in place by seven moons, rather than the single originally suspected shepherd, Janus. The small moons Pan, Atlas, Prometheus, Pandora, and Epimetheus play important roles, but Mimas, a larger Enceladus-sized world, does a lot of heavy lifting in the rings as well.

The moons’ combined influence keeps the A ring fairly compact. Without them, the ring particles would naturally spread out at that distance from the planet, causing the ring to disappear. Instead, the moons provide gravitational “confinement” to keep the ring in place. As they orbit, the moons maintain resonances (in which one moon completes X number of orbits, while another completes Y) that ensure the A-ring keeps up appearances, which some NASA researchers have compared to the grooves on a vinyl record. In reality, the grooves are actually density waves caused by the moons, as their resonances carve out these neatly delineated features.

Pan, Atlas, Prometheus, Pandora, Epimetheus, Mimas, and Janus are responsible for maintaining Saturn’s A ring.
NASA/JPL/David Seal
This is just one discovery in the thousands more that will come from the recently ended Cassini craft’s data. Cassini crashed into Saturn on September 15, but spent its final months swinging through the rings of the sixth planet, getting unprecedented closeups of the clumpy matter and icy particles that may be the remains of larger moons that got too close to the planet.

As new details emerge, researchers may grow closer to understanding how the rings interact with each other and the planet as well.