Ravioli moon NASA/JPL-Caltech/Space Science Institute

It doesn’t carry the weight of the world – or even of Saturn’s rings. The best images yet of Saturn’s small moon Atlas show that it is a fluffy oddball with less responsibility than we thought.

On 12 April, NASA’s Cassini spacecraft dove close to Atlas to bring us the best images of this odd little moon we’ll see. With a resolution of up to 84 metres per pixel, these new photographs improve our view of Atlas by a factor of two. This is the last and closest flyby of the moon before the Cassini mission ends in September.

“It’s very interesting to see it close up after decades of knowing it was there,” says Richard Terrile at NASA’s Jet Propulsion Laboratory, who discovered the moon in 1980. “Finally seeing it as a real piece of real estate is very, very interesting.”


Some of Saturn’s moons, called shepherd moons, use their gravity to keep the planet’s famous rings in check. But Atlas appears to be a shepherd moon that isn’t responsible for shepherding ring particles.

“At the time [we found it], we thought the satellite was holding out the edge of the A ring,” Terrile says. But instead, the ring’s shape is held by two other moons, Janus and Epimetheus. “This is a really interesting kind of dynamical dance that these moons do with the ring particles.”

We already knew Atlas has a UFO-like ridge around its equator, but surprisingly, the new images show that ridge is smooth.

“It looks more subdued than I expected,” Terrile says. “It looks like it’s covered in some kind of fluffy material.”

“What is especially interesting is the extent to which the soft material seems to bury and mute any crisp-looking topographic features even on the central ‘core’ structure,” says Paul Helfenstein at Cornell University in Ithaca, New York, who helped plan the flyby.

Another moon, Pan, also has an equatorial ridge, but another recent flyby showed its ridge is rough with tension cracks and craters. The two moons are approximately the same size, but Pan is embedded within a ring, and Atlas is along the outer edge.

The dominant theory for how the ridges form is that because the moons’ diameters are so much larger than the ring’s thickness, they gather material along their equators as they plow through stray ring particles. But it might not be that simple. “It may also be some kind of gravitational tidal effect from being near all this ring material,” says Terrile.

Understanding Pan and Atlas may be key to understanding gravity’s role in all of Saturn’s moons. “The same gravity that causes all these weird phenomena that we’re seeing on these little moons causes energy to be pumped into some of the larger ones,” says Terrile. “And that energy can create under-ice oceans, maybe even habitable zones.”