Cassini Swoops Past Saturn’s Moon Atlas for a Final Glorious Photoshoot
published during a waning gibbous moon.

During NASA’s Cassini spacecrafts’ flyby on April 12th, 2017, raw images of Saturn’s oddly-shaped moon, Atlas, were taken from a distance of about 11,000 kilometers. The images are the closest ever taken of Atlas and the last we’ll see before Cassini ends its mission in late 2017.

Like its sibling moon, Pan, Atlas has a thick equatorial ridge giving it an uniquely bulged appearance. But unlike Pan’s sharply turned belt, Atlas’ equatorial ridge is more rounded, as though the moon is bobbing around orbit wearing an inner tube. Scientists hypothesize that its ridge was formed by a build up of stray material that escaped from Saturn’s outmost ring, the A ring.


Cassini captures the closest-ever images of Saturn’s moon Pan from just 24,572 kilometers away. Credit: NASA/JPL-Caltech/Space Science Institute

While similar in shape, these new images reveal the moons have distinctly different textures. Pan’s surface is roughly cratered and its equatorial ridge is stressed with cracks, whereas Atlas’ is powdery smooth. These differences provoke new questions about Atlas’ geology and formation. “Does that mean it’s younger?” asks Matthew Tiscareno, a planetary scientist with the SETI Institute on the Cassini Imaging Team. “Or is Pan’s apron more icy and Atlas’s apron more snowy? But why would they be different?”


Raw image of Cassini’s Atlas flyby on April 12, 2017. Credit: NASA/JPL/Space Science Institute

First discovered in 1980 by Richard Terrile, a Voyager scientist at NASA’s Jet Propulsion Laboratory, Atlas was thought to be instrumental in giving Saturn’s A ring its neat edges.  Thus, Terrile named the moon after a Hero from Greek Mythology who was ordered by Zeus to uphold the entire Universe upon his shoulders, akin to how the little moon was thought to be holding Saturn’s enormous ring in place. But the real dynamics proved to be more complicated than that.

Atlas is known as a “shepherd” moon, a moniker describing how the gravitational field of the moon “shepherds” stray particles away from the planet’s rings–allowing the rings to stay neat and tidy, but Atlas isn’t shepherding the A ring after all. “It turns out that a lot of what was thought to be classic examples of shepherding are not actually that simple,“ says Tiscareno.

Instead, the outer edge of the A ring is dominated by the gravitational pull of Saturn’s larger and more distant moons, Janus and Epimetheus. “Atlas is a fly buzzing around the horse, and Janus is the person with a bit and bridle actually governing the horse,” says Tiscareno.