Atlas, Saturn's flying-saucer moon
Atlas is a small moon of Saturn, about 30 km in mean diameter, that orbits just outside the outer edge of the A ring, at about 137,700 km from the planet's center. Its defining feature is a broad, smooth equatorial ridge that makes it look like a flying saucer, an effect even more pronounced than on its neighbor Pan. The ridge is thought to be made of ring particles that Atlas has swept up over time.
Contents
Discovery by Voyager 1
Atlas was discovered by Richard Terrile of the Voyager imaging team in images taken by Voyager 1 in October 1980, a few weeks before the spacecraft's closest approach to Saturn. It was one of several small inner moons found during the Voyager flybys, along with Prometheus and Pandora. Its name comes from the Titan of Greek mythology who was condemned to hold up the sky, a reference to its position at the edge of the rings it was thought to support.
A saucer in orbit
Voyager could only resolve Atlas as a faint, elongated dot. Cassini images from 2004 onward showed a pointed equatorial profile, and on 12 April 2017, during the Ring-Grazing orbits, Cassini passed about 11,000 km from Atlas, obtaining its sharpest views. They revealed a lens-shaped body, roughly 41 km across at the equator but only about 19 km from pole to pole.
The ridge is large compared with the moon's core and appears smooth, with few visible craters, while the central body is more textured. That contrast is the key clue to its origin. Like Pan, Atlas orbits almost exactly in the ring plane, so the fine particles it captures fall mostly on its equator. Over time, those deposits pile up into a skirt. Researchers have estimated that the ridge could be geologically young compared with the core, possibly formed after the core itself took shape, although such ages remain uncertain.
The surface color of Atlas is similar to that of the A ring, which supports the idea that its outer layers are made of ring material. Its density of about 0.4 to 0.5 g/cm³ indicates a porous interior, probably a loose aggregate of icy particles.
A shepherd that turned out not to be one
When it was discovered, Atlas was thought to be the moon that keeps the outer edge of the A ring sharp, much as Prometheus and Pandora were thought to confine the F ring. It is indeed close: about 900 km beyond the edge, which lies at 136,775 km from Saturn's center.
Later analysis showed that the edge is controlled mainly by a resonance with the more massive pair Janus and Epimetheus, whose 7:6 resonance falls right at the edge of the ring. Atlas is far too small to do the job on its own. It does, however, influence the outer A ring in subtler ways, and the edge region displays complex patterns related to several moons. The general topic is covered on the shepherd moons page.
Atlas's own ring
Cassini found a faint, narrow ring of dust along the orbit of Atlas in 2004, designated R/2004 S 1. It is probably made of small particles knocked off Atlas and other nearby bodies by micrometeorite impacts, which remain on roughly the same orbit. Similar faint rings or arcs are associated with Janus and Epimetheus and with the small moons between Mimas and Enceladus.
Atlas and its orbital neighbors
Atlas's orbit is subtly perturbed by Prometheus, with which it is close to a 54:53 resonance, meaning the orbital periods are in that near-exact ratio. This interaction causes small, measurable changes in Atlas's orbit. Its environment is crowded: between the outer edge of the A ring and the orbit of Janus lie Atlas, Prometheus, the F ring and Pandora, all within roughly 14,000 km of one another.
Understanding the shapes of Atlas and Pan has implications beyond these two moons. If small moons near the rings grow by accreting ring particles, their ridges record how ring material moves today, and possibly how the rings have evolved, a question tied to the debate over the age of the rings.
FAQ
Why does Atlas look like a flying saucer?
Atlas has a broad equatorial ridge made of fine material it has accumulated from Saturn's rings. Because it orbits in the ring plane, particles land mainly around its equator, building a smooth rim that gives it a saucer shape.
How big is Atlas?
Atlas measures about 41 × 35 × 19 km, for a mean diameter of about 30 km. The large difference between its equatorial width and its polar height comes from its ridge.
Does Atlas shepherd Saturn's A ring?
Atlas was long thought to keep the A ring's outer edge sharp, but that role is now attributed mainly to a resonance with Janus and Epimetheus. Atlas only has a minor influence.
Who discovered Atlas?
Richard Terrile of the Voyager imaging team discovered Atlas in Voyager 1 images taken in October 1980.
Sources
- NASA Science: Saturn's moons
- NASA: Voyager mission
- Charnoz, S. et al., "The equatorial ridges of Pan and Atlas: terminal accretionary ornaments?", Science 318 (2007)
- Buratti, B. J. et al., "Close Cassini flybys of Saturn's ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus", Science 364 (2019)