Saturn's shepherd moons
Shepherd moons are small moons whose gravity confines a narrow ring or keeps a gap open, much as sheepdogs keep a flock together. At Saturn, the best known are Prometheus and Pandora, which flank the narrow F ring; Pan, which clears the Encke Gap; and Daphnis, which maintains the Keeler Gap. On a smaller scale, tiny moonlets embedded in the A ring create propeller-shaped disturbances.
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How shepherding works
In 1979 Peter Goldreich and Scott Tremaine proposed the mechanism to explain Uranus's very narrow rings, which collisions between particles should have spread out quickly. Their idea: a moon orbiting just inside a ring gives angular momentum to the particles and pushes them outward, while a moon just outside takes angular momentum away and pushes them inward. Squeezed from both sides, the particles stay confined to a narrow band. The following year, in 1980, Voyager 1 found two moons on either side of Saturn's F ring.
A moon embedded inside a broad ring does the opposite: it pushes material away on both sides and opens a gap. In both cases the moon itself slowly drifts away from its ring in exchange.
Prometheus and Pandora, guardians of the F ring
| Moon | Distance from Saturn's center | Mean size | Role |
|---|---|---|---|
| Atlas | about 137,670 km | about 30 km | just outside the A ring's edge |
| Prometheus | about 139,380 km | about 86 km | inner F ring shepherd |
| F ring | about 140,200 km | 30 to 500 km wide | narrow, braided ring |
| Pandora | about 141,720 km | about 81 km | outer F ring shepherd |
Prometheus, the more massive of the pair, is the more active. On every orbit, about every 14.7 hours, it approaches the F ring and draws out streamers of material, leaving dark channels behind it that repeat like the teeth of a comb. The effect peaks when its elliptical orbit brings it closest to the ring's core. Pandora, farther out, has a subtler influence. Their exact roles in confining the F ring are debated: some models suggest the core's position is set mainly by resonances with Prometheus.
The F ring itself is among the most changeable objects in the solar system. Cassini saw twists, knots and jets of material produced by kilometer-sized bodies plowing through its core, as well as the aftermath of recent collisions.
Pan and Daphnis, the gap moons
Pan, discovered in 1990 by Mark Showalter in Voyager images, orbits in the middle of the roughly 325 km wide Encke Gap. As it passes the gap's edges, it imprints regular wavy patterns and scallops. Cassini photographed it up close in 2017: its "ravioli" shape, with a pronounced equatorial ridge, is thought to come from ring material piling up on its equator.
Daphnis, discovered by Cassini in 2005, is only about 8 km across yet keeps the 40 km wide Keeler Gap open. Its slightly tilted, eccentric orbit raises waves along the gap's edges, both within the ring plane and vertically, up to about 1.5 km high. At the 2009 equinox, low-angle sunlight revealed these waves through the shadows they cast.
Atlas, the flying saucer
Atlas orbits just beyond the A ring's outer edge. It was long considered that edge's shepherd, but the edge is actually maintained by a 7:6 resonance with the co-orbital pair Janus and Epimetheus. Like Pan, Atlas has a broad equatorial ridge that makes it look like a flying saucer, probably built up from accreted ring particles.
Big moons at work too
Shepherds are not the only ring sculptors. Mimas, about 186,000 km from Saturn, sets the B ring's outer edge through a 2:1 resonance and helps maintain the Cassini Division. Janus, Epimetheus, Prometheus, Pandora and other moons launch hundreds of density and bending waves in the A ring through resonances. Rings and moons form a coupled system that constantly trades energy and angular momentum.
Propellers: moons in the making?
In 2006, Matthew Tiscareno's team announced the discovery in Cassini images of small, airplane-propeller-shaped structures in the A ring. Each betrays a moonlet 100 m to about 1 km across, too small to clear a full gap but massive enough to carve two offset grooves on either side of itself. Hundreds have been identified, and thousands probably exist. The largest, beyond the Encke Gap, were nicknamed after famous aviators: Blériot, Earhart, Santos-Dumont. Tracked for more than a decade, Blériot showed a slightly wandering orbit, a sign of its interactions with the ring.
In 2013, a small clump at the A ring's edge, nicknamed "Peggy," was interpreted as a moon possibly forming, or even escaping the rings. Such objects offer a miniature view of planet formation in disks of gas and dust, and support the idea that some of Saturn's small moons were born from the rings (see ring age and origin).
FAQ
What is a shepherd moon?
A small moon whose gravity confines a narrow ring or holds a gap open in a broader ring, pushing particles away through an exchange of angular momentum.
Which moons shepherd Saturn's F ring?
Prometheus on the inside and Pandora on the outside. Prometheus is the more influential, pulling streamers of material out of the ring on every orbit.
What is a propeller in Saturn's rings?
A propeller-shaped disturbance created by a moonlet 100 m to about 1 km across embedded in the A ring. Cassini found hundreds of them starting in 2006.
Why are Pan and Atlas shaped like flying saucers?
Their equatorial ridges are thought to be ring material that accumulated on their equators, where these moons cross the very thin ring plane.
Sources
- P. Goldreich and S. Tremaine, "Towards a theory for the Uranian rings," Nature, 1979
- M. S. Tiscareno et al., "100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures," Nature, 2006
- C. D. Murray et al., "How Prometheus creates structure in Saturn's F ring," Nature, 2005
- NASA Science, Saturn