Prometheus, the moon that carves streamers in the F ring
Prometheus is an elongated, potato-shaped moon of Saturn, about 136 km long and 86 km in mean diameter, that orbits just inside the narrow F ring. It is the inner of the F ring's two "shepherd" moons, with Pandora on the outside. Each time Prometheus swings out toward the ring on its slightly eccentric orbit, its gravity pulls out streamers of material and leaves dark channels, producing a repeating pattern that Cassini filmed in detail.
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Discovery and name
Prometheus was found in October 1980 in Voyager 1 images, shortly after the narrow F ring itself had been discovered by Pioneer 11 in 1979. The discovery was credited to the Voyager imaging team, with Stewart Collins and colleagues identifying the moon. Its orbital neighbor on the outside of the ring, Pandora, was discovered in the same images. In Greek mythology, Prometheus is the Titan who stole fire from the gods for humanity, and the brother of Epimetheus, whose name is borne by one of the co-orbital moons a little farther out.
A shepherd of the F ring
The F ring is a narrow, braided-looking ring about 140,200 km from Saturn's center, beyond the A ring. Its bright core is only tens of kilometers wide. Voyager scientists proposed that Prometheus and Pandora confine it from both sides, each moon nudging particles away from its own orbit. This "shepherding" model, first developed by Peter Goldreich and Scott Tremaine for the rings of Uranus, explains part of what is seen, but Cassini revealed a more complicated picture: the F ring is also shaped by collisions with small moonlets embedded in or near it, and Prometheus's role turned out to be far more visible than Pandora's. The broader topic is covered on the shepherd moons page.
Streamers and channels
Prometheus's orbit and the F ring are both slightly eccentric. Once per orbit, about every 14.7 hours, Prometheus reaches its farthest point from Saturn and comes closest to the ring. Its gravity then pulls a streamer of ring material inward, toward the moon. As Prometheus moves away, the disturbed particles, now on slightly different orbits, spread out and leave behind a dark channel, a lane depleted of material.
Because particles closer to Saturn orbit faster, each new channel is sheared by differential rotation and tilts over time. The result is a series of regularly spaced streamer-channel structures along the ring, one created on each orbit of the moon, like the strokes of a comb drawn repeatedly through the ring. Cassini's movie sequences show the pattern in progress, with the newest channels sharp and the older ones fading as they are drawn out.
The strength of the interaction varies over a longer cycle. The orientations of the eccentric orbits of Prometheus and the F ring precess at different rates, so the moon's closest approach to the ring becomes deeper or shallower over a period of roughly 17 years. Around 2009 the alignment brought Prometheus to its closest, and it may then have dipped into the outer parts of the ring material.
Clumps and jets
Prometheus's repeated disturbance may also help trigger the clumping of ring particles into small, temporary aggregates. Cassini observed bright "mini-jets" and clumps in the F ring, which researchers attribute to collisions between ring material and small bodies, some of which may form in Prometheus's wake.
A dynamic, chaotic orbit
Prometheus and Pandora interact gravitationally with each other. Their orbits are close to a 121:118 resonance, and their mutual tugs are strongest roughly every 6.2 years, when the two moons' orbits are most closely aligned. These kicks make their motions chaotic over the long term. The effect was noticed during the 1995 ring-plane crossing, when Hubble Space Telescope observations found Prometheus about 20 degrees behind the position predicted from Voyager data, while Pandora was ahead of its predicted position.
A porous, elongated body
Prometheus is strongly elongated, with its long axis pointed toward Saturn, as tidal forces arrange it. Cassini images show a surface marked by ridges, valleys and craters, partly blanketed with fine material. Its density of about 0.48 g/cm³ points to a porous body of loosely bound ice. The moon's elongated shape and low density are typical of the ring moons, a group that also includes Pan, Daphnis and Atlas.
FAQ
What does Prometheus do to Saturn's F ring?
On each orbit, when Prometheus comes closest to the F ring, its gravity pulls out a streamer of material and leaves behind a dark channel. The repeated process creates a regular pattern of tilted features along the ring.
How big is Prometheus?
Prometheus measures about 136 × 79 × 59 km, for a mean diameter of about 86 km. It is one of the larger inner moons, but still irregular in shape.
Is Prometheus a shepherd moon?
Yes. Prometheus orbits just inside the F ring and, together with Pandora outside it, has been called a shepherd of the ring. Cassini showed that its influence is the more visible of the two.
When was Prometheus discovered?
Prometheus was discovered in October 1980 in images taken by Voyager 1, along with Pandora.
Sources
- NASA Science: Saturn's moons
- NASA: Cassini mission
- Murray, C. D. et al., "How Prometheus creates structure in Saturn's F ring", Nature 437 (2005)
- Goldreich, P. and Rappaport, N., "Chaotic motions of Prometheus and Pandora", Icarus 162 (2003)