In Greek mythology, Prometheus stole fire from the Gods. Now, Saturn's tiny moon Prometheus is showing similar tendencies, repeatedly stealing material from planet's rings, according to new images taken by the Cassini probe.
The image was taken on 29 October 2004 from a distance of 791,000 kilometres. It shows a sliver of light about 300 km inside Saturn's F ring, which lies beyond its main ring system and contains at least three bright strands of ice and dust.
That sliver is the partially illuminated, potato-shaped moon Prometheus, which is about 150 km in length. Prometheus and another moon - Pandora, which orbits just outside the ring - bookend the ring and have been called "shepherd" moons because they appear to keep the ring in line.
But this image confirms that sometimes the moon also strips material from its neighbouring ring, as a stream of material appears to be drawn from the innermost bright strand toward the moon.
Dark lanes and streamers
This type of feature - called a streamer - was first seen around Saturn by the joint US-European Cassini spacecraft earlier in 2004. It is thought to occur when Prometheus - which travels in an elliptical orbit around the planet every 14 hours or so - reaches its closest point to the F ring. It is currently unclear to astronomers whether the wobbles in the bright central strand near Prometheus are associated with the moon.
The image also shows evidence of previous close approaches. A dark horizontal band in the upper right section of the image is thought to be the hole left behind from a previous pass in which Prometheus siphoned off ring material.
These dark lanes - called striations - were also first seen by Cassini earlier in 2004. But streamers and dark lanes have never been seen together in the same image.
"When we saw this we were blown away," says Mike Evans, an astronomer at Queen Mary, University of London, UK. "Before, we postulated streamers and dark lanes were connected and were effectively the same feature. But now you've got an image of Prometheus yanking material out and then these dark lanes in the same image."
"It's amazing to see something that looks just like what was seen in numerical simulations," agrees Luke Dones, a Cassini imaging team member at the Southwest Research Institute in Boulder, Colorado, US.
Earlier observations from NASA's Voyager spacecraft in 1980 and 1981 had hinted that the F ring might be divided into bright strands, but the ring looked different when it was observed at different times.
Then, observations from Earth when a star passed behind the ring in 1989 showed "a perfectly well-behaved narrow ring", Dones told New Scientist. He says the new Cassini data suggests "it's a complicated system" and not just one ring.
"Maybe there's a belt of little moonlets in the ring providing material to maintain the strands," suggests Evans. "We just don't know."
Cassini went into orbit around Saturn in July 2004.
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