From: Matthew Brown <[EMAIL PROTECTED]> Date: 2 November 2004 12:42:19 PM PST Subject: Prometheus caught stealing from Saturn's rings
http://www.newscientist.com/news/news.jsp?id=ns99996614
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.
Simulated success
"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.
