Cassini observations by several instruments have revealed the source of Saturn's broadest and faintest ring. Recent observations show that tiny particles of frozen water ice are streaming outward into space from the south polar region of the moon Enceladus.
The source of geological activity on Enceladus is a mystery. "We're amazed to see ice geysers on this little world that was thought to be cold and dead long ago," commented Dr. Dale Cruikshank of NASA Ames Research center, a member of the visual and infrared mapping spectrometer team. "Some unexpected process is vigorously heating the interior of Enceladus, especially the south polar region, and causing the ejection of the plumes of ice particles."
As the icy plumes jet out from the moon, the larger particles probably follow paths that mostly bring them back to the surface, while the smaller particles are nudged by sunlight into orbits around Saturn.
"Most of these small particles probably re-impact the moon, but the smallest ones eventually disperse as a result of radiation (light) pressure and interactions with Saturn's magnetosphere to form the broad E ring," said Dr. Mark Showalter of the SETI Institute, Mountain View, Calif. Thus, the E ring is currently being regenerated by some kind of geological activity in the interior of Enceladus.
During the Cassini spacecraft's flyby on Nov. 26, the visual and infrared mapping spectrometer instrument measured the spectrum of the polar plumes of Enceladus. "We see a very clear signature of small ice particles in the plume data, in the form of a strong absorption band at 2.9 microns in an otherwise featureless spectrum," said Dr. Phil Nicholson, professor of astronomy at Cornell University, Ithaca, N.Y. Nicholson is a member of the visual and infrared mapping spectrometer science team.
The visual and infrared mapping spectrometer images of Enceladus show not only the plume over the south pole, but also the dark side of the moon, silhouetted against a foggy background of light from the E Ring. Measurements of the spectrum show a very similar signature of small ice particles to that in the plumes, confirming earlier expectations that Enceladus is indeed the source of the E ring.
Preliminary analyses suggest that the average size of the particles in the plume is about 10 microns (1/100,000 of a meter), whereas the particles in the E ring are about three times smaller. The sunlit surface of Enceladus itself is also composed of water ice, but with a much larger grain size than the plume.