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Images like this one, showing bright wispy streaks thought to be ice revealed by subsidence of crater walls, are leading to the view that Phoebe is an icy-rich body overlain with a thin layer of dark material. Obvious downslope motion of material occurring along the walls of the major craters in this image is the cause for the bright streaks, which are over-exposed here. Significant slumping has occurred along the crater wall at top left.
The slumping of material might have occurred by a small projectile punching into the steep slope of the wall of a pre-existing larger crater. Another possibility is that the material collapsed when triggered by another impact elsewhere on Phoebe. Note that the bright, exposed areas of ice are not very uniform along the wall. Small craters are exposing bright material on the hummocky floor of the larger crater.
Elsewhere on this image, there are local areas of outcropping along the larger crater wall where denser, more resistant material is located. Whether these outcrops are large blocks being exhumed by landslides or actual 'bedrock' is not currently understood.
The crater on the left, with most of the bright streamers, is about 45 kilometers (28 miles) in diameter, front to back as viewed. The larger depression in which the crater sits is on the order of 100 kilometers (62 miles) across. The slopes from the rim down to the hummocky floor are approximately 20 kilometers (12 miles) long; many of the bright streamers on the crater wall are on the order of 10 kilometers (6 miles) long. A future project for Cassini image scientists will be to work out the chronology of slumping events in this scene.
This image was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 78 degrees, and from a distance of 11,918 kilometers (7407 miles). The image scale is approximately 70 meters (230 feet) per pixel. No enhancement was performed on this image.
In 2012, Cassini scientists published their findings, derived from theoretical modeling using data from a variety of Cassini instruments, that Phoebe almost surely is a large planetesimal which formed in the early Kuiper Belt before being captured into orbit around Saturn. The new analysis showed Phoebe likely was heated early in its history and consequently is nearly spherical and has denser rock-rich material concentrated near its center. Its average density, in fact, is about the same as Pluto, another object in the Kuiper Belt. Phoebe was likely captured by Saturn when it got close to the giant planet in the past.
[Caption updated April 26, 2012.]
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.