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All about Phoebe
27 May 2005
| | | | | | Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during the Cassini-Huygens fly-by on 11 June 2004.
The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. This phenomenon has been observed on other icy satellites, such as Ganymede around Jupiter.
When impacting bodies hit the surface of Phoebe, the collisions excavated fresh, bright material - probably ice - from under the surface layer. Further evidence for this can be seen on some crater walls where the darker material appears to have slid downwards, exposing more light-coloured material. Some areas of the image that are particularly bright - especially near lower right - are over-exposed.
This view was obtained from a distance of approximately 32 500 kilometres. The image scale is approximately 190 metres per pixel. No enhancement was performed on this image.
Credits: NASA/JPL/Space Science Institute | |  |  | |
| | | | | 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 been caused 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 kilometres in diameter, front to back as viewed. The larger depression in which the crater sits is on the order of 100 kilometres across. The slopes from the rim down to the ‘hummocky’ floor are approximately 20 kilometres long; many of the bright streamers on the crater wall are on the order of 10 kilometres 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 with an angle of 78 degrees between the Sun, Phoebe and the spacecraft, from a distance of 11 918 kilometres. The image scale is approximately 70 metres per pixel. No enhancement was performed on this image.
Credits: NASA/JPL/Space Science Institute | | | | |
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