Icy crater wall
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.
The Cassini-Huygens mission is a co-operative project of NASA, ESA and the Italian Space Agency.
Credits: NASA/JPL/Space Science Institute