
When impactors slammed into the surface of Phoebe, the collisions excavated fresh, bright material -- probably ice -- underlying 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-colored material. Some areas of the image that are particularly bright -- especially near lower right -- are over-exposed.
http://www.nasa.gov/multimedia/imagegallery/image_feature_639.html
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