An ancient, relatively bright, circular feature on the surface of a dark icy moon, such as Ganymede or Callisto. Palimpsests lack the relief associated with normal craters and are thought to be impact craters of which the topographic relief has been eliminated by viscous relaxation (creep) of the icy surface, probably during the impact itself. Typical is Ganymede's 340-km-wide Memphis Facula. Such structures hold important clues to the early thermal history and composition of the bodies on which they are found. The original meaning, "scraped again," refers to a manuscript on a waxen tablet or other writing material from which an early text was removed and then written over. The value of palimpsests, in both planetary astronomy and literature, is that, they preserve a record of the past in the form of something that is partly hidden from view.
