GEOLOGY & PLANETARY SCIENCE

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palimpsest

1. Memphis Facula, on Ganymede

   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.


2. In the traditional sense, a palimpsest is a painting that has been obscured and covered over by a second image. Palimpsests are sometimes found in the works of Old Masters who, dissatisfied by their own work, economized by simply painting over their first picture. They have also been used as a means of stealing a valuable old painting. A painting over a master work appears to be innocuous, or very bad, and can subsequently be removed to reveal the original work. [Thanks to Nikoli A. McCracken for this contribution.]

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