Palimpsest (planetary astronomy)

A palimpsest, in planetary astronomy, is an ancient crater on an icy moon of the outer Solar System whose relief has disappeared due to creep of the icy surface ("viscous relaxation") or subsequent cryovolcanic outpourings, leaving a circular albedo feature, perhaps with a "ghost" of a rim. Icy surfaces of natural satellites like Callisto and Ganymede preserve hints of their history in these rings. A typical example is Memphis Facula on Ganymede, a 340 km wide palimpsest.