Palimpsest (planetary astronomy)

A palimpsest , in planetary astronomy, is an ancient crater that has been degraded over time. They may also be referred to as "ghost craters", "degraded craters", or "pathological craters". Palimsests have been identified on Mercury, the Earth, the Moon, Mars, Ganymede, Callisto, and possibly even Titan. On Mars, these features are morphologically described as craters that are "flat-floored, rimless, extremely shallow, without central peaks, and would probably represent what remains after erosion."
On an icy moon of the outer Solar System, a palimsest is a crater 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.