Ariel appears to be fairly evenly cratered compared to other moons of Uranus; the relative paucity of large craters suggests that its surface does not date to the Solar System's formation, which means that Ariel must have been completely resurfaced at some point of its history. Ariel's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few of the craters are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the preexisting crustal structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters. If this is the case they would be similar to palimpsests on Jupiter's moon Ganymede. It has been suggested that a circular depression 245 km in diameter located at 10°S 30°E is a large, highly degraded impact structure.