Rayed craters on the far side of the Moon

A fresh crater on the Moon is associated with bright rays, which are made of fresh ejecta from the main and secondary craters. Rays of a lunar crater generally become indistinguishable from the background materials with the age, and the lifetime of a ray is supposed to be about 0.8 Gyr. Using Clementine 750-nm mosaic images, we identify rayed craters mainly on the far side of the Moon. A total of 235 rayed craters which are larger than 5 km in diameter are identified in an area of about 1.55 x 10^7 km^2. The size-frequency distribution for the rayed craters indicates a lack of small rayed craters, implying that the preservation of ray depends on the crater size.