Rayed craters on the moon and Mercury

Abstract

Listen

Translate article

Many fresh craters on the moon and Mercury exhibit systems of bright rays. Statistical comparison of the diameter/density distributions of fresh craters and rayed craters on these two bodies indicate that the rayed-crater populations are representative samples of the larger populations of fresh craters. The rayed-crater population on Mercury, however, is not similar either to the rayed- or to the fresh-crater population on the moon. Photogeological interpretation of the best available lunar imagery indicates that “ray material” is indeed ejected from the primary craters, and should be present in the lunar soil. Although the distribution of the ray material may be patchy, it probably occurs in deposits of thickness greater than the characteristic depth to which subsequent impact processes overturn the regolith. The time needed to darken a ray system thus may be more dependent on the thickness of the ray material than on the rates of the various darkening processes, and darkening rate may be a function of crater size. Ray darkening on the lunar surface is an inefficient process at present. The rays of craters more than 1 b.y. old are still bright, whereas the rays of craters older than Class 1 have generally faded to imperceptibility.