Retention time of crater ray materials on the Moon

Surfaces of planetary bodies are scarred with numerous impact craterings. Much of fresh material ejected from the impact cratering is deposited in the area surrounding the crater. These ejecta blanket reveals bright ray because the ejecta shortly after the impact cratering are immature (fresh). Adjacent to the crater rim, the ejecta typically forms a thick, continuous layer and shows brighter feature. At larger distances from the crater rim, the ejecta may occur as discontinuous clumps of materials. Lunar crater rays disappear over time, and it is suggested that these are the reason why space weathering that is the process of surface materials alternated by exposure to solar wind, cosmic rays, and micro-meteorite bombardments and impact gardening that is the mixing process of surface and subsurface materials. Wilhelms described that the presence of crater rays is considered as the marker to define the Copernican - Eratosthenian boundary, and the persistence of immature rays is less than about 1.1 Ga. The Copernican is one of the lunar geological timescale and runs from approximately 1.1 Ga to the present day. This is defined by impact craters having bright immature rays. It is important to estimate the crater rays retention time for the well-understanding of lunar geology. The purpose of this research is to investigate crater ray retention time of lunar craters using high-resolution data from Multiband Imager (MI) and Terrain Camera (TC) onboard Kaguya. We surveyed craters from 300 m to 10 km in diameter in lunar highland. The craters larger than 1 km in diameter are plotted above the 750 Ma isochron which was estimated by Werner and Medvedev (2010). However, the distribution cannot be fitted by a single isochron, this might suggest that the retention time of crater ray materials is longer than 750 Myr. As a point of space weathering which related to production of nano-phase iron (Sasaki et al., 2001), the cause of long time to disappear the crater ray on the highland might be the absence of iron content of the highland material.