Asymmetrical distribution of rayed craters on the Moon

The synchronous rotation of the satellite ought to cause a spatial variation in the cratering rate over its surface. The crater density is expected to be maximum at the apex of the orbital motion and decrease with the increase of the angular distance from the apex. The ratio of the density at the apex (maximum) to that of the antapex (minimum) depends on the average encounter velocity of impactors to the satellite. Although the Moon is also in a state of the synchronous rotation, it has been supposed that the asymmetry in the crater density on the Moon can be hardly observed. We report here a spatial variation in the density of rayed craters on the Moon, which may be associated with the synchronous rotation. Since the lifetime of a ray is relatively short (<0.8 billion years), the results provide information on recent impacts. Rayed craters are identified on Clementine 750-nm mosaic images. We investigate craters in a lower latitude zone from 42°N to 42°S. To avoid an effect of material difference on the ray preservation, we analyze craters on the highland from 70°E to 290°E in east longitude. A total of 222 rayed craters larger than 5 km in diameter are identified in the study area of about 1.4×107 km2. The average density of rayed craters on the leading side is substantially higher than that on the trailing side. The crater density decreases as a sinusoidal function of the angular distance from the apex. The observed ratio of the density at the apex to that at the antapex is about 1.5. The ratio suggests that recent craters on the Moon are formed mainly by near-Earth asteroids rather than comets with higher encounter velocities.