Lunar Apex-antapex Cratering Asymmetry and Origin of Impactors in the Erath-moon System

Introduction: The synchronous rotation of a planetary satellite should generate a spatial variation in the crater production rate on its surface [1-9]. The production rate that has the maximum at the apex (equator, 90 ̊W) of the orbital motion of the satellite decreases with increase in angular distance from the apex and becomes the minimum at the antapex (equator, 90 ̊E). The degree of the apex-antapex cratering asymmetry primarily depends on the mean encounter velocity of impactors with respect to the planetary system and the orbital velocity of the satellite [1-5, 9]. The ratio of the maximum rate to the minimum value increases with the decrease of the encounter velocity of impactors. This fact means that we can estimate the mean encounter velocity of impactors by observation of the apex-antapex cratering asymmetry. A validity of this technique has already been verified by Morota and Furumoto [7]. From investigation of the spatial distribution for rayed craters of the Moon, they have estimated the mean velocity of recent impactors in the Earth-Moon system to be about 15 km/s corresponding to that of near-Earth asteroids. This result is consistent with recent studies of the size-frequency distribution for near-Earth asteroids; The shape of the size distribution of near-Earth asteroids is similar to that of lunar craters of the past 3.8 Gyr [10, 11]. Terrain Camera (TC) installed on a Japanese lunar explorer, SELENE that will be launched in 2007, will take images of surface of the whole Moon with nominal spatial resolution of 10 m/pixel [e.g., 12-14]. The extensive high-resolution images are available for the statistical study of crater distribution. Therefore, it is necessary to establish the technique for presumption of the origin of impactors. In this paper, we purpose to derive the lunar apexantapex cratering asymmetry as functions of the mean encounter velocity of impactors and time, considering the temporal variation in the lunar orbital velocity last 4.0 Gyr.