Analytical formulation of lunar cratering asymmetries

Abstract

We formulate the lunar cratering distribution and verify the cratering asymmetries generated by the main-belt asteroids (MBAs) as well as the near-Earth objects (NEOs). Based on a planar model that excludes the terrestrial and lunar gravitations on the impactors and assuming the impactor encounter speed with Earth $v_{\rm{enc}}$ is higher than the lunar orbital speed $v_{\rm{M}}$, we rigorously integrated the lunar cratering distribution, and derived its approximation to the first order of $v_{\rm{M}}/v_{\rm{enc}}$. Numerical simulations of lunar bombardment by the MBAs during the late heavy bombardment were performed with an Earth-Moon distance $a_{\rm{M}}$ = 20--60 Earth radii in five cases. The analytical model directly proves the existence of a leading/trailing asymmetry and the absence of near/far asymmetry. The approximate form of the leading/trailing asymmetry is $(1 + A_1 \cos\beta)$, which decreases as the apex distance $\beta$ increases. The numerical simulations show evidence of a pole/equator asymmetry as well as the leading/trailing asymmetry, and the former is empirically described as $(1 + A_2 \cos2\varphi)$, which decreases as the latitude modulus $|\varphi|$ increases. The amplitudes $A_{1,2}$ are reliable measurements of asymmetries. Our analysis explicitly indicates the quantitative relations between cratering distribution and bombardment conditions (impactor properties and the lunar orbital status) like $A_1 \propto v_{\rm{M}}/v_{\rm{enc}}$, resulting in a method for reproducing the bombardment conditions through measuring the asymmetry. Mutual confirmation between analytical model and numerical simulations is found in terms of the cratering distribution and its variation with $a_{\rm{M}}$. Estimates of $A_1$ for crater density distributions generated by the MBAs and the NEOs are 0.101--0.159 and 0.117, respectively.