ar X iv : a st ro - p h / 06 08 37 3 v 1 1 7 A ug 2 00 6 Current Bombardment of the Earth - Moon System : Emphasis on Cratering Asymmetries

We calculate the current spatial distribution of projectile delivery to the Earth and Moon using numerical orbital dynamics simulations of candidate impactors drawn from a debiased Near-Earth-Object (NEO) model. Surprisingly, we find that the average lunar impact velocity is 20 km/s, which has ramifications in converting observed crater densities to impactor size distributions. We determine that current crater production on the leading hemisphere of the Moon is 1.29 ± 0.01 that of the trailing when considering the ratio of craters within 30 • of the apex to those within 30 • of the antapex and that there is virtually no nearside-farside asymmetry. As expected, the degree of leading-trailing asymmetry increases when the Moon's orbital distance is decreased. We examine the latitude distribution of impactor sites and find that for both the Earth and Moon there is a small deficiency of time-averaged impact rates at the poles. The ratio between deliveries within 30 • of the pole to that of a 30 • band centered on the equator is nearly unity for Earth (< 1%)(0.992 ± 0.001) but detectably non-uniform for the Moon (∼ 10%) (0.912 ± 0.004). The terrestrial arrival results are examined to determine the degree of AM/PM asymmetry to compare with meteorite fall times (of which there seems to be a PM excess). Our results show that the impact flux of objects derived from the NEOs in the AM hours is ∼ 2 times that of the PM hemisphere, further supporting the assertion that meteorite-dropping objects are recent ejections from the main asteroid belt rather than young fragments of NEOs.