Current bombardment of the Earth–Moon system: Emphasis on cratering asymmetries

Abstract

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-EarthObject (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.

Asymmetric impacts of near-Earth asteroids on the Moon

“…7b. The difference in cratering rate between at the polar and the equatorial regions is ∼10%; this is consistent with the analytical estimate by Le Feuvre & Wieczorek (2008), as well as the numerical result presented in Gallant et al (2009).…”

“…While our work was in progress, Gallant et al (2009) published a study with quite a similar motivation to ours, which also yielded a similar numerical result about the expected lunar cratering asymmetry from NEA impacts. Although a large part of our results overlap, the numerical models are different.…”

“…This larger value would still be insufficient to explain the difference of the degree of asymmetric distribution of the actual lunar craters record (apex/antapex ratio ∼1.65) from that of our numerical result (∼1.32) when we consider the semi-analytic estimate of the cratering rate as a function of the lunar orbital velocity (Zahnle et al 2001;Morota & Furumoto 2003). Similarly, Gallant et al (2009) performed a series of numerical simulations in order to check the effect of the smaller Earth-Moon distance, and found only a tiny change in hemispherical crater ratio for Earth-Moon distance as low as ∼90% of the current value (∼54 Earth radii). We must note, however, that the history of the lunar orbit is thus far predominantly based on theoretical models and is not especially well constrained by observations; there may exist an exciting possibility to place an observational constraint on the lunar orbital evolution by detailed modeling of the asymmetric lunar crater record.…”

“…2, it is clear that the Earth receives more NEA impacts on the leading (positive y) hemisphere than on the trailing hemisphere. This asymmetry leads to AM/PM asymmetries in NEA impact events, which is discussed in Gallant et al (2009) in detail.…”

“…2, our numerical model is straightforward and includes the orbit evolution of NEA-like particles from their current orbits up to their impacts, while Gallant et al (2009)'s study uses the NEA orbits without dynamical evolution. In this regard our paper serves as a complement to Gallant et al (2009). We also consider some additional important implications of the results that were not discussed previously.…”