Lunar ejecta as the missing piece to resolve the lunar cratering asymmetry

TL;DR

This study suggests lunar impact ejecta significantly contribute to the lunar cratering asymmetry, resolving a long-standing discrepancy between observations and models through numerical simulations.

Contribution

It introduces the hypothesis that lunar ejecta, previously overlooked, can explain the observed impact asymmetry, supported by simulation results.

Findings

~25% of ejecta re-impact within 3 Myr

~1.2% of ejecta impact the Moon

Ejecta cause a leading-trailing impact ratio of 5.9

Abstract

The leading-trailing asymmetry in lunar crater distribution provides a critical record of inner solar system dynamics, yet the long-standing discrepancy between the observed higher asymmetry and lower theoretical predictions indicates a gap in our understanding of the impactor population. This paper hypothesizes that lunar impact ejecta, which can enter Earth-like orbits and return, constitute a previously unaccounted-for component. Through numerical simulations, we find that ~25% of escaped ejecta will re-impact the Earth-Moon system within 3 Myr, with about 1.2% striking the Moon. Crucially, these lunar impacts exhibit an extreme leading-trailing asymmetry with a ratio of 5.9. Our results indicate that lunar ejecta, if comprising ~15% of total impactors, can fully explain the observed asymmetry, leading to their recognition as active agents shaping the lunar impact record. This work provides new constraints for understanding the impact environment of the Earth-Moon system, with direct relevance to the interpretation of lunar geology, the transport of lunar material to Earth, and ongoing space exploration missions.