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

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.

• The paper demonstrates that incorporating about 15% lunar ejecta into impact models reproduces the observed leading/trailing crater density ratios.
• High-resolution N-body simulations reveal that low-velocity ejecta maintain Earth-like orbits, creating an extreme asymmetry in impact flux.
• The study quantifies ejecta contributions to crater formation, suggesting revisions to lunar chronology and hazard assessments in the Earth–Moon system.

Lunar Ejecta and the Origin of Lunar Crater Asymmetry

Background and Problem Statement

The persistent leading-trailing hemispheric asymmetry in lunar crater distribution encodes essential information about both the ancient and current impact environments of the Earth-Moon system. Synchronous rotation ensures the lunar leading hemisphere intercepts more impactors at higher velocities, yet observed leading/trailing (L/T) density ratios ($\sim$1.4--1.9) consistently and substantially exceed those predicted by models considering only near-Earth objects (NEOs) as impactors (L/T $\sim$1.14). This longstanding discordance suggests a missing population of low-velocity, Earth-like orbital impactors required to reconcile models and observations.

Prior hypotheses have introduced several mechanisms for low-$V_{enc}$ impactor replenishment, such as the tidal disruption of small bodies and Earth's co-orbitals, but a comprehensive dynamical-quantitative evaluation of lunar ejecta’s contribution has been lacking.

Simulation Techniques and Orbital Analysis

The study performs high-resolution $N$-body simulations incorporating the Sun, all major planets, and the Moon. Ejecta initial positions uniformly sample the lunar surface via Fibonacci lattice, and launch velocity ($v_0$) regimes span 2--6 km/s, corresponding realistically to the majority of lunar ejecta. Orbital similarity to Earth is quantified using a Tisserand-parameter-based metric, and leading/trailing impact asymmetry is analytically computed as a function of $V_{enc}$ via established crater statistics.

A crucial finding is that low-$v_0$ ejecta occupy low-eccentricity, low-inclination, nearly Earth-like orbits immediately post-emission. However, the dynamic interaction with Earth's mean-motion resonances and subsequent secular perturbations produces progressive orbital excitation. Despite this, these particles retain $V_{enc}$ significantly below that of typical NEOs ($\sim$5 km/s after 1 Myr, compared to $>$20 km/s for standard NEOs), maintaining a fundamentally distinct impact signature.

Impact Fluxes and Asymmetry Quantification

The simulations demonstrate that $\sim$025% of ejected lunar material escapes the Earth-Moon system but returns to impact either the Earth or the Moon within 3 Myr, with only 1.2% of those re-impacts striking the Moon. This low lunar impact probability is rooted in gravitational focusing, which disproportionately increases Earth’s effective collisional cross-section at low velocities.

Most importantly, the resultant L/T ratio for returning lunar ejecta is extreme (L/T $\sim$15.9), in stark contrast to that expected for high-$\sim$2 populations. The high asymmetry persists for up to 100 kyr, declining as orbital memory dissipates. Critically, weighted combination with the NEO population indicates that if lunar ejecta compose only $\sim$315% of all impactors, the observed crater asymmetry ($\sim$4) is reproduced — an explicit quantitative match to observations.

Comparison with the markedly high L/T ratio observed during the Imbrian-Nectarian era ($\sim$53.1) suggests that periods of heightened large-scale lunar impacts and enhanced ejecta flux could account for the temporal evolution of asymmetry, with lunar ejecta contributing up to $\sim$670% of impactors in such epochs.

Implications for Lunar Geology and Solar System Dynamics

The findings necessitate a significant revision of lunar surface chronology models to explicitly account for ejecta-derived craters, especially on the leading hemisphere where ejecta re-impacts may represent up to $\sim$723% of features. This profoundly impacts interpretations of the lunar geological record, the calibration of relative crater ages, and estimates of the ancient impact flux.

The low-velocity nature and population dynamics of ejecta imply that they also form a significant component of Earth-impacting meteoroids, potentially explaining observed inconsistencies between the meteorite record and their presumed main belt or NEO origins. Observed variations in asymmetry as a function of crater/impactor diameter further imply size-dependent contributions, suggesting that microcratering and seismically-detected meteoroid impacts should be re-analyzed with a bias towards ejecta origins and low-$\sim$8 kinematics.

Practically, these results bear on lunar sample return mission design, interpretation of impact melt ages, and strategies for assessing meteoroid hazards for lunar infrastructure. The predicted episodic nature of large ejecta events highlights the need to consider lunar source material in future searches for Earth’s near-co-orbital asteroids.

Future Prospects

While this framework robustly elevates the role of lunar ejecta in shaping both lunar and terrestrial impactor environments, uncertainties remain in constraining the stochastic rate of high-energy lunar impacts, the absolute production rates of escaping ejecta, and the evolving size-frequency distribution of fragments. The potential influence of non-gravitational forces (e.g., Yarkovsky effect) on small ejecta and the explicit role of lunar orbital evolution over geologic timescales require additional investigation.

Upcoming missions, notably those targeting the quasi-satellite Kamo'oalewa as a candidate lunar ejecta body, can offer critical ground-truth for these dynamical models. Enhanced in situ monitoring of contemporary micro-impacts on the lunar surface may also help disentangle ejecta vs. exogenous sources in ongoing asymmetric cratering.

Conclusion

This study rigorously demonstrates that lunar impact ejecta are the previously overlooked population required to resolve the persistent excess in lunar leading/trailing crater asymmetry. Through detailed numerical experiments and semi-analytical reasoning, it establishes that modest inclusion ($\sim$915%) of returning lunar ejecta among all impactors suffices to reproduce observed cratering patterns. These findings fundamentally reframe the role of secondary lunar material as active agents in Solar System impact processes, mandating their explicit consideration in planetary surface chronology, meteoritic provenance, and near-Earth dynamic studies. Future work should focus on refining ejecta flux models, assessing the effect of stochastic large lunar impacts, and integrating observational campaigns designed to directly identify and characterize lunar-origin impactors within the Earth-Moon environment.