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arxiv: 2605.09884 · v1 · submitted 2026-05-11 · 🌌 astro-ph.EP

Recognition: no theorem link

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

Hailiang Li , Xiaoping Zhang , Li-Yong Zhou
Authors on Pith no claims yet

Pith reviewed 2026-05-12 04:29 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords lunar ejectacratering asymmetryleading-trailing asymmetryEarth-Moon systemimpact dynamicslunar geologyre-impacting ejectasolar system dynamics
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The pith

Lunar ejecta returning to the Moon can fully explain the observed leading-trailing crater asymmetry if they comprise about 15% of all impactors.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates the long-standing mismatch between the strong leading-trailing asymmetry seen in lunar craters and weaker predictions from standard impactor models. It proposes that ejecta blasted off the Moon can escape, enter orbits around Earth, and return to strike the Moon again. Numerical simulations reveal that these returning ejecta hit the Moon with a much stronger asymmetry ratio of 5.9 favoring the leading side. If such ejecta make up roughly 15 percent of the total impactors, the combined effect matches the observed crater distribution. This reframes lunar ejecta as an active part of the impact record rather than just byproducts.

Core claim

Numerical simulations show that about 25% of escaped lunar ejecta re-impact the Earth-Moon system within 3 Myr, with 1.2% striking the Moon, and these lunar impacts display an extreme leading-trailing asymmetry with a ratio of 5.9. The results indicate that lunar ejecta, if comprising ~15% of total impactors, can fully explain the observed asymmetry in the lunar cratering record.

What carries the argument

Orbital simulations of escaped lunar ejecta trajectories, which demonstrate that 1.2% return to strike the Moon with a leading-to-trailing impact ratio of 5.9.

If this is right

  • The total population of impactors on the Moon includes a recycled component from previous lunar material.
  • Interpretations of lunar surface ages and geology must incorporate self-generated impacts from ejecta.
  • Material exchange in the Earth-Moon system is more dynamic, with lunar material returning after orbiting Earth.
  • Space exploration missions need to consider ejecta as both hazards and samples of lunar history.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar ejecta recycling might occur on other airless bodies like Mercury or asteroids, affecting their crater records.
  • Compositional analysis of lunar craters could test for signatures of prior lunar origin in the impactors.
  • Refining the 15% fraction could come from better models of ejecta velocity distributions or long-term orbital stability.

Load-bearing premise

That the fraction of lunar impactors that are returning ejecta is close to 15% and that the orbital simulations accurately represent real ejecta trajectories without unmodeled perturbations or loss mechanisms.

What would settle it

A direct estimate of the actual fraction of lunar craters formed by material of lunar origin, for example through isotopic or mineralogical analysis that distinguishes re-impacting ejecta from external impactors.

Figures

Figures reproduced from arXiv: 2605.09884 by Hailiang Li, Li-Yong Zhou, Xiaoping Zhang.

Figure 1
Figure 1. Figure 1: Orbital dissimilarity metric across the a − e − i parameter space. Bluer colors indicate higher degrees of orbital similarity. of Earth (i.e., a = 1 au and e = i = 0). Generally, for a given a, a larger e leads to a greater ∆v. However, with a fixed e, the ∆v does not always reach its minimum precisely at a = 1 au, be￾cause the encounter geometry between the two objects becomes less favorable; a marginally… view at source ↗
Figure 2
Figure 2. Figure 2: Distributions of a, e, i, and ∆v of simulated ejecta at t = 100 yr. Points of different colors represent ejecta with different v0 values. As shown in [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Relationship between the orbits of lunar ejecta and their initial positions at different v0. From left to right, the panels represent e, i, and ∆v, respectively. Blank areas indicate ejecta that did not survive until t = 100 yr in the simulations. Black lines mark the lunar latitude and longitude grid, with longitude intervals of 45◦ and latitude intervals of 30◦ . =0° =90° =180° =270° leading side trailin… view at source ↗
Figure 4
Figure 4. Figure 4: Similar to [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: ∆v distributions at 100 yr, 100 kyr, and 1 Myr. Different colors representing distinct v0. Only three values are plotted for clarity. The middle panel of [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Distribution of time and venc for collision events on Earth-Moon system. The x-axis represents time on a logarithmic scale, while the y-axis shows the venc, which indicates the velocities of impactors as they enter the Hill radius of the Earth-Moon system. For clarity, impacts on the Moon are highlighted with larger markers. 2.38 3.00 4.00 5.00 6.00 vL(km/s) 0.0 0.2 0.4 0.6 0.8 1.0 Proportion Escape Probab… view at source ↗
Figure 7
Figure 7. Figure 7: Relationship between the proportion of ejecta escaping the Earth-Moon system and their vL. The launch frequency is provided by Jiao et al. (2024), while the escape probability is derived from simu￾lation results (represented by blue points). The dashed line on the left indicates the Moon’s escape velocity. C(t) = m(1 − exp( −ctk m )). (4) The derivative of Eq. 4 is given by dC(t) dt = cktk−1 exp( −ctk m ),… view at source ↗
Figure 8
Figure 8. Figure 8: Cumulative impact count of lunar ejecta on the Earth-Moon sys￾tem as a function of time. The light-colored points represent the simu￾lated cumulative impact counts, while the solid lines correspond to the fitted results based on Eq. 4. The black dashed line represents the over￾all result obtained after weighting different v0. L(t1) − L(t0) T(t1) − T(t0) = R ∞ 2.38 f(vL) × (C(vL, t1) − C(vL, t0)) × L/T(vL) … view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript claims that lunar ejecta returning to the Earth-Moon system can resolve the long-standing discrepancy between predicted and observed leading-trailing cratering asymmetry on the Moon. N-body simulations over 3 Myr show that ~25% of escaped ejecta re-impact the Earth-Moon system, with 1.2% striking the Moon at an extreme asymmetry ratio of 5.9. The authors conclude that lunar ejecta comprising ~15% of total impactors can fully account for the observed asymmetry, positioning ejecta as an active component in the lunar impact record.

Significance. If the 15% fraction can be independently justified, this would close a key gap in models of inner solar system dynamics and lunar geology by identifying a high-asymmetry impactor source. The quantitative outputs (25% re-impact, 1.2% lunar strikes, ratio 5.9) supply concrete numbers that could be tested against regolith transport models or meteorite delivery rates, strengthening constraints on Earth-Moon material exchange.

major comments (2)
  1. [Abstract] Abstract: The central claim that ejecta 'comprising ~15% of total impactors' fully explains the observed asymmetry is obtained by scaling the simulated 1.2% lunar strike rate against the known discrepancy. No section derives this fraction from ejecta production rates, crater scaling laws, or absolute impactor flux; it functions as a single free parameter chosen to reproduce the target observation.
  2. [Methods/Results] Simulation description (likely Methods/Results): The 3 Myr N-body runs report 25% re-impact and asymmetry ratio 5.9 without error bars, sensitivity tests on initial conditions, or inclusion of solar radiation pressure, Yarkovsky effects, and collisional grinding. These omissions directly affect the reliability of the return fraction and asymmetry used to support the 15% scaling.
minor comments (2)
  1. [Abstract] The abstract would benefit from a one-sentence description of the N-body integrator and initial ejecta velocity distribution to aid reader assessment of the simulation fidelity.
  2. Ensure that the term 'total impactors' is defined consistently when comparing the ejecta component to the background population (e.g., NEOs or main-belt asteroids).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which help clarify the scope and limitations of our work. We address each major comment below and outline the revisions we will make to strengthen the manuscript.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim that ejecta 'comprising ~15% of total impactors' fully explains the observed asymmetry is obtained by scaling the simulated 1.2% lunar strike rate against the known discrepancy. No section derives this fraction from ejecta production rates, crater scaling laws, or absolute impactor flux; it functions as a single free parameter chosen to reproduce the target observation.

    Authors: We agree that the ~15% figure arises from scaling the simulated 1.2% lunar re-impact rate to close the gap between predicted and observed asymmetry. This was presented as an indicative value to show that a modest ejecta contribution could resolve the discrepancy, rather than a first-principles derivation. In the revised manuscript we will (i) explicitly label the 15% value as the result of this scaling in both the abstract and main text, (ii) add a new subsection that combines published ejecta production rates, escape fractions, and crater scaling relations to estimate a plausible range for the ejecta fraction, and (iii) discuss the uncertainties and assumptions involved. These changes will make the scaling argument transparent while retaining the core dynamical result. revision: yes

  2. Referee: [Methods/Results] Simulation description (likely Methods/Results): The 3 Myr N-body runs report 25% re-impact and asymmetry ratio 5.9 without error bars, sensitivity tests on initial conditions, or inclusion of solar radiation pressure, Yarkovsky effects, and collisional grinding. These omissions directly affect the reliability of the return fraction and asymmetry used to support the 15% scaling.

    Authors: The referee is correct that the current text presents the 25% re-impact fraction and 5.9 asymmetry ratio without accompanying uncertainties or sensitivity tests. The 3 Myr integration length was chosen to capture the dominant gravitational return phase. In revision we will (i) report statistical uncertainties derived from the particle ensemble, (ii) add a set of sensitivity runs that vary initial ejecta velocity distributions and launch locations, and (iii) include a concise discussion of the neglected effects. Solar radiation pressure and Yarkovsky drift are expected to be secondary over 3 Myr for the relevant particle sizes, while collisional grinding operates on longer timescales; we will quantify these expectations and note that a full treatment is reserved for follow-up work. These additions will improve the robustness assessment without changing the reported central values. revision: partial

Circularity Check

1 steps flagged

The ~15% ejecta fraction is scaled to reproduce the observed asymmetry rather than derived independently

specific steps
  1. fitted input called prediction [Abstract]
    "Our results indicate that lunar ejecta, if comprising ~15% of total impactors, can fully explain the observed asymmetry"

    The 15% value is not obtained from first-principles ejecta production or independent flux estimates; it is the specific fraction required to scale the simulated 5.9 asymmetry ratio (from the 1.2% re-impact rate) so the composite matches the known observed discrepancy. The explanatory success is therefore enforced by construction once the parameter is chosen to fit the target.

full rationale

The paper's simulations independently yield a 1.2% lunar re-impact rate and 5.9 leading-trailing ratio for returning ejecta. However, the central claim that ejecta 'fully explain' the observed asymmetry is achieved by inserting a single free parameter (15% of total impactors) chosen to close the gap between background predictions and data. No section derives this fraction from crater scaling laws, regolith ejection rates, or absolute flux measurements; it functions as a fitted input whose value is set by the target observation. This matches the 'fitted input called prediction' pattern with partial circularity in the explanatory result, while the dynamical simulations themselves remain non-circular.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The explanatory power rests on one fitted fraction and standard but untested assumptions about long-term orbital stability of ejecta.

free parameters (1)
  • ejecta fraction of total impactors = ~15%
    Value of ~15% is selected so the simulated returning population accounts for the entire observed asymmetry discrepancy.
axioms (1)
  • domain assumption Numerical integration of ejecta trajectories over 3 Myr accurately captures real dynamics without significant unmodeled effects such as solar radiation pressure or planetary perturbations beyond the model.
    Invoked to justify the reported 25% re-impact rate and 1.2% lunar strike fraction.

pith-pipeline@v0.9.0 · 5488 in / 1387 out tokens · 40109 ms · 2026-05-12T04:29:57.511009+00:00 · methodology

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