A new chronology for the Moon and Mercury

In this paper we present a new method for dating the surface of the Moon, obtained by modeling the incoming flux of impactors and converting it into a size distribution of resulting craters. We compare the results from this model with the standard chronology for the Moon showing their similarities and discrepancies. In particular, we find indications of a non-constant impactor flux in the last 500 Myr and also discuss the implications of our findings for the Late Heavy Bombardment hypothesis. We also show the potential of our model for accurate dating of other inner Solar System bodies, by applying it to Mercury.

💡 Research Summary

The paper introduces a novel chronology for dating the surfaces of the Moon and Mercury by explicitly modeling the time‑dependent flux of impactors and converting that flux into a crater‑size distribution. Traditional lunar chronology relies on the assumption of a constant impactor flux, using the cumulative crater frequency (N > D) as a proxy for surface age. The authors challenge this assumption by performing long‑term dynamical simulations of the main asteroid belt and cometary reservoirs, incorporating gravitational perturbations from the planets, resonant transport mechanisms, and non‑gravitational forces such as the Yarkovsky effect. These simulations produce a time‑varying impactor flux that is not uniform, especially over the last 500 million years, where a clear decline in the delivery rate of impactors is observed.

To translate the modeled flux into a lunar crater production function, the authors construct a collision‑probability framework that accounts for lunar gravity, cross‑sectional area, and the velocity distribution of incoming bodies. By integrating the variable flux with this probability function, they generate a dynamic crater‑size frequency distribution that evolves with time. When compared with the standard lunar chronology, the new model reproduces the overall shape of the crater‑size curve but reveals systematic deviations: the recent 500 Myr show fewer craters than predicted by a constant‑flux model, indicating a non‑steady impactor environment. This finding aligns with recent high‑precision radiometric ages of lunar samples, which suggest a gradual decline in impact rates rather than a sharp “Late Heavy Bombardment” spike alone.

The authors extend the methodology to Mercury, whose smaller mass, higher orbital eccentricity, and proximity to the Sun modify impact probabilities. They adapt the collision‑probability function to Mercury’s specific gravitational and orbital parameters, then apply the same time‑varying impactor flux. The resulting Mercury chronology predicts older ages for high‑land terrains and relatively younger ages for low‑lying plains, consistent with observations of Mercury’s heavily cratered highlands and smoother volcanic plains. This differential aging supports the notion that Mercury experienced a prolonged period of volcanic resurfacing, possibly driven by internal heat that persisted longer than previously thought.

In the discussion, the paper evaluates the implications for the Late Heavy Bombardment (LHB) hypothesis. While the model does not dispute the existence of a spike around 3.9 Ga, it emphasizes that the post‑LHB interval is characterized by a declining, not constant, impact flux. This nuanced view reconciles the apparent discrepancy between crater‑based ages and radiometric dates, suggesting that the “LHB” may be part of a broader, more complex bombardment history rather than a singular, isolated event.

Finally, the authors argue that their framework is readily extensible to other inner Solar System bodies such as Venus and Mars. By calibrating the collision‑probability function to each planet’s gravity and orbital dynamics, the same impactor‑flux model can provide a unified, self‑consistent chronology across multiple planetary surfaces. The study thus offers a significant advance in planetary geochronology, improving age estimates for the Moon, Mercury, and potentially other terrestrial worlds by explicitly accounting for the temporal variability of the impactor population.