A Sawtooth-like Timeline for the First Billion Year of Lunar Bombardment

Abstract: We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and "narrow" impact spike some 3.9 Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near 4.1 Gy ago. The impact flux at the beginning of this weaker cataclysm was 5-10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since about 4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov (1994)). Overall we expect that the sawtooth event accounted for about 1/4 of the total bombardment suffered by the Moon since its formation. Consequently, considering that about 12-14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was about 45-50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on Pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38-4.42 Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.

• The paper proposes a sawtooth-like timeline that revises traditional exponential decay and sharp cataclysm models of early lunar bombardment.
• It integrates crater record analysis, radiometric dating, and HSE constraints to refine the chronology of lunar surface formations.
• Findings indicate a 5–10× surge in impact flux around 4.1 billion years ago, attributing about one-quarter of the lunar bombardment to this phase.

Sawtooth-like Timeline for Early Lunar Bombardment: Insights and Implications

The detailed analysis and modeling of the timeline for lunar bombardment presented in "A Sawtooth-like Timeline for the First Billion Years of Lunar Bombardment" offers a nuanced understanding of the early evolution of the Moon's impact history. This comprehensive study revisits the established view of a simple exponential decline or a sharp lunar cataclysm event and proposes a more complex sawtooth-like profile of impact events.

Overview and Methodology

The authors utilize a combination of modeling, based on plausible sources of impactors and their dynamical decay rates, alongside constraints derived from the lunar crater record, radiometric ages, and the abundance of highly siderophile elements (HSEs) in lunar samples. The study challenges the conventional lunar cataclysm hypothesis, suggesting instead a timeline marked by an uptick in impact flux around 4.1 billion years ago (Gy), defying the previously assumed narrower impact spike approximately at 3.9 Gy ago.

The integration of various constraints led to the deduction that this "sawtooth-like" timeline includes a 5-10 times increase in impact flux during the early phase of the so-called weak cataclysm, which differs markedly from prior models. The research presents a decline in impact rates post-4.1 Gy adhering to classic crater chronology models established by Neukum and Ivanov (1994), indicating about one-quarter of the total lunar bombardment occurred during this sawtooth event, leading to an estimated 45-50 basin formations throughout lunar history.

Key Findings

The study refines the timeline for the formation of the Moon's basins and suggests a new lunar chronology model that better suits Pre-Nectarian surface units. The research also argues that many ancient lunar terrains are approximately 4.38-4.42 Gy old, while the South Pole-Aitken basin predates the cataclysm, contradicting its affiliation with a geologically defined lunar cataclysm.

1. Impact Flux Analysis: The analysis reveals a sawtooth pattern with significant implications for understanding the frequency and scale of impact events in the early lunar history.
2. Chronology Enhancement: This refined chronology offers improved accuracy for dating lunar surfaces, ranging from the oldest crustal units to the demarcation of basin-forming events.
3. Weak Lunar Cataclysm: While earlier models emphasized a strong cataclysm at 3.9 Gy, the study supports a milder event beginning as early as 4.1-4.2 Gy.

Theoretical and Practical Implications

The research presented makes bold claims that impact the theoretical framework for the early solar system and how we interpret planetary development. It aligns with the Nice model's scenarios and resonates with the E-belt hypothesis, suggesting plausible projectiles' migration and distribution in the solar system's history. Additionally, the study's geochemical constraints posed by HSEs provide robust cross-links in understanding the material composition transferred post-moon formation.

Practically, these findings improve our capacity to compare lunar surface ages accurately, assist in providing context regarding Earth's early history, and, more broadly, aid in refining the evolutionary models of other terrestrial planetary bodies.

Future Directions

This exploration prompts several lines of further investigation. Continued analysis using enhanced lunar sample data and advancing dating methods will help further validate or refine these chronological estimates. Additionally, examining the potential biosphere implications on Earth during such impact phases could enhance our holistic understanding of habitability and planetary development.

In summary, by presenting a sawtooth-like profile of early lunar bombardment, this paper contributes a significant revision to the lunar timeline understanding, supporting an intermediate impact flux model between the previously polarized concepts of smooth decay and sharp cataclysm. The implications of this study are vast, offering fresh insights into lunar history's complexity and, by extension, informing models of planetary development across the solar system.