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

TL;DR

This study proposes a sawtooth-like impact flux timeline for the Moon's first billion years, challenging the traditional lunar cataclysm model by suggesting a gradual decline with a mild uptick around 4.1 Gy ago.

Contribution

It introduces a new impact flux timeline for early lunar history, combining modeling with lunar crater and geochemical data, and refines lunar chronology estimates.

Findings

Impact flux had a sawtooth profile with an uptick near 4.1 Gy ago.

The lunar cataclysm was not a narrow spike but a mild increase in impact rate.

Most lunar basins, including South Pole Aitken, predate 4.3 Gy.

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.