Introduction: The impact cratering shaped Moon’s landscapes offer a fossil record of the Near Earth Object populations. This record goes back to the formation and cooling of the lunar crust [1-3] when the crust was sufficiently viscous to retain the cratering “scars”. The crater densities of highland and mare surfaces testify a substantially higher impact flux rate during the first ~1.5 Ga compared to that for the last 3 Ga of our Solar System. The temporal evolution of the impact flux (i.e., the bombardment timeline) is the topic of intense scientific debate for at least the past 40 years. The end-member interpretations are 1) the “Early Heavy Bombardment” (EHB) that assumes an exponentially declining impact flux rate until it oscillated around a much lower value during the last 3 Ga, and 2) the “Late Heavy Bombardment” (LHB) referring to a brief “terminal lunar cataclysm” with a sharply rising and falling peak centered around 3.9 Ga ago [1,2].
On current state of concepts: While the EHB may still be considered the least assumptions model consistent with our fuzzy sample-based knowledge of the bombardment timeline, it lacks a proper physical (orbital) explanation. In light of the recent knowledge from lunar, terrestrial and asteroid belt samples, the classical LHB scenario is no longer attractive [1,2]. In addition, it appears highly inconsistent with orbital models [4-5]. This particularly stems from the difficulty or even impossibility to properly correlate the Apollo and Luna samples to their bedrock or crater ejecta source. However, present re-interpretation of old data together with acquisition of new data from Apollo and Luna samples suggest that intermediate views that consider a complex bombardment timeline with moments of heavy bombardment are a better venue to consensus. Two events able to initiate moments of heavy bombardment are:
1) The giant Moon forming impact event created an extremely massive heliocentric debris disk; meaning a new projectile population with high impact probability onto the Earth and the Moon for the following few hundred million years [6-7].
2) The reorganization of the planetary system architecture as proposed by the updated “Nice”-model basically explains the extended tail-end of the heavy bombardment as testified by the crater density on mare basalts and possibly the Archean spherule layers on Earth . However the level of resolution in dynamical models is not yet equal to data from samples.
3) Other impactor populations (planetary left overs, comets, debris discs, asteroid belts) that resulted from single events or dynamical processes leading to additional spikes in the impact flux cannot be excluded. One should be aware that “you don’t know what you don’t know”. Therefore, additional work on current samples and/or new samples is required.
Acknowledgments: All this work has been carried out in collaboration with Jörg Fritz and Stephanie Werner.
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