Can Planetesimals Leftover from Terrestrial Planet Formation Produce the Lunar Late Heavy Bombardment?

Details Can Planetesimals Leftover from Terrestrial Planet Formation Produce the Lunar Late Heavy Bombardment? Can Planetesimals Leftover from Terrestrial Planet Formation Produce the Lunar Late Heavy Bombardment?

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6601b&link_type=abstract

American Astronomical Society, DPS meeting #38, #66.01; Bulletin of the American Astronomical Society, Vol. 38, p.616

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The Lunar Late Heavy Bombardment (LHB) describes a time 3.8 to perhaps 4.1 Gy ago when the basins on the Moon with reasonably well-constrained ages were formed (e.g., Imbrium, Orientale). Some argue these basins were produced by a terminal cataclysm that caused a spike in the inner Solar System impactor flux 3.8-4.0 Gy ago. Others suggest the basins were formed by the tail end of a monotonically-decreasing impactor population produced by planet formation processes. Here we investigate whether the declining bombardment model of the LHB is consistent with constraints provided by planet formation models and the large lunar basins with known ages. We assumed the post-planet formation population (PPP) that existed after the Moon-forming event on Earth had an initial orbital distribution comparable to high inclination asteroids from the observed near-Earth object population. We tracked PPP bodies using numerical integration codes until most were dynamically eliminated. The remnants were then repeatedly cloned and restarted through multiple generations until the survivors reached 1-4 billion years of simulation time. The dynamical decay rate and lunar impact rate of the PPP was computed from this data. Both decreased by more than 4 orders of magnitude over the first 700 My of our simulation. By including the effects of mutual collisions between PPP bodies, we reduced the PPP by an additional order of magnitude. Overall, we found this depletion so extreme that the declining bombardment model could not produce the large lunar basins with known ages unless the initial PPP contained more than 10 Earth masses of material in the form of diameter D < 1000 km bodies. This estimate is both implausible and highly inconsistent with planet formation models. Accordingly, we conclude that the declining bombardment model was extremely unlikely to produce the LHB.

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