Planetary Science

Lunar Cratering

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Science  27 Apr 2001:
Vol. 292, Issue 5517, pp. 605
DOI: 10.1126/science.292.5517.605c

Life in the early solar system was probably like a food fight at the end of a long week at summer camp—all of the leftovers were thrown around and produced a large number of craterlike impact features. The moon carries a signature of these events, technically referred to as the Late Heavy Bombardment (LHB), in the form of well-preserved impact craters that have not been wiped clean by later tectonic activity. Because it is difficult to date lunar surface features accurately, debate on when the LHB occurred continues. It may have lasted from 4.4 to 3.6 billion years ago, with an exponential decline in the number of impactors, but there is evidence of a sharp increase during the period from 4.0 to 3.8 billion years ago (called the lunar cataclysm).

Morbidelli et al. estimate the supply rate of impactors to the Moon using orbital dynamic simulations, which they start just after the terrestrial planets have formed and before all of the planetesimals have been destroyed. They find that a small percentage of these planetesimals survive in highly inclined orbits. These survivors are then used to simulate a LHB of the Moon. The leftover planetesimals hit the Moon with an exponentially decaying rate consistent with a slow and steadily decreasing LHB. Thus, another mechanism may be required to account for the lunar cataclysm. — LR

Meteorit. Planet. Sci.36, 371 (2001).

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