High-velocity collisions from the lunar cataclysm recorded in asteroidal meteorites

• Published in: Nature geoscience Vol. 6; no. 4; pp. 303 - 307
• Main Authors: Marchi, S., Bottke, W. F., Cohen, B. A., Wünnemann, K., Kring, D. A., McSween, H. Y., De Sanctis, M. C., O’Brien, D. P., Schenk, P., Raymond, C. A., Russell, C. T.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.04.2013; Nature Publishing Group
• Subjects: 639/33/445/847; 639/33/445/848; 639/33/445/849; Asteroids; Degassing; Earth Sciences; Earth System Sciences; Geochemistry; Geology; Geophysics/Geodesy; Moon; Radiometric dating
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo1769

The Moon experienced an intense period of impacts about 4 Gyr ago. This cataclysm is thought to have affected the entire inner Solar System and has been constrained by the radiometric dating of lunar samples: 40 Ar– 39 Ar ages reflect the heating and degassing of target rocks by large basin-forming impacts on the Moon. Radiometric dating of meteorites from Vesta and the H-chondrite parent body also shows numerous 40 Ar– 39 Ar ages between 3.4 and 4.1 Gyr ago, despite a different dynamical context, where impacts typically occur at velocities too low to reset geochronometers. Here we interpret the 40 Ar– 39 Ar age record in meteorites to reflect unusually high impact velocities exceeding 10 km s −1 . Compared with typical impact velocities for main-belt asteroids of about 5 km s −1 , these collisions would produce 100–1,000 times more highly heated material by volume. We propose that the 40 Ar– 39 Ar ages between 3.4 and 4.1 Gyr ago from Vesta, the H-chondrite parent body and the Moon record impacts from numerous main-belt asteroids that were driven onto high-velocity and highly eccentric orbits by the effects of the late migration of the giant planets. We suggest that the bombardment persisted for many hundreds of millions of years and affected most inner Solar System bodies. Lunar samples suggest that the inner Solar System was bombarded by asteroids about 4 Gyr ago. Radiometric ages of meteorites suggest an unusual number of high-velocity asteroids at this time, consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits.