PerspectivePlanetary Science

Fragments of the Lunar Cataclysm

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Science  15 Jun 2012:
Vol. 336, Issue 6087, pp. 1390-1391
DOI: 10.1126/science.1224184

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Summary

The flux of extraterrestrial materials impacting the Earth is not representative of potential source bodies in the solar system. Except for the Moon and Mars, chunks of planets and moons are absent from our meteorite collections. And aside from tiny interplanetary dust particles, comets are also missing; they strike Earth at high relative velocities and do not leave identifiable remains. Although asteroidal fragments face a less harrowing journey to Earth, they have to endure launch from their parent body by hypervelocity collisions, impacts by micrometeorites in interplanetary space, and frictional stresses involved in passing through Earth's atmosphere. Major collisions on asteroids are stochastic events; for example, an asteroid-shattering impact 470 million years ago on a particular chondritic body (1) disgorged a clutch of material so massive that numerous fossil meteorites from that body have been found in Ordovician sedimentary rocks (2). The flux of material that reaches the top of Earth's atmosphere is the same as that impacting the lunar surface. Although we cannot identify intact meteoritic debris in terrestrial rocks that are billions of years old, there is a chance that such material would survive on the Moon. On page 1426 in this issue, Joy et al. (3) identify ancient projectile fragments in Apollo 16 lunar rocks that were consolidated sometime between 3.8 and 3.4 billion years ago (Ga). This time interval corresponds to the tail end of the epoch when large lunar impact basins like Imbrium were being formed (4) (see the figure).