Lunar impact history from (40)Ar/(39)Ar dating of glass spherules
Lunar spherules are small glass beads that are formed mainly as a result of small impacts on the lunar surface; the ages of these impacts can be determined by the (40)Ar/(39)Ar isochron technique. Here, 155 spherules separated from 1 gram of Apollo 14 soil were analyzed using this technique. The data show that over the last approximately 3.5 billion years, the cratering rate decreased by a factor of 2 to 3 to a low about 500 to 600 million years ago, then increased by a factor of 3.7 +/- 1.2 in the last 400 million years. This latter period coincided with rapid biotic evolutionary radiation on Earth.
Culler
TS
Department of Geology and Geophysics, Department of Physics, University of California, Berkeley, CA 94720, USA. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, U.
Becker
TA
Muller
RA
Renne
PR