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Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules
Timothy S. Culler,13
Timothy A. Becker,4*
Richard A. Muller,23*
Paul R. Renne14*
Lunar spherules are small glass beads that are formed mainly as a
result of small impacts on the lunar surface; the ages ofthese impacts
can be determined by the 40Ar/39Ar isochron
technique. Here, 155 spherules separated from 1 gramof Apollo 14 soil
were analyzed using this technique. The datashow that over the last
~3.5 billion years, the cratering ratedecreased by a factor of 2 to
3 to a low about 500 to 600 millionyears ago, then increased by a
factor of 3.7 ± 1.2 in the last400 million years. This latter
period coincided with rapid bioticevolutionary radiation on Earth.
1 Department of Geology and Geophysics,
2 Department of Physics, University of California,
Berkeley, CA 94720, USA.
3 Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA.
4 Berkeley Geochronology Center, 2455 Ridge Road,
Berkeley, CA 94709, USA.
*
These authors are listed alphabetically.
To whom correspondence should be addressed. E-mail:
ramuller{at}lbl.gov
