HR: 0800h
AN: SH51B-0272    [Abstracts]
TI: Apollo 12 Glass Spherule Ages and the Meteoroid Bombardment History of the Moon
AU: * Levine, J
EM: jlevine@socrates.berkeley.edu
AF: Department of Physics, University of California, Berkeley, CA 94720 United States
AU: Becker, T A
EM: tbecker@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Muller, R A
EM: ramuller@lbl.gov
AF: Department of Physics, University of California, Berkeley, CA 94720 United States
AU: Muller, R A
EM: ramuller@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720-8160 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AB: With 5 g of soil collected by the Apollo 12 astronauts, we are continuing our study of the meteoroid bombardment history of the Moon and the inner solar system. The Moon, by virtue of its lack of water and air, preserves evidence of ancient meteoroid impacts that would quickly be eroded away or otherwise obscured on Earth. We use the $^{40}$Ar/$^{39}$Ar isochron technique to determine the ages of individual glass spherules which are produced in meteoroid impacts on the Moon. The spherules in our study are $\sim$200 microns in diameter, and typically contain $\sim$100 fmol of radiogenic $^{40}$Ar. In Culler et al. (Science 287, p. 1785, 2000), we reported the ages of spherules from an Apollo 14 soil sample. The distribution of spherule ages from a well-mixed sample of the lunar regolith is a proxy for the impactor flux to the Moon since the end of mare basalt volcanism, over 3 billion years ago. The surprising abundance of spherules with young ages that we found in the Apollo 14 soil suggests that the meteoroid bombardment rate in the inner solar system increased suddenly, by a factor of approximately 4, about 400 million years ago. We are testing this hypothesis by examining spherules from a second lunar location; our Apollo 12 samples were collected 180 km away from the Apollo 14 spherules studied by Culler et al. (2000). We have so far analyzed over 100 spherules from Apollo 12 sample 12023. Geochemical analyses of each spherule indicate that the vast majority were formed in impacts, rather than in volcanic eruptions. Preliminary indications are that we are confirming an overabundance of spherules with ages younger than 400 million years. We will present final, calibrated geochronological results for over 150 spherules at the conference. Observing an apparent increase in spherule production at two widely spaced locations on the Moon favors the hypothesis of increased meteoroid bombardment, and disfavors the possibility that special geological circumstances at particular lunar locations led to an increase in spherule production or preservation 400 million years ago. Understanding this meteoroid flux increase will be an interesting future direction in planetary science.
UR: http://jlevine.lbl.gov
DE: 5420 Impact phenomena (includes cratering)
DE: 5455 Origin and evolution
DE: 6250 Moon (1221)
DE: 1035 Geochronology
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2004 AGU Fall Meeting