Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p11a..06l&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P11A-06
Physics
1035 Geochronology, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5420 Impact Phenomena (Includes Cratering), 6250 Moon (1221)
Scientific paper
We are using lunar glass spherules to measure changes in the meteoroid bombardment of the inner solar system through the last four billion years. Spherules, which range in size up to a few hundred microns in diameter, are formed in meteoroid impacts and volcanic fire-fountaining events on the surface of the Moon. The ages of formation of the impact spherules, which we determine using the 40Ar/39Ar isochron technique, record the timing of the impacts that produced them. The distribution of 155 ages of spherules taken from Apollo 14 soil sample 14163 (which we reported in 2000 in Science 287, 1785) showed a gradually decreasing abundance of spherules formed from 3.0 to 0.4 Ga as the solar system matured, and then a surprising fourfold jump in spherule production in the last 400 Myr. We now seek to learn whether the recent increase in spherule production is due to increased meteoroid bombardment since that time, an increase in the efficiency of spherule creation in impacts, or a change in preservation conditions at the Apollo 14 landing site. To determine whether the abundance of young spherules truly implies a Solar System-scale phenomenon, we are repeating the experiment with samples from a different location on the Moon. We have selected 178 spherules from Apollo 12 soil 12023 which, like 14163, was taken from the ejecta of a recent bedrock-penetrating impact, and therefore should offer a representative selection of spherules from various depths in the regolith. These spherules are each larger than 180 microns in diameter, and, because precision of our age measurements is limited by the scarcity of K, the larger size of these spherules relative to those in our previous study should compensate for the somewhat lower K content of Apollo 12 soils. Chemical compositions, which we measured using energy dispersive x-ray spectrometry, imply that nearly all the Apollo 12 spherules were produced in impacts rather than in volcanic eruptions. Impact origin is indicated by Fe-Ni surface grains, chemical heterogeneity, and low Mg/Al ratios. Radioisotopic age measurements on the spherules are now under way. If the Apollo 12 measurements confirm our earlier observation that the meteoroid bombardment has been four times higher over the last 400 Myr than previously, we shall need to consider where a new population of impactors could have come from, how long such epochs of high meteoroid flux last, and how significant such periods might be for the evolution of life on Earth.
Levine Jerome
Muller Richard A.
Renne Paul R.
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