| Glass spherules are droplets of melted rock which quenched while in free flight. On the Moon, these are created in volcanic eruptions and, more commonly, in meteoroid impacts. Radioisotopically determined ages of impact spherules from a well-mixed soil sample are a probe of the meteoroid bombardment history of the Moon. The Earth and its natural satellite have experienced similar histories of bombardment, but evidence of ancient impacts is better preserved on the dry, airless, and tectonically inactive Moon.; We collected 179 glass spherules from a soil sample returned to Earth by the Apollo 12 astronauts. Geochemical investigations with the scanning electron microscope showed that nearly all the spherules were of impact origin. The ages of 83 of these impact spherules, which we determined by the 40Ar/39Ar isochron technique, are consistent with models in which the lunar regolith is stratified, with younger soils generally overlying older layers, and with impacts locally inverting the pre-existing stratigraphy.; The analysis of the argon isotopic data we acquired in this experiment forced us to reconsider assumptions and algorithms of 40Ar/ 39Ar dating, as applied to these lunar samples. We illustrate subtleties of the analysis with data from Apollo 14 spherules, studied by Culler et al. (Science 287, 1785, 2000). In our analysis, the distribution of spherules from the Apollo 14 sample, like the set of Apollo 12 spherule ages, is consistent with an increase in the meteoroid bombardment flux in the inner solar system over the last few hundreds of millions of years. However, neither suite of spherules requires this interpretation, and we discuss alternative models.; Lunar spherules are also potentially valuable specimens for the implanted solarwind and cosmogenic gases that they hold. We attempted to constrain the initial distribution of each argon isotope in the Apollo 14 spherules by inverting the measured argon releases. Notwithstanding the utility of spherules for geochronology, we find that most have three-dimensionally complex structures that prevent such inversions, either because of bubbles, mineral inclusions, or chemical heterogeneity. |
