COSMOGENIC NUCLIDES IN EXTRATERRESTRIAL MATERIALS

The nuclides made in extraterrestrial materials by cosmic rays help reveal the histories both of the irradiated objects and of the cosmic rays. Improvements in measurement techniques, especially in accelerator mass spectrometry, have greatly reduced detection limits. Thanks to several extensive series of measurements in meteorites, a comprehensive picture has taken shape of how cosmogenic nuclide production depends on size, shape, and composition. Complementary to this work, (1) the irradiation in space of meteoroids has been simulated by means of accelerator experiments both with very thick and with spherical targets, and (2) various models for calculating production rates of cosmogenic nuclides have been developed or refined. New classes of material—meteorites recovered in Antarctica and tiny meteorites from the stratosphere and deep‐sea sediments, for example—have become more widely available for analysis. From their cosmogenic nuclide contents we have learned about the exposure histories of lunar meteorites, of SNC meteorites, which may come from Mars, and of interplanetary dust particles. Cosmogenic nuclides have been put to use in unfolding increasingly complex exposure histories. Some grains in gas‐rich meteorites were irradiated in at least two episodes, and others perhaps even before the solar system formed. Continuing measurements of lunar samples reveal the past behavior of energetic solar particles. We now know from cosmogenic nuclide measurements that many meteorites found in Antarctica have been there for ∼0.1–1 million years and that the age distributions vary from site to site. Copyright 1990 by the American Geophysical Union.