Irradiation Records, Cosmic-Ray Exposure Ages, and Transfer Times of Meteorites

During the 4.56-G.y. history of the solar system, every meteorite experienced at least one exposure to cosmic rays as a meter-sized meteoroid. The cosmic-ray exposure (CRE) age of a meteorite measures the integral time of exposure to galactic cosmic rays (GCRs). A fraction of meteoritic material was also irradiated by cosmic rays before ejection from kilometer-sized parent bodies (pre-irradiation); studies of pre-irradiation effects yield information about surface processes on planetary objects. In this review we discuss some methods of calculation for CRE ages of meteorites and present CRE age histograms for asteroidal, martian, and lunar meteorites. Compositional, formation-age, and CRE records indicate that probably the ~18,000 meteorites in our collections come from about 100 different asteroids. Stone meteorites exhibit CRE ages ≤120 m.y., while those of iron meteorites are generally, but not always, longer (up to 1500 m.y.); these CRE ages also show evidence for long-term GCR flux variations in the solar neighborhood. The CRE age differences of the various classes either signal different source regions, variability in the size of the Yarkovsky effect, or different resistance against crushing. Rocks blasted off the Moon and Mars by asteroidal or cometary impacts represent surface areas unlikely to be sampled by manned or automated missions; their CRE ages indicate that they come from some eight different sites on the Moon and also on Mars.