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The results obtained from mass-spectrometry analyses of the noble gases He, Ne, Ar, Kr, and Xe in a 182-mg chip of the largest Apollo 15 rock 15555 are presented. The spallation krypton data indicate a well-shielded location through most of the time during which the rock was exposed to cosmic rays. Gas retention ages are estimated. No evidence for the presence of products from plutonium-244 or iodine-129 was found.
The results of the radioactive dating and the discovery of gas-rich meteorites on the Moon surface are reviewed. Special attention is paid to the extinct radioactivity iodine-129. This radioactivity is produced by r-process of nucleosynthesis and it decays with a half-life of 17 m.y. It provides a clock sensitive to small changes in the early years of the solar system.