The Determination of the Age of Iron and Stony Meteorites from their Radioactivity

Abstract

Observed meteor and meteorite velocities (Opik, Boothroyd) indicate that a small fraction, perhaps in the neighborhood of 15 percent, of all meteorites striking the earth are of extra-solar origin. Helium-radium age measurements (Paneth, Urry, & Koeck) on 23 iron meteorites, none of which was of known heliocentric velocity, disclose a uniform distribution of last-solidification ages with no ages clearly greater than the age of the earth. Thus, either the specimens are all from the solar system, as Paneth proposed, or the galactic system is of the same age as the earth. Two critical experiments are in progress, based on the following evidence. The abundance-ratio of the C, O, Si, Cl, Fe, Co, and Ni isotopes in meteorites has been studied by various workers, and is found to be the same as in the terrestrial elements. Moreover, the ratio of the two independent isotopes of uranium, 92U235 and 92U238, is the same in all terrestrial specimens, regardless of the geologic age of the mineral in which they are found. It is therefore taken as a reasonable premise, but regarded as a working assumption only, that the original isotopic ratio of any element is a constant of nature and is independent of its place of origin in the galactic system. Measurements of the actinouranium vs. uranium I activity-ratio, and of the specific activity of potassium, are in progress on stony meteorites selected for their known heliocentric velocities. Some of these are of extra-solar origin while others are solar-system meteorites. The final results of the analyses give the difference between the age of the atoms in the meteorites and the same type of atoms in the earth. The method is independent of all influence from varying chemical and physical conditions during the history of the meteorite, as it deals entirely with isotopic ratios. The study of both the AcU, UI ratio and the K40, K41 ratio provides an independent check on the basic assumption of the method. The apparatus, technique, and mathematical background for these experiments are fully described. The announcement of the numerical results of these analyses is deferred until all the observations and control runs have been repeated many times, in order to assure their complete reliability.