The age of the earth: the approach of Clair Cameron Patterson

This article is a practical physics course designed for high school students that illustrates the Patterson's approach to determine the age of the earth. In 1953, the result published by Patterson (4.55 ± 0.07)*109 y was the first correct estimation of the age of the earth. His approach is based on the idea that the isotopic composition of meteorites can give information on the origin of the solar system. He assumed that meteorites and earth were formed at the same time and from a common homogeneous material (in terms of lead and uranium isotopic compositions). Patterson determined the isotopic lead composition of meteorite samples and inferred the age of the earth. The first part of the article gives the kinetics equations of Pb206 and Pb207, these 2 lead isotopes were present originally and have been created since through the decay of U238 and U235 respectively. The second part present the application to experimental data from 5 meteorite samples. A conclusion is drawn that highlights the limits of the method: the measurement of the lead composition of the samples is very delicate to do because the meteorites contain very low amounts of lead (about 1 ppm) and the crossing of the atmosphere could have contaminated them with lead from humane activities. (A.C.)