Getting Older

THE AGE OF THE EARTH as suggested by a comparison of the ratios in meteorites of two isotopes of lead has been pushed back to 4.5 billion years, which is the earliest date yet obtained by analyzing the elements in the solar system. Studies made with a mass spectrometer at the University of Chicago by H. Brown, M. Inghram, C. Patterson, and G. Tilton on the lead content of meteorites have provided better agreement with astronomical estimates of the earth’s age (about four billion years) than has other geophysical evidence. Comparisons of uranium to lead ratios in rocks, for example, have indicated an age no greater than 3.3 billion years. The present results, reported at a recent conference on geophysics and described in the University’s publication Reports, depend on the fact that calculable amounts of lead 206 result from the radioactive decay of uranium 238 while larger amounts of lead 207 result from the more rapid decay of uranium 235.