238 The half-lives of 235U and U are 7.04 × 10° years and 4.47 x 10° years, respectively, and the present abundance ratio is 238U/235U = 137.7. It is thought that their abun- dance ratio was 1 at some...

Extracted text: 238 The half-lives of 235U and U are 7.04 × 10° years and 4.47 x 10° years, respectively, and the present abundance ratio is 238U/235U = 137.7. It is thought that their abun- dance ratio was 1 at some time before our earth and solar system were formed about 4.5 × 10° years ago. Estimate how long ago the supernova occurred that supposedly produced all the uranium isotopes in equal abundance, including the two longest lived isotopes, 238U and 235U. Using the result of problem 29 and the accepted age of the earth, 4.5 x 10° yr, calculate the 238U/235U ratio at the time the earth was formed.