The material that formed the earth was created in a supernova explosion approximately 6 billion years ago. The two most common isotopes of uranium, \({ }^{238} \mathrm{U}\) and \({ }^{235} \mathrm{U}\), were created in roughly equal amounts. Today, the earth contains more than 100 times as much \({ }^{238} \mathrm{U}\) as \({ }^{235} \mathrm{U}\). Which isotope has a longer half-life?