where did the uranium on earth come from
Uranium Along with all elements having atomic weights higher than that of iron, uranium is only naturally formed in supernovae.[52] Primordial thorium and uranium are only produced in the r-process (rapid neutron capture), because the s-process (slow neutron capture) is too slow and cannot pass the gap of instability after bismuth.[53][54] Besides the two extant primordial uranium isotopes, 235U and 238U, the r-process also produced significant quantities of 236U, which has a shorter half-life and has long since decayed completely to 232Th, which was itself enriched by the decay of 244Pu, accounting for the observed higher-than-expected abundance of thorium and lower-than-expected abundance of uranium.[55] While the natural abundance of uranium has been supplemented by the decay of extinct 242Pu (half-life 0.375 million years) and 247Cm (half-life 16 million years), producing 238U and 235U respectively, this occurred to an almost negligible extent due to the shorter half-lives of these parents and their lower production than 236U and 244Pu, the parents of thorium: the 247Cm:235U ratio at the formation of the Solar System was (7.0 ± 1.6) × 10−5.[56]