Implications for Complex Life: Rarity of Actinide Nucleosynthesis

New limit of 244Pu on Earth points to rarity of actinide nucleosynthesis

Authors:

Wallner et al

Abstract:

Half of the heavy elements including all actinides are produced in r-process nucleosynthesis whose sites and history still remain a mystery. If continuously produced, the Interstellar Medium (ISM) is expected to build up a quasi-steady state of abundances of short-lived nuclides (with half-lives less than 100My), including actinides produced in r-process nucleosynthesis. Their existence in today's ISM would serve as a radioactive clock and would establish that their production was recent. In particular 244Pu, a radioactive actinide nuclide (81 My half-life), can place strong constraints on recent r-process frequency and production yield. Here we report on the detection of live interstellar 244Pu, archived in Earth's deep-sea floor during the last 25 My, at abundances lower by about two orders of magnitude than expected from continuous production in the Galaxy. This large discrepancy may signal a rarity of actinide r-process nucleosynthesis sites, compatible with neutron-star mergers or with a small subset of actinide-producing supernovae.