Possible cosmic dust origin of terrestrial plutonium-244

Abstract

THE discovery of ²⁴⁴Pu by Hoffman et al.¹ in bastnaesite, a Precambrian rare earth fluorocarbonate ore from the Mountain Pass deposit in California, has raised the question of whether the so-called extinct nuclide, with a half life of 82 m.y., is part of the extant (or almost extinct) nuclide group or whether it was brought to Earth from a cosmic-ray source before the formation of the ore^1,2. I have reinvestigated the implications of its present existence in nature in terms of three possibilities: (1) survival of the primaeval ²⁴⁴Pu, (2) influx as a heavy cosmic-ray component, and (3) inflow as a cosmic dust component from supernova remnants. The result shows that there are some difficulties with the first two possibilities, both of which were thought to be plausible at the time of the discovery. Although it is still a speculative proposition at present that any part of the cosmic dust falling on the Earth originated outside of the Solar System, a calculation suggests that the ²⁴⁴Pu may have been carried by such dust. The ¹²⁹Xe excess detected by Srinivasan et al.³ in an old Australian iodyrite, AgI, is also not inconsistent with the present hypothesis.