Measurements of radioactive 60Fe and 244Pu deposits on Earth and Moon

Earth is exposed to nearby cosmic events. The solar system moves through the
interstellar medium and collects interstellar dust particles that contain unique
signatures of ongoing nucleosynthesis in nearby supernovae or rare cosmic
explosions. Such particles may accumulate over million years in deep-ocean
archives and in lunar soil, imprinting isotopic fingerprints of specific interstellar
radionuclides in the geological record. The most prominent radionuclides are
60Fe (t1/2 = 2.6 Myr) and 244Pu (81 Myr). Both do not exist naturally on Earth.
They can be measured with high sensitivity via accelerator mass spectrometry
(AMS). Indeed, both nuclides have been found on Earth and 60Fe also on the
Moon.
AMS measurements of 60Fe from terrestrial and lunar archives demonstrate
a clear exposure of Earth to recent (<10 Myr) cosmic explosions, suggesting
close-by supernova activity ∼2–3 and ∼7 million years ago. In addition, recent
detection of interstellar 244Pu, exclusively produced by the rapid neutron capture
(r-)process, allows to link supernovae and r-process. This latter process is far
from being fully understood. Interstellar 244Pu can place constraints on r-process
frequency and production yields over the last few 100 Myr. The measured
244Pu influx was found lower than expected if supernovae dominate r-process
nucleosynthesis, implying contributions from additional sources.
In this chapter, the journey of such radionuclides is followed, after their
production in massive stars or compact objects, from the interstellar medium
to their incorporation in geological archives, resulting finally in the detection of
a few atoms via direct atom counting. These experimental results provide unique
insights into recent and nearby nucleosynthesis events.