New Messengers From Cosmic Explosions: Live Radioisotopes on the Deep Sea and Moon

The geological and lunar records represent natural archives that contain new messengers from cosmic explosions: live (not decayed) radioisotopes ejected in these outbursts. Events sufficiently nearby will drive blast waves that engulf the solar system and deliver the radioisotopes to the Earth and Moon. Thanks to their low or terrestrial backgrounds, these astrophysical radioisotopes can be detected with ultrasensitive accelerator mass spectrometry (AMS) techniques. The first such messenger is now well-established: a wealth of ⁶⁰Fe (half-life 2.6 Myr) detections around the Earth and on the Moon demonstrate that at least two supernovae exploded near the Earth around 3 and 7 Myr ago. Recently, a new messenger has come into our grasp: the r-process actinide ²⁴⁴Pu (half-life 80 Myr) has been detected a deep-sea sample where ⁶⁰Fe is also found. We will review the status of these detections and present models of r-process nucleosynthesis to interpret their significance. Available deep-sea data allows for the ²⁴⁴Pu to originate either in one or both of the recent supernova(e), or in a separate nearby kilonova event. We will discuss how future observations can discriminate these possibilities and thus shed new light on the origin of the r-process. In particular we will highlight the value of upcoming lunar samples returned by the Chang'e and Artemis missions.