Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

Abstract: There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope 60Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as 26Al and 244Pu. The unique information provided by geological and lunar detections of radioactive 60Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs.

• The paper demonstrates that supernovae near Earth, detected via Iron-60 isotopes, occurred within 100 parsecs around 2-3 and 8 million years ago.
• The study utilizes accelerator mass spectrometry to identify isotope peaks, providing key insights into stellar nucleosynthesis and cosmic-ray sources.
• The findings promote interdisciplinary research by encouraging the search for additional isotopic markers to refine our understanding of local astrophysical events.

Overview of Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

This paper, presented to the 2020 Decadal Survey on Astronomy and Astrophysics, explores the compelling evidence of supernova explosions occurring within 100 parsecs of Earth in the recent geological past. The identification of radioactive isotopes, especially Iron-60 ($^{60}\text{Fe}$), in deep-ocean samples and lunar regolith has provided a robust foundation for understanding such cataclysmic astrophysical events' effects on Earth. This evidence signifies a seminal example where a dated astrophysical phenomenon outside the Solar System has unmistakably impacted our planet, ushering new opportunities for interdisciplinary scientific exploration.

Evidence of Supernova Proximity

The detection of $^{60}\text{Fe}$, an isotope with a half-life of 2.6 million years, is central to confirming the presence of supernova-derived material on Earth. Two significant temporal peaks have been identified in the distribution of $^{60}\text{Fe}$ in geological samples—a primary peak around 2-3 million years ago, with indications of an additional peak near 8 million years ago. These measurements, obtained through accelerator mass spectrometry (AMS), provide compelling evidence that core-collapse supernovae (CCSN) were sources of these isotopes, with their proximity to Earth estimated to be approximately 100 parsecs. This distance, beyond the "kill radius" for mass extinctions, suggests possible but not catastrophic perturbations to Earth's biosphere.

Implications and Research Opportunities

The implication of supernova events in the proximal solar neighborhood extends across fields such as nucleosynthesis, cosmic-ray astrophysics, and heliophysics. From a theoretical standpoint, terrestrial detections of $^{60}\text{Fe}$ afford a unique empirical probe of stellar evolution and nucleosynthesis processes—especially concerning the synthesis of heavy elements. Additionally, the presence of $^{60}\text{Fe}$ alongside cosmic-ray observations necessitates reevaluation of local supernovae as significant cosmic-ray sources influencing both Earth’s atmospheric chemistry and its biosphere.

The study also underscores the necessity for interdisciplinary research to fully decipher the data. Future work should focus on identifying additional isotopic markers beyond $^{60}\text{Fe}$, such as $^{244}\text{Pu}$, to garner insights into the nucleosynthetic yields from nearby supernovae and, potentially, neutron-star mergers as alternative sources of heavy elements. Furthermore, pinning down precise supernova remnants and associating them with current astronomical structures like the Local Bubble could enhance our understanding of stellar dynamics in our galactic neighborhood.

Conclusion

This investigation into near-Earth supernova explosions epitomizes a nascent scientific frontier, one interwoven with diverse disciplines ranging from nuclear physics to evolutionary biology. It calls for sustained funding and collaborative efforts in probing this multifaceted domain, leveraging astronomical observations and terrestrial data. As new cases are mapped and additional isotopes analyzed, the insights gained promise to enrich the theoretical frameworks governing astrophysical events, leading to a deeper understanding of the universe’s impact on Earth across geological timescales.