How a nearby supernova left its mark on Earth life

        When a massive star explodes as a supernova, it does more than release an extraordinary amount of energy. Supernovae explosions are responsible for creating some of the heavy elements, including iron, which is blasted out into space by the explosion.On Earth, there are two accumulations of the iron isotope 60Fe in sea-floor sediments that scientists trace back to about 2 or 3 million years ago and to about 5 to 6 million years ago.

The explosions that created the iron also dosed Earth with cosmic radiation.

In new research submitted to The Astrophysical Journal Letters, scientists examine how much energy reached Earth from these explosions and how that radiation may have affected life on Earth.

The paper is titled "Life in the Bubble: How a nearby supernova left ephemeral footprints on the cosmic-ray spectrum and indelible imprints on life," and it is available on the arXiv preprint server. The lead author is Caitlyn Nojiri from UC Santa Cruz.

"Life on Earth is constantly evolving under continuous exposure to ionizing radiation from both terrestrial and cosmic origin," the authors write. Terrestrial radiation slowly decreases over billions of years. But not cosmic radiation. The amount of cosmic radiation that Earth is exposed to varies as our solar system moves through the galaxy.

"Nearby supernova (SN) activity has the potential to raise the radiation levels at the surface of the Earth by several orders of magnitude, which is expected to have a profound impact on the evolution of life," they write.The authors explain that the 2-million-year-old accumulation is directly from a supernova explosion, and the older accumulation is from when Earth passed through a bubble.

The Local Bubble is not a quiet place. It took multiple supernovae to create it. The authors write that it took 15 SN explosions over the last 15 million years to create the LB. "We know from the reconstruction of the LB history that at least nine SN exploded during the past 6 Myrs," they write.

The researchers took all the data and calculated the amount of radiation from multiple SNe in the LB. "It is not clear what the biological effects of such radiation doses would be," they write, but they do discuss some possibilities.