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Researchers then scoured the globe for thin layers of radioactive isotopes in rock strata and in 1999 struck figurative gold: Samples from beneath the ocean revealed some hard metallic layers, known as ferromanganese crusts that form slowly over millions of years, containing iron-60, an isotope with a half-life of 2.6 million years -- so short that the material must be much younger than Earth. The iron-60 was in a stratum laid down 2.2 million years ago. Similar layers of iron-60 have since been found elsewhere in the oceans. Astronomers have also been scouring the skies for groups of stars that...

Radioactive elements synthesized in massive stars are ejected into space via stellar winds and supernova explosions. Our Solar System moves through the interstellar medium and collects these extrasolar products. One such product is iron-60. Because it is not naturally produced on Earth, the presence of this radioactive isotope is a sensitive indicator of supernova explosions within the last few million years. Australian National University’s Dr. Anton Wallner and colleagues previously found traces of iron-60 at about 2.6 million years ago, and possibly another at around 6 million years ago, suggesting our planet had traveled through fallout clouds from nearby supernovae....