A deep-ocean ferromanganese crust, of the kind used to identify radioactive supernova debris. The German coin is for scale. Image from the Munich AMS group.
How can we know our planet was the subject of these cosmic insults?
The key to "supernova archaeology" was found by Ellis, Fields, and Schramm (1996), who considered the impact of a supernova on the Earth. If an event were close enough, the powerful blast wave could penetrate the solar system. As a result, debris from the explosion would literally be dumped on the Earth. This material would be a mix of interstellar gas, and more importantly, fresh supernova-produced atoms. Some of these atoms would have been radioactive. This is useful, because radioactive material is unstable, and naturally exists on the Earth in very low amounts. Consequently it is easy to detect even a modest amount of supernova debris from its deposition of radioactivity. Ellis, Fields, and Schramm (1996) thus arrived at their prediction: an unusually high level of radioactive atoms in geological strata represents the "gold-plated signature" of a nearby supernova.
Several years ago, the pioneering work of Knie, Korschinek, Faestermann, Wallner, Scholten, & Hillenbrandt in July 1999 Physical Review Letters (electronic version) presented the first evidence of just such a signature. These results were obtained in a lab at Technical University of Munich, in Germany. A companion paper by Fields & Ellis, in New Astronomy (at LANL preprint server) discussed these results. We find that it is indeed possible to interpret the iron-60 results in terms of a "recent" supernova exploding near the Earth.
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A deep-ocean ferromanganese crust, of the kind used to identify radioactive supernova debris. The German coin is for scale. Image from the Munich AMS group. |
The Munich group has now published new results which spectacularly confirm their original findings, and open the door to "radioactive supernova astronomy." The new study by Knie et al (2004) is a high-precision assay of ancient, deep-sea material: a crust of manganese and iron deposits formed over millions of year on a rock in the deep ocean. The German group analyzed the iron content of this crust with ultrasensitive techniques (accelerator mass spectrometry) that can detect the presence of single atoms of rare species. They found that iron-60 was present. This unstable, radioactive isotope of iron has a half-life of 1.5 million years, and thus had to be recently created. The basic method was similar to the original 1999 results, but used a different crust from a different location in the Pacfic; the detection of iron-60 in the new sample thus shows that this radioactive anomaly was widespread, consistent with the supernova scenario. Moreover, both the deep-ocean sample, and the laboratory techniques, were better suited for a high-resolution measurement. Knie et al (2004) showed that the iron-60 atoms are isolated in a single layer 2.8 million years old. They estimated that a supernova exploding at that time, and a distance of about 120 light years, could lead to the radioactive signal they found.
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Finding atomic needles in sedimentary haystacks: accelerator mass spectrometry laboratory, Max Planck Institute, Germany |
