Blast From the Past

Scientists say they have found, for the first time, traces of an ancient supernova explosion imbedded in Earth. Large amounts of an unusual radioactive version of iron at the bottom of the South Pacific Ocean, described in the 5 July Physical Review Letters, could only have come from a supernova, the researchers say.

Although the universe is now replete with heavy metals--what astronomers call all elements heavier than helium--it didn't start out that way. The earliest generations of stars probably formed out of nearly pure hydrogen, which fused into helium, carbon, and oxygen as the stars burned. Theoretical models predict that heavier elements like iron are produced only in the cauldrons of supernovae, the spectacular explosions of certain large stars at the ends of their lives. A supernova's immense energy scatters iron and other metals far and wide. Although scientists believe that virtually all heavy elements--from the aluminum in beer cans to the iron in refrigerator magnets--were formed in stars, the rarity of nearby supernova explosions and the constant geologic mixing in the Earth's crust have prevented them from connecting specific atoms to particular supernovae. Until now.

Looking for supernovae remnants in three several-million-year-old layers of sediment from the ocean floor near Mona Pihoa in the South Pacific, a team led by astrophysicist Gunther Korschinek of the Max Planck Institute for Astrophysics in Garching, Germany, discovered iron-60, a radioactive isotope, in two of the three millimeter-thick layers. Cosmic rays and interstellar dust can deposit trace amounts of iron-60 on Earth, but the concentrations found by Korschinek's team are a hundred times too high to be explained by either source. A supernova explosion is the only possible explanation, Korschinek's group says. To produce the observed iron-60 density, the metal must have formed roughly 5 million years ago and was released from a supernova about 100 light-years from Earth, the team estimates.

"The data look suggestive, but not definitive," says supernova expert Adam Burrows of the University of Arizona in Tucson. He points out that a supernova would spread iron-60 over the entire globe, so these measurements need to be confirmed at other locations. The German team now wants to search for iron from the same supernova in the polar ice caps, where the slow rate of geological mixing may have preserved the iron in distinct layers.

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