For the first time, scientists have found traces of matter that came to Earth from another star--a supernova that exploded with such power that it powdered Earth with its ashes.
Included in this blast of stardust was an exotic form of radioactive iron that a team of German scientists says it has detected in samples lifted from the floor of the Pacific Ocean.
"This is a stunning discovery," said Donald Clayton, a Clemson University astrophysicist. He said that if the German researchers indeed found this rare form of iron, called iron-60, he has little doubt it came from a supernova.
To have left such a noticeable record on Earth, the researchers estimate, the supernova must have been relatively close--about 90 light-years. That would have been 1,000 times closer than the recent supernova 1987A, which thrilled scientists because it was the brightest observed in 400 years and was faintly visible to the naked eye.
The scientists estimate the older supernova would have appeared a million times brighter than 1987A.
By dating the sediments, the researchers estimate that the star exploded about 5 million years ago. It probably lit up the sky like a full moon, perhaps even catching the attention of man's apelike ancestors.
If it had been any closer, the supernova might have bombarded Earth with life-threatening gamma rays and X-rays, said Gunther Korschinek of the Technical University in Munich. He and his colleagues will publish their finding in next week's issue of the journal Physical Review Letters.
Supernovas have been crucial to the origin of the solar system and Earth. The heavy elements that make up much of the matter on Earth originated in supernovas that exploded before the solar system was born. Beyond that, Clayton and other scientists suspect that one or more of these explosions triggered the formation of the sun and planets from a cloud of gas.
Supporting that idea, scientists have detected a few long-lived radioactive elements on Earth that appear to have come from stars exploding around the time the solar system was born.
But this is the first finding of supernova dust that came after Earth was born. Korschinek and his colleagues decided to look for iron-60 because it is unlikely to come from anything other than the heat, pressure and nuclear activity of a supernova.
Its half-life of about 1.5 million years helps to date the supernova's explosion.
Because of the tendency of metals such as iron to concentrate in certain rocks, researchers knew that to find the minuscule traces of iron-60, they would have to sift through an iron-rich area. They knew that others had dredged up iron-rich samples from the Pacific floor.
To look for iron-60, they used a supersensitive machine called an accelerator mass spectrometer, which can detect extraordinarily diluted elements.