An exploding star may have destroyed part of Earth?s protective ozone layer two million years ago, killing off some forms of ancient marine life. Narciso Benitez, of Johns Hopkins University, says the ?missing smoking gun? of a mass extinction that occurred far in the past was the revelation that a stellar cluster with many large, short-lived stars prone to producing supernovae passed near Earth?s solar system several million years ago.

That discovery, made by Space Telescope Science Institute astronomer Jesus Maiz-Apellaniz, led Benitez to check the scientific record for potential effects of nearby supernovae on Earth. ?Nobody had realized that this cluster of stars that Jesus had tracked, which is known as the Scorpius-Centaurus OB association, could have been so close to Earth during the past several million years,? he says. ?And when I did a search, one of the first things that popped out was a 1999 finding where a team of German astronomers led by Klaus Knie detected the presence of a highly unusual isotope of iron in samples of the Earth?s crust drilled from the deep ocean bottom.?

Knie had proposed that the iron isotope was debris from a supernova explosion that took place close to Earth. But astronomers didn?t know of any nearby explosion until they discovered Maiz-Apellaniz?s work. Benitez compared data produced by Maiz-Apellaniz and Knie?s results, and found ?very good agreement, both in the amount of iron deposited and in its time distribution.?

Benitez consulted with his wife, Matilde Canelles, an immunologist at the U.S. National Institutes of Health, to learn if the paleontological record included an extinction that had characteristics that suggested a link to a supernova. ?Such an extinction would have had especially pronounced effects on the plankton and the marine organisms,? he says. Canelles found evidence of a widespread extinction of plankton and other marine organisms about two million years ago, and says scientists are still debating the possible causes of the event.

?Based on the minimal distance we expected for a supernova in the Scorpius-Centaurus association at that time, I then did some calculations to explore the potential effects on Earth,? says Benitez. He found that cosmic ray emissions from a supernova could have had a devastating effect on Earth?s ozone layer, which absorbs harmful ultraviolet emissions from the sun and other sources.

?People study supernovae using telescopes and supercomputer simulations,? says Benitez ?In the future, some of the most relevant information in this field may be found in the deep ocean floor.?

Benitez and Maiz-Apellaniz say there?s no need to worry about another supernova in the Scorpius-Centaurus group affecting Earth in the near future. The next star due to explode, Antares, is now located almost 500 light-years away, which is too far to have an effect on Earth.

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