The new theory brings together puzzling clues from several different fields of research, including paleontology, geology and astronomy.

Narciso Ben�tez, an associate research scientist in astronomy in the Krieger School of Arts and Sciences at The Johns Hopkins University, says the "missing smoking gun" that brought the clues together was the revelation that a stellar cluster with many large, short-lived stars prone to producing supernovae had passed near Earth's solar system several million years ago.

That discovery, made by co-author and Space Telescope Science Institute astronomer Jes�s Ma�z-Apell�niz, led Ben�tez to check the scientific record for potential effects of nearby supernovae on the Earth.

"Nobody had realized that this cluster of stars that Jes�s had tracked, which is known as the Scorpius-Centaurus OB association, could have been so close to Earth during the past several million years," Ben�tez 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 recent supernova explosion that took place close to Earth. But astronomers had no plausible candidates for such a nearby explosion until Ma�z-Apell�niz's work with the Scorpius-Centaurus association, which is also being presented at this week's meeting of the American Astronomical Society.

Ben�tez compared data produced by Ma�z-Apell�niz and Knie's results, and found "very good agreement, both in the amount of iron deposited and in its time distribution."

Ben�tez consulted with his wife, Matilde Ca�elles, an immunologist at the National Institutes of Health who had done her master's thesis on microscopic algae, to learn if the paleontological record included an extinction that had unusual characteristics suggestive of a potential link to a supernova.

"Such an extinction would have had especially pronounced effects on the plankton and the marine organisms," Ben�tez explains.

Ca�elles pointed out that evidence existed for a widespread extinction of plankton and other marine organisms about 2 million years ago, and noted that 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 Ben�tez.

He found that cosmic ray emissions from a supernova could have had a potentially devastating effect on the Earth's ozone layer, an upper layer of the atmosphere that absorbs harmful ultraviolet emissions from the sun and other sources.

"This would have produced a significant reduction in phytoplankton abundance and biomass, with devastating effects on other marine populations, such as bivalves," Ben�tez says.

Ben�tez emphasizes that the theory, while provocative, is consistent with the paleontological evidence, and also with the pattern of movement of the Scorpius-Centaurus group, which would have been at its closest to Earth at that time.

He concedes, though, that more evidence will be needed to firmly establish the theory. In particular, more detailed searches for supernova-produced isotopes in the geological record would show whether there was a tight temporal correspondence between the supernova explosion and the extinction event. Isotope searches could also offer crucial information about the physical processes involved in supernova explosions.

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

While the new theory may further heighten concern about human impacts on the ozone layer today, Ben�tez and Ma�z-Apell�niz 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 in the association, Antares, is now located at a distance of almost 500 light-years, which is too far away to have a significant effect on our planet.

This research was funded by an Advanced Camera for Surveys grant from NASA, the Johns Hopkins Center for Astrophysical Sciences, and a grant from the Space Telescope Science Institute.

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