Supernova dealt deaths on earth? Stellar blasts may have killed ancient marine life. (This Week).

A group of young, hot stars in and near the constellation Scorpius shine brightly, making a memorable sight in the southern night sky. But if Narciso Benitez of Johns Hopkins University in Baltimore and his colleagues are correct, this collection of stars could have a lot to answer for. One of its members may have exploded in the solar system's neighborhood 2 million years ago, causing the widespread destruction of a variety of marine species here on Earth.

About 35 of the thousands of stars in this group, known as the Scorpius-Centaurus (Sco-Cen) OB association, weigh several times as much as the sun. These heavyweights--the group's brightest stars--tend to end their relatively brief lives with a powerful bang called a supernova. Other astronomers have determined that previous supernovas within the Sco-Cen association carved out the Local Bubble, a low-density cavity of interstellar gas that extends about 150 light-years around the sun (SN: 4/20/96, p. 248).

The Sco-Cen stars now lie some 450 light-years from Earth, too far away for a supernova explosion to harm our solar system. But by using data from the Hipparcos satellite to measure the current positions and velocities of stars in the association, Benitez' team traced the stars' paths back in time. The researchers found that the Sco-Cen members were quite a bit closer a few million years ago. Some could have passed within 130 light-years of Earth.

That's near enough that if one of the stars went supernova, cosmic rays from the explosion would have destroyed much of Earth's ozone layer. Then, the sun's harmful ultraviolet radiation would have penetrated the atmosphere and could have led to the destruction of plankton, mollusks, and other marine life at the so-called Pliocene-Pleistocene boundary 2 million years ago, the team suggests.

Benitez and his colleagues Jesus Maiz-Apellaniz of the Space Telescope Science Institute in Baltimore and biologist Matilde Canelles of Silver Spring, Md., describe their analysis in an upcoming Physical Review Letters.

Two key pieces of evidence support their proposal, the researchers note. A study by Maiz-Apellaniz indicates that the Sco-Cen association has had 20 supernova explosions over the past 11 million years. Further, an analysis of deep-ocean crusts by other teams shows an excess of iron-60 in samples that date roughly from the time of the Pliocene-Pleistocene extinction. Iron-60, an isotope produced by supernovas, could have become incorporated into ocean sediment if dust from a stellar explosion bombarded Earth.

The extinction at the Pliocene-Pleistocene boundary is considered a modest one. Several researchers over the past 25 years have proposed that nearby supernova explosions could have caused one or more of the most massive extinctions, such as the dinosaur die-off. However, the new analysis shows that unless a supernova came within a few light-years of Earth, its radiation wouldn't be intense enough to cause such an extinction.

Geologist Luann Becker of the University of California, Santa Barbara concurs with Benitez' team that a supernova is indeed a plausible but unproven candidate for the marine extinction. She notes that supernova dust isn't the only type of iron-rich space debris to pelt Earth. Analyzing more samples of ocean sediment to pinpoint the amount and timing of the iron-60 excess could provide a more rigorous test of the supernova model, Becker adds.