When stars attack

Posted by David Eicher
on Wednesday, May 30, 2007

On Tuesday morning, May 29, I sat in one of many of the week's paper sessions at the American Astronomical Society meeting in Honolulu, Hawaii. Not all of them turn out to be unusually interesting; this one did. "I'm going to talk about cosmic WMDs," said Brian Fields, an astronomer at the University of Illinois. His target was supernovae, nearby supernovae in particular, and gauging how dangerous they could be — and have been — to living things on Earth.

"This is a hobby," said Fields, explaining that he and his colleagues work on this idea in an unfunded way because they're curious about the answers. Fields offers a novel way of investigating the deadliness of supernovae — dredging up sea sludge. Sediments from the ocean floor, in this way, can be used as a probe of past supernovae. How is this possible? Bear with me for a moment ...

It's important to know about the supernova record because a nearby exploding star could offer up big problems for us. Ionizing radiation could wipe out civilization; ultraviolet, X rays, and gamma rays, and diffuse cosmic rays, would present a big problem. The radiation itself would not be the primary problem, though. Instead, the damage done to the ozone layer would mean that, following the supernova, our own Sun would destroy us once our atmosphere was destroyed. There's another idea that a shower of cosmic rays could seed cloud formation and offer up a "cosmic ray winter." But that's debatable. Moreover, the flood of neutrinos from a nearby supernova would damage DNA.

The minimum safe distance for a supernova is 8 parsecs, about 25 light-years. Anything beyond that would be a welcome curiosity, says Fields, anything closer a big problem. Within 30 parsecs (100 light-years) of us, a supernova explodes about every 10 million years. Within a few parsecs, according to Fields, "multiple events have occurred during the 4.5-billion-year history of the solar system."

So where's the smoking gun? Is there really supernova debris on Earth? Fields and his colleagues, as well as other working groups, have studied sea sediments that may contain supernova particles because past blasts have temporarily overwhelmed the solar wind, depositing it on Earth. (Such a record could exist in ice cores, too.) The fingerprint scientists look for in this "supernova archaeology" comes in the form of so-called live radioactive isotopes. Studying the deep ocean crust, a working group led by Klaus Knie of the Technical University of Munich, found a slab of solidified sediment, a ferromanganese crust, with live 60Fe. This radioactive material was deposited by a supernova blast 2.2 million years ago. In 2004, the same group found 10Be, another live radioactive marker, in another rock that indicates a supernova blast 2.8 million years ago.

Stay tuned. This is an exciting, new field of multi-disciplinary research that we will hear much more about.

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