"A nearby supernova would bathe our planet in high-energy particles -- cosmic rays -- with potentially disastrous effects," said Brian Fields, a visiting professor of astronomy at the U. of I. "Increased cosmic-ray bombardment could have affected Earth's biosphere by enhancing the penetration of harmful solar ultraviolet radiation and by increasing the global cloud cover, leading to a 'cosmic-ray winter' and a mini-extinction."

In a paper scheduled to appear in New Astronomy, Fields and colleague John Ellis, a theoretical physicist at CERN, claim the measured iron-60 abundance cannot be explained by known sources, but could be the radioactive ashes of a supernova -- the explosion of a massive star. This cataclysmic explosion occurred 100 light-years from Earth about 5 million years ago, Fields and Ellis estimate.

The scientists based their analysis upon iron-60 data collected by a German team led by Gunther Korschinek of the Technical University of Munich.

"If these data are confirmed, together with their extraterrestrial origin, the implications are profound," Fields said. "They would constitute the first direct evidence that a supernova occurred near Earth in the fairly recent geologic past, with detectable effects on our planet."

Those effects include a huge increase in global cloud cover and a dramatic decrease in the protective ozone layer.

"A correlation has been observed between solar activity -- namely the sunspot cycle -- and the extent of Earth's cloud cover," Fields said. "Some researchers believe this correlation is due to modulation of the normal cosmic-ray flux observed today, caused by variations in the solar wind."

The enhanced cosmic-ray bombardment from a nearby supernova could create a large increase in global cloud cover, Fields said, significantly reducing Earth's surface temperature and triggering a cosmic-ray winter that could last for thousands of years.

Besides global cooling, an increased cosmic-ray flux would produce additional stratospheric nitric oxide that could deplete the ozone and expose the biosphere to the harmful effects of ultraviolet radiation.

"There is fossil evidence for a couple of mini-extinctions during the Cenozoic Era," Fields said. "One occurred about 13 million years ago; the other occurred about 3 million years ago. Marine animal families near the bottom of the food chain -- such as zooplankton and echinoids -- were impacted the most. The pattern of extinction is consistent with a major reduction in marine photosynthesis."

The supernova origin of the observed iron-60 would be confirmed by finding additional radioactive nuclei, such as plutonium-244. "This would open up a whole new era of supernova studies using deep-ocean sediments as a telescope," Fields said.

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