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>Date: Fri, 30 Jul 1999 23:56:51 GMT
>From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>To: astro@lists.mindspring.com
>Subject: [ASTRO] Nearby Supernova may Have Caused Mini-Extinction,
Scientists Say
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>Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
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>August 1999
>
>Nearby supernova may have caused mini-extinction, scientists say
>
>CHAMPAIGN, Ill. -- The recent discovery of the rare radioactive isotope
>iron-60 in deep-sea sediments could be the telltale sign of a killer
>supernova, a University of Illinois researcher says.
>
>"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.
>
>