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Asteroids May Have Played Role in the Origins of Life

Asteroids May Have Played Role in Origins of Life

         SAN FRANCISCO (Reuter) - Asteroids and comets hitting earth
billions of years ago could have intermittently thawed its
frozen oceans, causing reactions that gave rise to the first
living organisms, U.S. researchers said Saturday.
         The theory contradicts the widely accepted view that the
atmosphere of early earth was rich in carbon dioxide, forming a
heat-trapping blanket that prevented the oceans from freezing.
         Jeffrey Bada and Stanley Miller, scientists at the
University of California, San Diego (UCSD), outlined their
theory at the annual meeting of the American Association for the
Advancement of Science here.
         Scientists have debated for decades why the earth did not
become a permanently frozen planet early in its history. Climate
models show that if the atmosphere of the early earth was the
same as today's, a decrease of only a few percent in the sun's
luminosity would cause the oceans to freeze and global surface
temperatures to drop to minus 40 degrees Fahrenheit.
         Yet solar evolution models indicate that the young sun was
20 to 30 percent less luminous than today, the researchers said.
         To overcome this paradox, scientists have proposed that the
atmosphere must have contained a high concentration of carbon
dioxide that kept the planet warm and stopped the oceans
freezing. But a carbon-dioxide-rich atmosphere would have
prevented organic material from being synthesized on earth, the
scientists said.
         Bada, Miller and another researcher, Charles Bigham,
theorize that the top 1,000 feet of the early ocean was frozen.
         They argue that the impact of extraterrestrial objects such
as meteorites and asteroids with diameters of about 60 miles
would have had enough energy to melt the ocean's frozen surface.
         The group suggests that the oceans probably thawed and then
refroze in between a series of impacts.
         Such freeze-melt cycles could have played an important role
in the beginnings of life by trapping material deposited from an
asteroid or comet in the waters beneath the ice, Bada said.
         In that way, materials important for the synthesis of
organic molecules required for the origin of life were not lost
to the atmosphere, where they would have been destroyed.