A new look at the possible origin of life on earth.
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Study shows early Earth atmosphere hydrogen-rich, favorable to life
A new University of Colorado at Boulder study indicates Earth in its
infancy probably had substantial quantities of hydrogen in its
atmosphere, a surprising finding that may alter the way many scientists
think about how life began on the planet.
Published in the April 7 issue of Science Express, the online edition of
Science Magazine, the study concludes traditional models estimating
hydrogen escape from Earth's atmosphere several billions of years ago
are flawed. The new study indicates up to 40 percent of the early
atmosphere was hydrogen, implying a more favorable climate for the
production of pre-biotic organic compounds like amino acids, and
ultimately, life.
The paper was authored by doctoral student Feng Tian, Professor Owen
Toon and Research Associate Alexander Pavlov of CU-Boulder's Laboratory
for Atmospheric and Space Physics with Hans De Sterk of the University
of Waterloo. The study was supported by the NASA Institute of
Astrobiology and NASA's Exobiology Program.
"I didn't expect this result when we began the study," said Tian, a
doctoral student in CU-Boulder's Astrobiology Center at LASP and chief
author of the paper. "If Earth's atmosphere was hydrogen-rich as we have
shown, organic compounds could easily have been produced."
Scientists believe Earth was formed about 4.6 billion years ago, and
geologic evidence indicates life may have begun on Earth roughly a
billion years later.
"This study indicates that the carbon dioxide-rich, hydrogen-poor Mars
and Venus-like model of Earth's early atmosphere that scientists have
been working with for the last 25 years is incorrect," said Toon. In
such atmospheres, organic molecules are not produced by photochemical
reactions or electrical discharges.
Toon said the premise that early Earth had a CO2-dominated atmosphere
long after its formation has caused many scientists to look for clues to
the origin of life in hydrothermal vents in the sea, fresh-water hot
springs or those delivered to Earth from space via meteorites or dust.
The team concluded that even if the atmospheric CO2 concentrations were
large, the hydrogen concentrations would have been larger. "In that
case, the production of organic compounds with the help of electrical
discharge or photochemical reactions may have been efficient," said Toon.
Amino acids that likely formed from organic materials in the
hydrogen-rich environment may have accumulated in the oceans or in bays,
lakes and swamps, enhancing potential birthplaces for life, the team
reported.
The new study indicates the escape of hydrogen from Earth's early
atmosphere was probably two orders of magnitude slower than scientists
previously believed, said Tian. The lower escape rate is based in part
on the new estimates for past temperatures in the highest reaches of
Earth's atmosphere some 5,000 miles in altitude where it meets the space
environment.
While previous calculations assumed Earth's temperature at the top of
the atmosphere to be well over 1,500 degrees F several billion years
ago, the new mathematical models show temperatures would have been twice
as cool back then. The new calculations involve supersonic flows of gas
escaping from Earth's upper atmosphere as a planetary wind, according to
the study.
"There seems to have been a blind assumption for years that atmospheric
hydrogen was escaping from Earth three or four billion years ago as
efficiently as it is today," said Pavlov. "We show the escape was
limited considerably back then by low temperatures in the upper
atmosphere and the supply of energy from the sun."
Despite somewhat higher ultraviolet radiation levels from the sun in
Earth's infancy, the escape rate of hydrogen would have remained low,
Tian said. The escaping hydrogen would have been balanced by hydrogen
being vented by Earth's volcanoes several billion years ago, making it a
major component of the atmosphere.
In 1953, University of Chicago graduate student Stanley Miller sent an
electrical current through a chamber containing methane, ammonia,
hydrogen and water, yielding amino acids, considered to be the building
blocks of life. "I think this study makes the experiments by Miller and
others relevant again," Toon said. "In this new scenario, organics can
be produced efficiently in the early atmosphere, leading us back to the
organic-rich soup-in-the-ocean concept."
In the new CU-Boulder scenario, it is a hydrogen and CO2-dominated
atmosphere that leads to the production of organic molecules, not the
methane and ammonia atmosphere used in Miller's experiment, Toon said.
Tian and other team members said the research effort will continue. The
duration of the hydrogen-rich atmosphere on early Earth still is
unknown, they said.
Source: University of Colorado at Boulder
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http://www.physorg.com/news3633.html
--
John Hachmann aa #1782
"Those who can make you believe absurdities can make you commit atrocities"
-Voltaire
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