Meteorite Analysis Favor Reducing Atmosphere for Early Earth
- From: baalke@xxxxxxxxxxxxx
- Date: 7 Sep 2005 09:16:14 -0700
http://news-info.wustl.edu/news/page/normal/5513.html
Calculations favor reducing atmosphere for early earth
Was Miller-Urey experiment correct?
By Tony Fitzpatrick
Washington University in St. Louis
September 7, 2005
Using primitive meteorites called chondrites as their
models, earth and planetary scientists at Washington University in St.
Louis have performed outgassing calculations and shown that the early
Earth's atmosphere was a reducing one, chock full of methane, ammonia,
hydrogen and water vapor.
In making this discovery Bruce Fegley, Ph.D., Washington University
professor of earth and planetary sciences in Arts & Sciences, and Laura
Schaefer, laboratory assistant, reinvigorate one of the most famous and
controversial theories on the origins of life, the 1953 Miller-Urey
experiment, which yielded organic compounds necessary to evolve
organisms.
Chondrites are relatively unaltered samples of material from the solar
nebula, According to Fegley, who heads the University's Planetary
Chemistry Laboratory, scientists have long believed them to be the
building blocks of the planets. However, no one has ever determined
what
kind of atmosphere a primitive chondritic planet would generate.
"We assume that the planets formed out of chondritic material, and we
sectioned up the planet into layers, and we used the composition of the
mix of meteorites to calculate the gases that would have evolved from
each of those layers," said Schaefer. "We found a very reducing
atmosphere for most meteorite mixes, so there is a lot of methane and
ammonia."
In a reducing atmosphere, hydrogen is present but oxygen is absent. For
the Miller-Urey experiment to work, a reducing atmosphere is a must. An
oxidizing atmosphere makes producing organic compounds impossible. Yet,
a major contingent of geologists believe that a hydrogen-poor, carbon
dioxide-rich atmosphere existed because they use modern volcanic gases
as models for the early atmosphere. Volcanic gases are rich in water,
carbon dioxide, and sulfur dioxide but contain no ammonia or methane.
"Geologists dispute the Miller-Urey scenario, but what they seem to be
forgetting is that when you assemble the Earth out of chondrites,
you've
got slightly different gases being evolved from heating up all these
materials that have assembled to form the Earth. Our calculations
provide a natural explanation for getting this reducing atmosphere,"
said Fegley.
Schaefer presented the findings at the annual meeting of the Division
of
Planetary Sciences of the American Astronomical Society, held Sept. 4-9
in Cambridge, England.
Schaefer and Fegley looked at different types of chondrites that earth
and planetary scientists believe were instrumental in making the Earth.
They used sophisticated computer codes for chemical equilibrium to
figure out what happens when the minerals in the meteorites are heated
up and react with each other. For example, when calcium carbonate is
heated up and decomposed, it forms carbon dioxide gas.
"Different compounds in the chondritic Earth decompose when they're
heated up, and they release gas that formed the earliest Earth
atmosphere," Fegley said.
The Miller-Urey experiment featured an apparatus into which was placed
a
reducing gas atmosphere thought to exist on the early Earth. The mix
was
heated up and given an electrical charge and simple organic molecules
were formed. While the experiment has been debated from the start, no
one had done calculations to predict the early Earth atmosphere.
"I think these computations hadn't been done before because they're
very
difficult; we use a special code" said Fegley, whose work with Schaefer
on the outgassing of Io, Jupiter's largest moon and the most volcanic
body in the solar system, served as inspiration for the present early
Earth atmosphere work.
NASA's Astrobiology Institute supported this work.
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