[meteorite-list] Chondrites support Miller-Urey experiment atmo assumptons

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed Sep 7 11:46:54 2005
Message-ID: <up2uh1t94qe2vun673dmd6iq18s8fage12_at_4ax.com>

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


Sept. 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.
Received on Wed 07 Sep 2005 11:48:06 AM PDT