Using primitive meteorites called chondrites as their models, earth andplanetary scientists at Washington University in St. Louis haveperformed outgassing calculations and shown that the early Earth'satmosphere was a reducing one, chock full of methane, ammonia, hydrogenand water vapor.
In making this discovery Bruce Fegley, Ph.D., Washington Universityprofessor of earth and planetary sciences in Arts & Sciences, andLaura Schaefer, laboratory assistant, reinvigorate one of the mostfamous and controversial theories on the origins of life, the 1953Miller-Urey experiment, which yielded organic compounds necessary toevolve organisms.
Chondrites are relatively unaltered samples of material fromthe solar nebula, According to Fegley, who heads the University'sPlanetary Chemistry Laboratory, scientists have long believed them tobe the building blocks of the planets. However, no one has everdetermined what kind of atmosphere a primitive chondritic planet wouldgenerate.
"We assume that the planets formed out of chondritic material,and we sectioned up the planet into layers, and we used the compositionof the mix of meteorites to calculate the gases that would have evolvedfrom each of those layers," said Schaefer. "We found a very reducingatmosphere for most meteorite mixes, so there is a lot of methane andammonia."
In a reducing atmosphere, hydrogen is present but oxygen isabsent. For the Miller-Urey experiment to work, a reducing atmosphereis a must. An oxidizing atmosphere makes producing organic compoundsimpossible. Yet, a major contingent of geologists believe that ahydrogen-poor, carbon dioxide -rich atmosphere existed because they usemodern volcanic gases as models for the early atmosphere. Volcanicgases are rich in water , carbon dioxide, and sulfur dioxide butcontain no ammonia or methane.
"Geologists dispute the Miller-Urey scenario, but what theyseem to be forgetting is that when you assemble the Earth out ofchondrites, you've got slightly different gases being evolved fromheating up all these materials that have assembled to form the Earth.Our calculations provide a natural explanation for getting thisreducing atmosphere," said Fegley.
Schaefer presented the findings at the annual meeting of theDivision of Planetary Sciences of the American Astronomical Society,held Sept. 4-9 in Cambridge, England.
Schaefer and Fegley looked at different types of chondritesthat earth and planetary scientists believe were instrumental in makingthe Earth. They used sophisticated computer codes for chemicalequilibrium to figure out what happens when the minerals in themeteorites are heated up and react with each other. For example, whencalcium carbonate is heated up and decomposed, it forms carbon dioxidegas.
"Different compounds in the chondritic Earth decompose whenthey're heated up, and they release gas that formed the earliest Earthatmosphere," Fegley said.
The Miller-Urey experiment featured an apparatus into whichwas placed a reducing gas atmosphere thought to exist on the earlyEarth. The mix was heated up and given an electrical charge and simpleorganic molecules were formed. While the experiment has been debatedfrom the start, no one had done calculations to predict the early Earthatmosphere.
"I think these computations hadn't been done before becausethey're very difficult; we use a special code" said Fegley, whose workwith Schaefer on the outgassing of Io, Jupiter's largest moon and themost volcanic body in the solar system, served as inspiration for thepresent early Earth atmosphere work.
NASA's AstrobiologyInstitute supported the Washington University research. Fegley is amember of the National Aeronautics and Space Administration's GoddardAstrobiology team.
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