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Project Progress

This progress report summarizes astrobiology research done during the past 12 months (June 2007 — July 2008) at Washington University in St. Louis under the direction of Professor Bruce Fegley, Jr. This research is part of the NASA Goddard Center for Astrobiology (GCA) Team.

During the past year we (Professor Fegley and Ms. Laura Schaefer) continued modeling the outgassing of chondritic material and now have a complete picture of the history of Earth’s atmosphere during its formation. During the past years we modeled chemistry of outgassed volatiles during accretion of the Earth. Accretion of the Earth, and especially the Moon-forming impact, heats the Earth to temperatures high enough to melt and vaporize its silicate crust and mantle. The Earth has a silicate atmosphere during this phase of its history. As the Earth cools, the silicate atmosphere collapses and a steam atmosphere forms. This atmosphere is not pure steam, but contains H₂O, H₂, CO, CO₂, CH₄ in varying proportions depending on temperature, pressure, and oxygen fugacity. Further cooling leads to a collapse of the steam atmosphere and a gaseous atmosphere forms. This is an early reducing atmosphere with CH₄, H₂, NH₃, and water vapor.

Our work on the silicate atmosphere is described in several posters presented at scientific meetings and remains to be written up for publication. Our work on the steam atmosphere was done during the past year and is currently being written up for submission to Icarus. This is discussed in more detail below. Our work on the early reducing atmosphere appeared in Icarus last year (Schaefer and Fegley 2007 “Outgassing of Ordinary Chondritic Material and Some of its Implications for the Chemistry of Asteroids, Planets, and Satellites.” Icarus 186, 462-483). This work is in the vanguard of a new paradigm and other groups have jumped into the field since our paper was published.

Our results on the chemistry of Earth’s putative steam atmosphere show that the atmosphere was probably not steam in most cases (see Table 1). Only CI and CM carbonaceous chondritic material produces steam-rich atmospheres (Figure 1 for CI material), while H chondritic material (closer in composition to the bulk Earth) produces a H₂-rich atmosphere (see Figure 2).

Our results have several important implications. One is that the massive greenhouse effect of a steam atmosphere is absent in the H₂ + CO atmospheres we predict. Hydrogen is a dismal greenhouse gas, CO is a good greenhouse gas, but not nearly as good as steam and CO₂. Consequently, it may be very difficult or impossible to maintain a global magma ocean on the early Earth. Therefore we believe it would be wise for researchers interested in magma oceans to investigate the effect of different atmospheric compositions on the formation and lifetime of the putative magma ocean.

Another important implication is that volatilization of rock-forming elements such as Mg and Si will be different in atmospheres that are mainly H₂ + CO instead of steam-rich. This is important for the Mg/Si ratio of the Earth. The geochemical behavior of minor and trace elements that can be volatilized at moderately high temperatures is also different in the H₂ + CO versus steam-rich atmospheres and is important for geochemical models of Earth evolution.

Finally, we also have written and are writing two invited reviews. One review chapter is about the cosmochemistry of the biogenic elements and is in press. Another review chapter about the cosmochemistry of all naturally occurring elements is in preparation.

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