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Project Progress

Executive Summary. This progress report summarizes astrobiology research done at Washington University in St. Louis under the direction of Professor Bruce Fegley, Jr. This research is part of the NASA Goddard Astrobiology Node. Our outgassing models predict the composition of first-formed atmospheres on the Earth and other rocky planets in our own and other solar systems formed from chondritic material. Chondritic material forms in any protoplanetary disk with solar or near-solar composition.

Our research resulted in one invited talk and three contributed presentations at three scientific meetings – the March 2007 American Chemical Society national meeting, the March 2007 Lunar and Planetary Science Conference, and the August 2006 Meteoritical Society meeting (Fegley 2007, Fegley and Schaefer 2006, Schaefer and Fegley 2006, Schaefer and Fegley 2007d), two refereed papers (Schaefer and Fegley 2007a, b), a letter in Science (Bada et al. 2007), and one paper in preparation (Schaefer and Fegley 2007c).

Technical Description. During the past year we (Professor Fegley and Ms. Laura Schaefer) continued modeling the outgassing of ordinary chondritic material. The thermal outgassing of volatiles during and after planetary accretion is a question of paramount importance for the origin of planetary atmospheres (# 2-4), the origin of life (#1), thermal metamorphism of meteorites (#5-6), the chemistry and mineralogy of asteroids (#3, 7-8), the survival of presolar material in asteroids, satellites, and other bodies, the detection of extrasolar planetary atmospheres, and many other topics.

Earth and the other terrestrial planets have secondary atmospheres that originated through the outgassing of volatile-bearing material during and/or after planetary accretion and which are depleted in volatiles relative to solar and chondritic abundances (Figs. 1-2). The outgassed volatiles, which are compounds of H, C, N, O, F, Cl, and S, formed the Earth’s early atmosphere and oceans and the early atmospheres of Mars and Venus. The nature of the early terrestrial atmosphere is of particular interest because it was in this atmospheric environment that the first life forms originated between 4.5 and 3.8 Ga ago. Whether Mars possessed an early atmosphere conducive for the origin of life is also a matter of intense interest and speculation. Understanding the nature of Venus’ early atmosphere is important for understanding whether Venus once had water and lost it or has been a desert planet throughout its entire history.

We used chemical equilibrium calculations to model thermal outgassing of ordinary chondritic material as a function of temperature, pressure, and bulk composition. The calculations include ~1,000 solids and gases of the elements Al, C, Ca, Cl, Co, Cr, F, Fe, H, K, Mg, Mn, N, Na, Ni, O, P, S, Si, and Ti. The major volatiles outgassed from ordinary chondritic material are CH₄, H₂, H₂O, N₂, and NH₃ (the latter at conditions where hydrous minerals form) (see Fig. 3). We also studied the effect of core formation by removing Fe metal and FeS from the average H chondrite composition, which simulates segregation of Fe into the core. As shown in Fig. 4, CH₄ remains a major outgassing product, particularly at low pressures. Our results are insensitive to variable total pressure, variable volatile element abundances, and kinetic inhibition of C and N dissolution in Fe metal. Our results predict that Earth’s early atmosphere contained CH₄, H₂, H₂O, N₂, and NH₃; similar to that used in Miller – Urey synthesis of organic compounds.

To date we studied outgassing of ordinary chondritic material, the most abundant type of meteoritic material, and the most abundant type of material accreted by the Earth. During the next year we plan to study the composition of outgassed volatiles from more oxidized (carbonaceous) and more reduced (enstatite) chondritic material. Our initial results show that outgassing of carbonaceous chondritic material gives oxidized carbon gases (CO2) whereas outgassing of enstatite chondritic material gives reduced carbon gases (CH4). Then we plan to study the composition of outgassed volatiles from mixtures of carbonaceous, enstatite, and ordinary chondritic material. This work is relevant to the origin of terrestrial planetary atmospheres in our solar system and in other planetary systems.

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