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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 research resulted in two invited talks at the Origins of Solar Systems Gordon Conference (Fegley 2005, Lodders 2005), two poster presentations at the September 2005 DPS meeting (Fegley and Schaefer 2005, Schaefer and Fegley 2005b), a talk and a poster at the August 2006 Meteoritical Society Meeting (Fegley and Schaefer 2006, Schaefer and Fegley 2006a), two refereed papers (Lodders and Amari 2005, Schaefer and Fegley 2005a), and a paper being submitted to Icarus later this month (Schaefer and Fegley 2006b). A major new conclusion of the past year’s work is that methane (CH₄) is the major carbon — bearing gas produced by the outgassing of chondritic material (Fegley and Schaefer 2006, Schaefer and Fegley 2005b, 2006a,b).

(1) Outgassing of chondritic material

Professor Fegley and Laura Schaefer, a technical staff member in his group, are using chemical equilibrium and where relevant chemical kinetic calculations to model the thermal outgassing of chondritic material, i.e., matter with the average chemical composition of ordinary chondrites. The chondrites are stony meteorites containing metal + sulfide + silicate. The ordinary (H, L, LL) chondrites constitute about 97% of all chondrites. The chondrites are primitive material from the solar nebula and were the building blocks of the Earth and other rocky asteroids, planets and satellites. Chondritic material released gases containing volatile elements (e.g. H, O, C, N, S) as the Earth and other rocky bodies were heated during and after their accretion. These outgassed volatiles became the primordial atmospheres on Earth, Venus, and Mars.

Professor Fegley’s and Ms. Schaefer’s objective is to model this important process which has not been studied before. They started with the H group ordinary chondrites, which are the most abundant ordinary chondrites and thus the single most abundant group of meteorites. As reported in several abstracts (Fegley and Schaefer 2006, Schaefer and Fegley 2005b, 2006a) and a paper being submitted to Icarus (Schaefer and Fegley 2006b), Professor Fegley and Ms. Schaefer found that the major out-gassed volatiles are CH₄, H₂, H₂O, N₂ and NH₃ (the latter at temperatures and pressures where hydrous minerals form). Contrary to widely held assumptions, neither CO nor CO₂ are major carbon — bearing gases during the outgassing of ordinary chondritic material (see Fig. 1).

{{ 1 }}

Figure 1 shows chemical equilibrium abundances of gases along a thermal profile for the asteroid 6 Hebe, which is thought to be the H chondrite parent body. Methane dominates over CO and CO2 because ordinary chondritic material contains iron metal alloy, olivine, and pyroxene. The coexisting olivine and pyroxene regulate the thermodynamic activity of quartz via the reaction

2 (Mg_xFe_1-x)SiO₃ (pyroxene) = (Mg_xFe_1-x)2SiO₄ (olivine) + SiO₂ (quartz) (1)

The olivine — pyroxene — metal alloy mineral assemblage regulates (buffers) the oxygen partial pressure (fugacity) at low values characteristic of the quartz — fayalite — iron (QFI) buffer

Fe₂SiO₄ (fayalite) = 2 Fe (metal) + SiO₂ (quartz) + O₂ (g) (2)

The fayalite and Fe metal in reaction (2) are dissolved in olivine and Fe — rich metal alloy, respectively. Figure 2 compares the oxygen fugacities (fO2) measured during the heating of ordinary chondrites with our calculated values. The oxygen fugacity for the QFI buffer involving pure fayalite, pure iron, and pure quartz is also shown for reference.

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Professor Fegley and Ms. Schaefer did a large number of calculations studying the sensitivity of the results to the different types of ordinary chondrites (H, L, LL), variations in P and T, variations in the abundances of the volatile elements H, C, N, O, S, kinetic inhibition of solid solution formation, solubility of C and N in metal, and open (vs. closed) system behavior. They found that CH₄ remained the dominant carbon gas for all three ordinary chondrite groups. Methane was the major C gas formed at pressures and temperatures expected for planetary thermal profiles, i.e. higher pressures go hand in hand with higher temperatures deeper inside a rocky planet. Likewise, CH₄ remained the major carbon gas over the range of elemental abundances (H, C, N, O, S) observed in ordinary chondrites, and whether or not solid solution formation was kinetically inhibited (e.g., pure minerals vs. solid solutions, C dissolved in metal vs. Fe carbide, and N dissolved in metal or not allowed to do so). Finally CH₄ remained much more abundant than CO (or CO₂) for either open or closed system outgassing. In other words, the conclusion that CH₄ is the major carbon — gas from outgassing of chondritic material is robust. This important result predicts that outgassing of a chondritic Earth produced a reducing atmosphere with CH₄ that favored synthesis of organic compounds by Miller — Urey type reactions initiated by lightning, UV light, and heat.

(2) Chemical and isotopic composition of presolar grains found in chondritic meteorites

The presolar grains found in chondritic meteorites are remnants of the material accreted by the solar nebula during formation of the solar system. Our knowledge of presolar grains helps us to understand the origin of our solar system and of other extrasolar planetary systems. Klaus Keil invited research associate professor Lodders and senior research scientist Dr. Sachiko Amari to write a paper about the origin and nebular processing of carbonaceous presolar grains. Dr. Lodders has written several highly cited papers about the condensation chemistry of presolar grains around AGB stars. Dr. Amari is one of the co-discovers of presolar TiC grains and has studied presolar grains in meteorites since their discovery in 1987. Their invited review paper entitled “Presolar Grains from Meteorites: Remnants from the Early Times of the Solar System” was published in the March 2005 issue of Chemie der Erde (Lodders and Amari 2005 Chem. Erde 65, 93-166). In this paper they describe the search for presolar grains, the different types of presolar grains, the chemical and isotopic composition of the different types of grains, and their origin from different stellar sources.

(3) Atmospheric chemistry during the accretion of Earth-like planets.

Planetary accretion models show temperatures of several thousand degrees during accretion of the Earth. The high temperatures result from conversion of gravitational potential energy into heat. The thermodynamic properties of iron, and the major silicates (such as olivine (Mg,Fe)2SiO4) that make up the Earth are sufficiently well known that the energy required for heating, melting, and vaporization can be calculated accurately. Professor Fegley and Laura Schaefer used thermochemical equilibrium calculations to model the chemistry of silicate vapor and steam-rich atmospheres formed during accretion of the Earth and Earth-like exoplanets. The codes used in this work are the MAGMA code (Fegley and Cameron 1987 EPSL 82, 207-222, Schaefer and Fegley 2004 Icarus 169, 216-241, Schaefer and Fegley 2005a Earth, Moon and Planets DOI 10.1007/s11038-005-9030-1) and the CONDOR code (Fegley and Lodders 1994 Icarus 110, 117-154). Our results predict spectroscopically observable gases that can be used to search for Earth-like planets forming in other planetary systems. In particular we find that silicon monoxide (SiO) gas is the major species in silicate vapor atmospheres for T > 3080 K, and monatomic Na gas is the major species for T < 3080 K. During later, cooler stages of accretion (1500 K), the major gases (abundances >1%) in a steam-rich atmosphere are H2O, H2, CO2, CO, H2S, and N2. Carbon monoxide converts to CH4 as the steam atmosphere cools.

(4) Gordon conference talks

Professor Fegley and Dr. Lodders gave invited talks entitled “Nebular Processes” and “Short Lived Isotopes and Solar System Formation”, respectively, at the “Origins of Solar Systems” Gordon Research Conference at Connecticut College on June 26 – July 1, 2005. These talks are based on work funded under this grant.