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Project Progress

This progress report summarizes astrobiology research done during the first year of funding from the NASA Astrobiology Institute 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 focused on two topics: (1) chemical and isotopic composition of presolar grains found in chondritic meteorites, and (2) atmospheric chemistry during the accretion of Earth-like planets. Our work on these two topics is described in two refereed papers (Lodders and Amari 2005, Schaefer and Fegley 2005a) and two abstracts (Fegley and Schaefer 2005, Schaefer and Fegley 2005b).

(1) 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.

(2) 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.

Professor Fegley and Laura Schaefer also calculated the composition of volatiles out-gassed from chondritic planetary bodies. The considered appropriate mixtures of the common chondritic meteorites (CI, CM, CV, H, L, and EH chondrites), which are widely regarded as the building blocks of the Earth. For example, the oxygen — isotope mixing model (Lodders and Fegley 1997 Icarus 126, 373-394, Lodders 2000 Space Sci. Rev. 92, 342-354) predicts a composition of 70% EH, 21% H, 5% CV, and 4% CI chondritic matter for the early Earth. Professor Fegley and Ms. Schaefer found that the major out-gassed volatiles for these starting compositions are CH₄, N₂, NH₃, H₂, and H₂O. This important result predicts that Earth’s earliest permanent atmosphere was a reducing atmosphere that favored synthesis of organic compounds by Miller — Urey type reactions initiated by lightning, UV light, and heat.

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