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C.B. Pilcher (Astron. & Physics Div., Office of Space Science, NASA Headquarters)
A major long-term goal of NASA's Origins Program is to search for signs of life, i.e., biosignatures, on planets around other stars. The Terrestrial Planet Finder is planned to be the first Origins mission capable of detecting some potential biosignatures on Earth-size extrasolar planets. Consideration of these biosignatures has tended to focus on spectral features of gases present in Earth's modern atmosphere, particularly ozone, the photolytic product of biogenically produced oxygen. But it is generally believed that life existed on Earth for about 1-1/2 billion years before the atmosphere became oxygenated. Earth's earliest biosphere provides clues to potential biosignatures on Earth-like planets before oxygenic photosynthesis leads to a detectable oxygen signature. The close phylogenetic and biochemical relationships of methane generation and sulfur reduction (which produces H2S), two of the earliest terrestrial metabolisms, suggest that organo-sulfur compounds may have been among the products of early life on Earth. Detection of these compounds in a mildly reduced extrasolar planet atmosphere with significant quantities of methane may be an indicator of the presence of life.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.