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Project Progress

We have pursued three related avenues meant ultimately to lead to a coherent model of how detectable biosignatures may be produced on extrasolar planets. First, we have continued our assessment of life in aquifers and springs associated with terrestrial serpentinizing peridotite bodies. Several organisms have been isolated and a baseline for annual and seasonal ariations in geochemical conditions is emerging. This project should yield an understanding of the sorts of nutrient limitations faced by and the resultant biosignatures produced by life that might develop associated with rocks at the surface of an undifferentiated water-rich terrestrial planet. We hope to use such understanding to constrain models of life on earliest Earth and on hypothetical young extrasolar planets.

Along the second avenue we have been developing an anaerobic early Archean ecosystem model. This work by Kharecha, Siefert and Kasting has thus far focused on biological productivity and methane production prior to the origin of oxygenic photosynthesis. Without oxygenic photosynthesis, carbon fixation was limited by the availability of reductants. In the early Archean, this is essentially equivalent to being limited by the volcanic flux of hydrogen, which today is about 5 × 10¹² mol/year. The model explicitly accounts for cycling of hydrogen and carbon through 4 processes: primary production by hydrogen utilizing methanogens, fermentation, methanogenesis and photolysis. We assume that methanogens must derive ≤30kJ/mol (~1 adenosine triphosphate (ATP) equivalent) in making 1 mol of methane. A series of scenarios have been run using an atmospheric chemistry model coupled with a stagnant boundary layer model to capture the surprising diffusion-limitation on delivery of hydrogen to the marine biota and methane to the atmosphere. Initial results indicate that if we assume the modern volcanic flux of hydrogen, then the atmosphere could contain up to ~350 ppmv of methane if hydrogen utilizing methanogens were the primary producers.

We have also initiated a dynamical model of a microbial ecosystem (Tinetti, Storrie-Lombardi), which includes interaction with the external environment, including the stellar flux. In this multicomponent model, individual microbial species interact with each other, and their growth is governed by available energy and chemical resources, and limited by space. We continue to explore the stability of these equations, for both the autonomous and environment-coupled system. We are also working on expansion of the model to include stellar-driven photosynthetic life. In the case of photosynthetic life, the population growth and its periodicity will be determined by the available wavelength-dependent solar flux.