Biology
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p22b0553r&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P22B-0553
Biology
0325 Evolution Of The Atmosphere, 1615 Biogeochemical Processes (4805), 4805 Biogeochemical Cycles (1615), 4840 Microbiology, 6207 Comparative Planetology
Scientific paper
The search for life extraterrestrial life has rapidly expanded during the past several years. In addition to missions to Mars and Europa, NASA now envisions launching an orbiting telescope, Terrestrial Planet Finder (TPF), capable of resolving Earth-sized planets around stars as far away as 50 parsecs within the next 10-15 years. By that time we need to develop our understanding of the effects of life on such planets in order to confidently distinguish inhabited planets from barren ones. Our group is in the process of developing a fully coupled generalized 1-D radiative transfer-atmospheric chemistry model. Around this core we are building the Virtual Planetary Laboratory (VPL) to generate synthetic spectra of hypothetical extrasolar terrestrial planets. Computational modules mimicking the influence of life on atmospheric chemistry/climate are of central importance for analyzing data from TPF and related missions. Here we describe our rationale and initial efforts to parameterize the effects of life using a Virtual Microbial Community (VMC). At first glance, the task of modeling hypothetical inhabited planets appears intractable. However, we may assume that most planets settle into a fairly small number of stable climate/chemistry regimes during their history. These regimes are maintained by negative feedback loops. Transitions from one stable solution to another are singularities, times during which the system is unregulated and may vary wildly. In this context, life is one of several processes modifying the chemical composition of a planetary atmosphere, potentially modifying climate. We seek to elucidate those processes and signatures unique to life and visible from space. The VMC is a first attempt at quantifying the possible range of effects of life on the atmosphere of a planet. We start from the presumption that kinetics and thermodynamics are the same throughout the universe. Given the remarkable metabolic diversity of life on Earth, we assume that all available energy sources may be used by biology on detectably colonized planets. Simple feedback loops such as those governing Lovelock?s famous Daisyworld or the Walker CO2 feedback, offer starting points for thinking about global scale feedbacks. Feedbacks in microbial communities, e.g. those postulated in anaerobic methane oxidation communities, involving the use of one or more organisms? waste products as nutrients by another, hint at the local complexity from which we need to scale up. Our first attempt at bridging this gap involves describing the processes that may have helped stabilize the Archean climate. Archean biogenic methane production could have been rapid enough to provide 100s ppm atmospheric CH4. At such CH4 levels Earth would have remained ice free. Sudden increases in CH4 production might have led to runaway greenhouse conditions. However, if CH4/CO2 > 1 a UV absorbing aerosol haze should form. UV-labile ammonia could have accumulated in the atmosphere under the haze, quickly making rain pH > 7, dramatically slowing chemical weathering on the continents and interrupting vital phosphate delivery to the oceans. The residence time of P is ca.10,000 years. Thus, over a time scale of ca.10,000 years primary productivity dropped sharply. Biogenic methane production, near the base of the trophic ladder, suffered disproportionately. With little CH4 production CH4/CO2 fell to < 1. The UV screen and atmospheric NH3 disappeared in a few years. Rain pH dropped. Weathering restarted. Biological productivity recovered. The above testable scenario serves as an example of a plausible feedback involving interplay between biological, geochemical, atmospheric and stellar processes. Feedback loops of this sort will be central features of the fully realized VMC module for the VPL.
Rye Rob
Storrie-Lombardi Michael
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