Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v11d1997s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V11D-1997
Mathematics
Logic
[0315] Atmospheric Composition And Structure / Biosphere/Atmosphere Interactions, [0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [0343] Atmospheric Composition And Structure / Planetary Atmospheres, [0428] Biogeosciences / Carbon Cycling
Scientific paper
The accepted model for early atmospheres of terrestrial planets has settled on a CO2-N2 composition. Unfortunately, while it is largely based on a brilliant geological analysis by Rubey, there is no compelling evidence whatsoever for such a composition as the first “permanent” atmosphere for Earth or any other planet. In fact, geological discoveries of the past 50+ years reveal several problems with a CO2-N2 atmosphere, some of which Rubey recognized in his own analysis. He clearly addressed the problem of timing of degassing, concluding that early massive degassing of CO2 would produce readily observed and profound effects, which are not evident. Modeling and constraints on the timing of planetary accretion and core formation indicate massive early degassing. If early degassing emitted CO2-N2, the effects are concealed. Plate tectonic recycling is not a solution, as conditions would have persisted beyond the time of the earliest rocks, which do not show the effects. Attempts to return degassed CO2 to the mantle are not only ad hoc, but inconsistent with early thermal structure of the Earth. Second, production of prebiotic organic compounds from a CO2-N2 atmosphere has been a nagging problem. At best this has been addressed by invoking hydrogen production from the mantle to provide reducing capacity. While hydrogen may be emitted in volcanic eruptions, it is exceedingly difficult to imagine this process generating enough organics to yield high concentrations in a global ocean. The recent fashion of invoking organic synthesis at deep-sea vents suffers from the same problem: how to achieve sufficient concentrations of organics in a global ocean by abiotic synthesis when hydrothermal activity stirs the solution and carries the prebiotic products off to great dilution? Suggesting life began at deep-sea vents, and continues to carry on chemosynthesis there, begs the question. Unless you get high enough concentrations of prebiotics by abiotic processes, you simply don’t get life. Third, solutions invoking high atmospheric CO2 as the answer to the faint young sun problem encounter exactly the same problems associated with prompt degassing of CO2. Fourth, the carbon isotope record, in which early carbonaceous deposits show signs of photosynthetic fractionation of carbon are problematical if most of the primary surface and near-surface carbon was CO2. Finally, the delay in oxidation of Earth’s surface following oxygenic photosynthesis is problematical if early photosynthesizers had a vast CO2 source from early degassing. Given an abundant food supply, what prevented early cyanobacteria from rapidly oxidizing the surface? Although discovery of CO2-rich atmospheres on Venus and Mars seem to support CO2 as the primary surface carbon reservoir, these atmospheres are the result of atmospheric evolution, particularly loss of H to space. ALL of these problems disappear with early degassing of reduced gases such as CH4 and NH3. Considerations of meteoritic compositions, accretionary processes, and early atmosphere/hydrosphere chemical processing all support reduced gases as dominant in the early atmospheres of the terrestrial planets. The time has come to revert to any earlier idea of strongly reducing conditions on early terrestrial planet surfaces, followed by long-term evolutionary trends toward oxidation.
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