Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........16s&link_type=abstract
Thesis (PH.D.)--CALIFORNIA INSTITUTE OF TECHNOLOGY, 1995.Source: Dissertation Abstracts International, Volume: 56-07, Section: B
Mathematics
Logic
3
Basalt Glass, Diopside
Scientific paper
Given the hypothesis that Mars once possessed a much denser CO_2 atmosphere, we investigated carbonate formation as a way to explain the small current pressure. We exposed powders of basalt glass, diopside, and other minerals to Mars-like conditions ( {~}7-1000 mb, {~}245-300 K, and H_2 O contents equivalent to <1 to >5000 monolayers on particle surfaces), and used a sensitive manometer to monitor CO_2 pressure. Long-term (minutes to days) uptake of gas suggested that CO_2 reacted with powders to form carbonate. Fits to P(t) = D log 10 (1 + t/t_ {rm o}) gave reaction rates of D = 0.01-2 monolayers CO_2 per log 10t, and amounts of {~}0.005-10 monolayers. Rates varied with sample composition (basalt and diopside > olivine > plagioclase and quartz), and increased with H_2O content, temperature, and CO_2 pressure. Mid-infrared (2.5-12.5 μm) reflectance spectra were obtained for experimental powders, and ratioed to starting spectra for maximum sensitivity to added phases. Prominent features near 7 mu m in basalt and diopside coincided with CO _3^{-2} absorptions at 6.9 μm for calcite (additional consistent absorptions occurred near 6.1 μm in basalt and 4.0 μm in diopside). The positive correlation between absorption ratios and experimental CO_2 uptakes (and hence H _2O content) strengthened the conclusion that carbonate formed. Application to Mars indicated that this process may be insufficient to reduce atmospheric pressure by {~}1 bar over geologic time. Carbonate growths greater than a monolayer occurred in a regime of logarithmic reaction kinetics, where CO _2 uptake is limited by declining surface area available for reaction. For a global layer of basalt powder, only high specific surface area (> 1 m^2/g), a deep regolith (>100 m), or plentiful H_2 O (equivalent to films >5 monolayers thick) allow total CO_2 stored as carbonate to exceed ~10-100 mb. Alternatively, diffusion-limited kinetics, not ruled out for very long timescales, could account for storage of an early Martian atmosphere. Also, other mechanisms for the loss of CO _2 may contribute in the transition to the present surface pressure, or Mars may simply never have had a dense CO_2 atmosphere.
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