Carbonate formation on Mars : experiments and models

[摘要] NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.The experiments reported in this thesis were motivated by a desire to explain the small present [...] pressure in the Martian atmosphere, given the hypothesis that Mars once possessed a much denser [...] atmosphere. We adopted the premise that carbonate production on the surfaces of regolith particles, mediated by small amounts of [...], might explain the decline in the surface pressure over geologic timescales.We exposed powders [...] of basalt glass, and of monominerallic diopside, olivine, plagioclase, and quartz, to conditions simulating the past and present surface of Mars ([...], and [...] contents equivalent to <1 to >5000 monolayers on particle surfaces). A sensitive manometer was used to acquire precise measurements of pressure over periods of [...] days. Initial pressure drops were attributed to adsorption of [...] on particle surfaces and dissolution of [...]. Continuing uptake of gas in most experiments suggested that [...] reacted with powders to form carbonate. Fits to [...] after [...] day gave [...], implying logarithmic reaction kinetics (i.e., reaction rate, [...]. Subsequent fits to [...], incorporating adsorption and dissolution, gave rates of D = 0.01-2 monolayers [...] per [...], with [...] day. Reaction amounts totaled [...] monolayers. Parabolic kinetics, arising from diffusion through a product layer, probably did not exceed P(t) [...]Rates varied with sample composition (basalt and diopside > olivine > plagioclase and quartz). Basalt glass was not more reactive than diopside. Basalt powder pretreated with weak acid displayed rates reduced by over an order of magnitude (although diopside did not), suggesting something removed by acid contributes to [...] uptake. Rates increased with [...] content, temperature, and CO2 pressure, vA fit to data for basalt at 295 K gave [...], where H is [...] content in monolayers and D is in monolayers [...] per log [...]t; at 248-263 K, the-effect of [...] is stronger:[...].[...] for "dry" experiments; no lower limit was put on [...] required for reaction.Reflectance spectra at mid-infrared wavelengths [...] were obtained for all "dry" and "vapor" experimental powders, and ratioed to starting spectra for maximum sensitivity to added phases. Prominent spectral features near [...] in basalt and diopside coincided with [...] absorptions at 6.9 [...] for calcite. Additional absorptions near 6.1 [...] in basalt and 4.0 [...] in diopside were also consistent with the presence of calcite. Absorption ratios near 7 [...] ranged from [...] (corresponding to perhaps 3 wt% added calcite) to as little as [...], below which we were unable to identify added phases. There was a clear positive correlation between absorption ratios and experimental [...] uptakes (and hence [...] content), strengthening the conclusion that carbonate--probably calcite but perhaps magnesite or dolomite--formed in pressure-drop experiments.Our experiments suggested that this process does occur, but modeling and application to Mars indicated that it may be insufficient to explain the carbonate production required to reduce atmospheric pressure by [...] bar over geologic time. The principal difficulty lies in the lack of evidence for the reaction proceeding easily beyond one monolayer of product. Although some experiments displayed evidence for the growth of more than a monolayer, they also showed that this arises in a regime of logarithmic reactionkinetics, where [...] uptake is limited by declining surface area available for reaction. For a global layer of basalt powder, only high specific surface area [...], a deep regolith (>100 m), or plentiful [...] (equivalent to films >5 monolayers thick) allow total [...] stored as carbonate to exceed [...] mb. Diffusion-limited kinetics were not ruled out for timescales much longer than experimental durations, and models with thicker carbonate growths show that this could account for storage of anearly Martian atmosphere.Other mechanisms for the loss of [...] may also contribute in the transition to the present surfacepressure.Or, Mars may simply never have had a dense, [...] atmosphere, although this requires explanations other than a [...] greenhouse for the morphological features used to support the hypothesis of a warm, wet early Mars. If large amounts of carbonate minerals do represent a sink for atmospheric [...], the question of their location on the surface still remains since they have not been confirmed spectroscopically.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Planetary Science and Geophysics