Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13c..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13C-01
Mathematics
Logic
[1060] Geochemistry / Planetary Geochemistry, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
The sedimentary rocks examined by the Mars Exploration Rover Opportunity reveal the existence of a remarkable ancient (ca. 3.8 Ga) environment preserved in the geological record of Mars. The data collected thus far have led to the working hypothesis that the formation, mobilization, and deposition of sand-sized sediment grains, composed of a mixed saline-siliciclastic mineral assemblage, occurred in a dune-interdune-shallow subaqueous sedimentary environment. The ultimate source of sediment grains is proposed to be an evaporative aqueous environment, perhaps contemporaneous playa lakes, which were sourced by upwelling groundwater. The presence of the mineral jarosite in these sedimentary rocks requires that the fluids, from which the Meridiani Planum sediments were derived, were both acidic (pH = ca. 2.0-4.0) and sulfate-rich. One of the key lingering questions that remain to be answered with respect to this hypothesis about the genesis of the Meridiani Planum sediments is: how did the groundwaters that sourced the mixed saline-siliciclastic grains attain their low-pH condition? Related to this question: if the rocks examined by Opportunity are representative of the entire sulfate-bearing sedimentary section that has been observed from orbit, some ~2x105km2 in area and ~800km in thickness, how was acidity maintained over such a large area of the Martian surface? Conventional wisdom holds that any groundwater system circulating through the basaltic subsurface on Mars should rapidly evolve to circum-neutral or alkaline pH, as is the case for basaltic groundwaters on Earth. The answers to these questions are ultimately rooted in surface redox chemistry. Using the bulk chemistry and Fe-mineral composition of Meridiani outcrop from alpha-particle X-ray and Mössbauer spectroscopic data, respectively, we have determined that acidic conditions could have been maintained solely by Fe(II)-oxidation and ferric iron mineral paragenetic processes. As the most likely source of oxidants on Mars are atmospheric O2 and UV photons, we suggest that oxidative and acid-generating processes were near-surface phenomena, and that the subsurface fluids migrating to the surface had the redox potential condusive to the transport of dissolved Fe2+, and indeterminate, but not acidic, pH. Accordingly, groundwater upwelling towards the surface evolved to a low pH state as a result of oxidation of a significant component of Fe2+ dissolved in the migrating fluid. We will further demonstrate that the available rates of Fe-oxidation by molecular O2 or UV photons are consistent with an approximately syn-depositional near-surface oxidation and acidification process, and that the oxidant budget required to generate the observed proportion of ferric iron in outcrop implies an active cycle of oxidant production and consumption on the late Noachian surface of Mars.
Fischer Werner
Hurowitz Joel A.
Milliken Ralph
Tosca Nicholas J.
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