Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13c..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13C-02
Other
[3617] Mineralogy And Petrology / Alteration And Weathering Processes, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
The detection of clay minerals (phyllosilicates) in ancient Noachian-aged terrains on Mars and their relative paucity in younger Hesperian terrains has led to the hypothesis that the planet transitioned to a drier and more acidic environment. Converting a largely basaltic planet, which will naturally tend to buffer acidic solutions, to an acid-dominated system is dependent on competition between acid production and the available volume of water. Clay-bearing strata associated with deltas (e.g., Eberswalde, Jezero) and other fluvial features are strong evidence for sediment transport of previously altered basaltic material, whereas some clay-bearing units may represent neoformation of smectites and other clay minerals. In either of these cases, the presence of 2:1 clays suggests that water was present for fluvial transport and pH levels were moderate to alkaline. Interestingly, most of the clay deposits detected thus far host primarily Fe/Mg-rich 2:1 clay minerals, suggesting alteration of a basaltic crust at relatively low water-to-rock ratios (or short-lived water-rock interaction) and minimal leaching. With a few exceptions, large exposures of late-stage Al-rich weathering products such as kaolinite or gibbsite are rare. It has also recently been noted that the production of smectite via dissolution of basalt leads to an excess of cations that implies the formation of coeval sedimentary salts (carbonates, sulfates, chlorides, etc.). However, sulfates are found primarily in Hesperian terrains and such salts are rarely observed in older clay-bearing units. Coupled with in situ observations by rovers, the orbital detection of these younger sulfate deposits has been used to suggest that Mars transitioned to an acidic planet during the Hesperian. Acid can be produced on Mars by the oxidation of Fe(II) in fluids in contact with the atmosphere, where UV photons or atmospheric O2 are likely sources of oxidant. Such processes would occur throughout Mars’ history, but the net decrease in the available amount of water through time would make it easier to maintain local acidity. Acidic conditions would be promoted in regions with limited water close to sources of S- and Cl-bearing volcanic gases. Conversely, locations with abundant water poor in Fe(II) and distal to volcanic sources would evolve towards more moderate pH levels and would be capable of nondestructive transport and/or formation of clay minerals during the Hesperian. This scenario is consistent with the presence of clays in several Hesperian-aged terrains, suggesting that acidic conditions were not global during this time. Martian clays would also be susceptible to burial diagenesis, and the survival of numerous smectite-bearing outcrops on Mars over several billion years suggests limited interaction with fluids over this timescale, providing a natural case study for long-term smectite stability. In summary, the apparent spatial-temporal distribution of clays (and sulfates) on Mars may not reflect a global temporal change from alkaline to acidic conditions as much as it reflects spatial variations in the local volume, availability, and Fe(II) concentration of near-surface water.
Bish David L.
Fischer Werner
Hurowitz Joel A.
Milliken Ralph
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