Biology
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p41b1278l&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P41B-1278
Biology
0406 Astrobiology And Extraterrestrial Materials, 1039 Alteration And Weathering Processes (3617), 1051 Sedimentary Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 6225 Mars
Scientific paper
Newly discovered mineral crusts similar in composition to outcrops studied by the MER Opportunity at Meridiani Planum provide clues on possible past Martian aqueous activity. These highly oxidized, iron- and sulfate-rich crusts are found in intracrater sediments of the Haughton impact crater, Devon Island (75 degrees North). The crusts occur in discrete layers within gray-brown carbonate-siliciclastic paleolacustrine sediments of the Haughton Formation (Miocene) exposed along a streambed to a depth of at least 3 m. These crusts are generally platy and discontinuous, approximately a few mm to 2 cm in thickness and are visible as reddish-orange patches. The crusts also occur as elongated nodules (1-5 cm in length) showing distinct zones or gradients of oxidation. Dark gray to black platy layers in the sediments are rich in pyrite, presumably of diagenetic origin. The crusts are of variable composition, but generally rich in gypsum and goethite. Hematite, lepidocrosite and magnetite have also been identified in lesser amounts. The hydrated Mg-sulfate hexahydrite occurs in at least one sample, though in minor amounts. Jarosite (and other hydrated Fe-sulfates) appears as late-stage coatings or powdery masses. Variable amounts of dolomite, calcite, quartz and other silicates are also present and are likely derived primarily from the host sediments. Electron microscopic investigation shows that pyrite grains are typically surrounded by acicular, bladed or colloform gypsum and fine grained Fe-oxides, the latter often surrounding the gypsum. Framboidal pyrite also shows signs of oxidation to Fe-oxides/hydroxides. In some cases, selective dissolution of the gypsum, likely at cold temperatures, has produced distinctive honeycombs or rosettes of Fe-oxides. Multiple generations of Fe-oxides appear to have precipitated. Jarosite likely formed in acidic microenvironments following the dissolution of gypsum and other soluble sulfates. The presence of jarosite in a well-buffered carbonate sediment suggests that jarosite formation at Meridiani Planum did not require a highly acidic body of water as has been proposed, but merely localized acidic environments, which could have been produced by dissolution of more soluble Ca-Mg-sulfate minerals. Detailed electron microscopy, geochemical and spectroscopic analyses, multiple isotopic systems (C, O, S, Fe), and geochemical modeling are being used in combination to elucidate the (bio) geochemical processes involved in mineral formation and dissolution as well as to search for possible biomarkers in these deposits.
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