HR: 09:41h
AN: P31A-05    [Abstracts]
TI: Moessbauer Mineralogical Evidence for Water- and Sulfate-Bearing Phases at Gusev Crater and Meridiani Planum: The View After One Year
AU: * Morris, R V
EM: richard.v.morris@nasa.gov
AF: NASA Johnson Space Center, Code KR, NASA-JSC, Houston, TX 77058 United States
AU: Klingelhoefer, G
EM: klingel@mail.uni-mainz.de
AF: Mainz University, Staudinger Weg 9, Mainz, D-55099 Germany
AB: A major science goal of the Mars Exploration Rover (MER) mission is to search for mineralogical evidence of water activity in the form of water- and sulfate-bearing phases and other phases that form in aqueous environments. The Moessbauer spectrometers on the MER rovers at Gusev Crater (Spirit rover) and Meridiani Planum (Opportunity rover) have measured the relative abundances of iron with respect to both oxidation state and iron-bearing phase. The assemblage of phases identified by Moessbauer spectroscopy indicates aqueous alteration processes at both landing sites. Although the rock and soil of the Gusev Crater plains are dominated by Fe(2+) in olivine-bearing basalt, a Fe(3+)-rich component (nanophase ferric oxide, np-Ox) has significant abundance in surface soils and in the surface coatings/rinds of certain rocks (up to about 40% of total Fe) but not in rock interiors exposed by grinding (less than 6%). The mode of occurrence of np-Ox implies that it is the product of oxidative alteration of Fe(2+) silicate and oxide phases in the presence of H2O. Using both Moessbauer data and elemental concentrations from the APXS, the concentration of Fe associated with np-Ox has a positive correlation with S (Fe to S ratio of about 0.6), implying a sulfate or formation in direct proportion to sulfates. This ratio is low compared to values for the acid sulfate weathering product schwertmannite (Fe to S ratio of about 4.6 to 8) but is comparable to that for rhomboclase ((H3O)Fe(SO4)2(H2O)2. There is currently insufficient evidence, however, to refine the mineralogical identification beyond np-Ox. A possible weathering agent is volcanic emanations rich in H2O and SO2. Depending on how np-Ox is modeled, the basaltic soils in the Gusev plains have the equivalent of up to approx. about 1 wt% H2O. Generally, rocks in the Columbia Hills are significantly more altered than those in the Gusev plains, with a higher proportion of Fe(3+)-bearing phases compared to Fe(2+)-bearing phases. The oxyhydroxide mineral goethite (FeOOH) has been identified in at least one rock, imparting the equivalent of about 1 wt% H2O. Because its structure contains the hydroxide anion, the Moessbauer detection of the hydroxide sulfate jarosite (K,Na)Fe3(SO4)2(OH)6 (about 28% of total Fe) in outcrops at Meridiani Planum is direct mineralogical evidence for oxidative aqueous alteration of basaltic precursors under acid sulfate conditions. We calculate that average outcrop has the equivalent of about 1-2 wt% H2O contained in the jarosite. Another Fe(3+) doublet component is also present in outcrop materials, and it can be associated with the equivalent of an additional about 0.5 wt% H2O. Modeling using APXS elemental abundances and hydrated phases suggests total H2O contents of about 10 wt% are possible. Acid-sulfate conditions could arise through different pathways, including oxidation of sulfide minerals by aqueous fluids and volcanic emanations with a high SO2/H2O ratio. Hematite is pervasive and is found within the outcrop matrix (about 35% of total Fe) and in spherules (interpreted as concretions) dispersed throughout the outcrop. Presumably, the process of spherule formation involved aqueous transport processes. As at Gusev Crater, basaltic surface soils at Meridiani Planum have significant concentrations of nanophase ferric oxide and the equivalent of up to about 1 wt% H2O.
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 5410 Composition
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2005 Joint Assembly