Mathematics – Logic
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p21c..05r&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P21C-05
Mathematics
Logic
3934 Optical, Infrared, And Raman Spectroscopy, 5464 Remote Sensing, 6225 Mars
Scientific paper
MarsExpress OMEGA showed that many of the Interior Layered Deposits (ILDs) in Valles Marineris contain sulfates and proposed the sulfates as indicators of past aqueous activity in the Theiikian period (Gendrin etal, 2005; Bibring etal, 2005; Bibring etal, 2006). Better discrimination of the sulfate assemblages present and the stratigraphic relationships within the ILD is critical to understanding the environment during and since their formation. We present a method for identifying classes of sulfates present in a multi-sulfate exposure with MRO CRISM data. Multiple mineral phases can be defined by diagnostic absorptions in spatially distinct wavelength regions. Combinations of minerals phases is more complicated but can be resolved by identifying superposed absorption feature and assuming linear mixing. We focus on four wavelength regions: (a) 2.4 and 2.1 μm, (b) 2.2 μm, (c) 1.9 and 1.4 μm, and (d) 0.9 μm, in a methodical classification of possible sulfate types present. While there is some overlap in the wavelength regions, absorptions are sufficiently separate to be recognizable. Additionally, care must be taken to select geologically feasible minerals assemblages. (a) Hydrated sulfates have an absorption near 2.4 um due to probable interactions between the H2O and SO3 molecules (Cloutis etal, 2006). Monohydrated sulfates have a distinct absorption near 2.1 μm due to combinations of H2O stretch and rotation vibrations of the single water molecule in a sulfate structure (Cloutis etal, 2006) which shifts with cation. Thus minerals such as kieserite (MgSO4 H2O) and szomolnokite (Fe2+SO4 H2O) can be distinguished in CRISM data. (b) The 2.21-2.26 μm region is generally convex in sulfates, but gypsum (CaSO4 2H2O ) and jarosite group members (MFe3(SO4)2(OH)6) have absorptions there. The minimum within this wavelength region depends on the mineral present. (c)The ~1.9 μm is due to the OH stretch and H2O bend combination tone and the ~1.4 μm absorption is due to the 1st overtone of the OH stretch. Sulfates or other minerals with 2+ structural H2O are necessary for the deep water and hydroxyl absorptions in many spectra we observe. (d) Ferric and ferrous minerals have wide absorptions near 0.9 μm due to charge transfer and electronic transition processes. Presence of a ~0.9 μm absorption could indicate either a iron-bearing sulfate, a co-existing iron oxide, or both. Ferrous minerals such as olivine and pyroxene can be excluded by the position and width of their 1.0 and 2.0 μm absorptions. An eastern Candor Chasma ILD has a multiple sulfate assemblage including mono- and polyhydrated sulfates with a variety of cations that are uniform within a specific layer. The sulfates are intimately or spatially mixed on the meter scale. The relative strengths of mono and polyhydrated sulfate absorptions vary with layering, indicating a degree of independence. Mineralogy and geomorphology is consistent with an evaporite sequence or groundwater alteration of ash or aeolian deposits. In addition, iron oxide spectral features overprint the sulfate spectra in some places and cut across layering in others, suggesting the iron-bearing phase may be either separate from the sulfate occurrences or spatially redistributed. Future work will better characterize the assemblage there and in other chasmata ILDs.
Arvidson Ray E.
Bishop Janice L.
Lichtenberg Kim A.
Milliken Ralph E.
Morris Richard V.
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