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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.09.006 Byline: Joseph Michalski (a), Francois Poulet (a), Jean-Pierre Bibring (a), Nicolas Mangold (b) Keywords: Mars, Surface; Mineralogy; Spectroscopy; Infrared observations Abstract: Diverse phyllosilicate deposits discovered previously in the Nili Fossae region with near infrared reflectance data are a window into the complex history of aqueous alteration on Mars. In this work, we used thermal infrared data from the Thermal Emission Spectrometer (TES) in combination with near infrared data from the Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) to better constrain the mineralogy and geologic origin of these deposits. We developed a TES spectral index for identification of clay minerals, which correctly identifies the phyllosilicates in the Nili Fossae area and points to several other interesting deposits in the Syrtis Major region. However, detailed inspection of the TES spectral features of Nili Fossae phyllosilicates shows a feature at low wavenumbers (350-550cm.sup.-1) that is not an exact match to any specific Fe.sup.3+-, Al-, or Mg-rich phyllosilicate phase. Instead, the feature is more similar to basaltic glass and may indicate that the phyllosilicates in this region are: (1) rich in Fe.sup.2+ (based on similarity to trends seen in laboratory data of clay minerals), (2) poorly crystalline/extremely disordered, and/or (3) present within a matrix of actual basalt glass. This feature is similar to spectral features seen in altered rocks in the Columbia Hills region of Gusev Crater by previous authors. By calibrating measured spectral index values against mathematical spectral mixtures of typical martian dark surfaces and known abundances of alteration minerals, we are able to estimate an enrichment in abundance of alteration minerals in the altered surfaces. Many dark, Noachian deposits in the Nili Fossae area are enriched phyllosilicates by 20-30% ([+ or -]10-15%) relative to dark, volcanic surfaces in the same region. The distribution and abundance of these phases indicates that alteration in the region was pervasive, but did not completely erase the original mineralogy of what was likely an Fe-rich basalt protolith. As a group, the Nili Fossae phyllosilicate deposits are fundamentally different from those found in the Mawrth Vallis region. Nili Fossae deposits have strong thermal infrared features related to admixed pyroxene, plagioclase, and occasionally olivine, whereas the Mawrth Vallis deposits contain no mafic minerals. Comparison of TES and OMEGA data also illustrates some more general differences between the datasets, including the impact of physical character of the martian surface on detectability of minerals in each spectral range. Author Affiliation: (a) Institut d'Astrophysique Spatiale, BAcentstiment 121, Universite Paris Sud, Orsay Cedex 91405, France (b) Lab. de Planetologie et Geodynamique, UMR6112, CNRS et Universite de Nantes, 2 rue de la Houssiniere, BP 92208, 44322 Nantes Cedex 3, France Article History: Received 2 May 2009; Revised 28 August 2009; Accepted 3 September 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.08.016 Byline: Joseph R. Michalski (a), Robin L. Fergason (b) Keywords: Mars; Mineralogy; Infrared observations Abstract: Clay mineral-bearing deposits previously discovered on Mars with near infrared (I'=0.3-5 [mu]m) remote sensing data are of major significance for understanding the aqueous history, geological evolution, and past habitability of Mars. In this study, we analyzed the thermal infrared (I'=6-35 [mu]m) surface properties of the most extensive phyllosilicate deposit on Mars: the Mawrth Vallis area. Clay mineral-bearing units, which in visible images appear to be relatively light-toned, layered bedrock, have thermal inertia values ranging from 150 to 460 Jam.sup.-2aK.sup.-1as.sup.-1/2. This suggests the deposits are composed of a mixture of rock with sand and dust at 100-meter scales. Dark-toned materials that mantle the clay-bearing surfaces have thermal inertia values ranging from 150 to 800, indicating variable degrees of rockiness or induration of this younger sedimentary or pyroclastic unit. Thermal Emission Spectrometer (TES) spectra of the light-toned rocks were analyzed with a number of techniques, but none of the results shows a large phyllosilicate component as has been detected in the same surfaces with near-infrared data. Instead, TES spectra of light-toned surfaces are best modeled by a combination of plagioclase feldspar, high-silica materials (similar to impure opaline silica or felsic glass), and zeolites. We propose three hypotheses for why the clay minerals are not apparent in thermal infrared data, including effects due to surface roughness, sub-pixel mixing of multiple surface temperatures, and low absolute mineral abundances combined with differences in spatial sampling between instruments. Zeolites modeled in TES spectra could be a previously unrecognized component of the alteration assemblage in the phyllosilicate-bearing rocks of the Mawrth Vallis area. TES spectral index mapping suggests that (Fe/Mg)-clays detected with near infrared data correspond to trioctahedral (Fe.sup.2+) clay minerals rather than nontronite-like clays. The average mineralogy and geologic context of these complex, interbedded deposits suggests they are either aqueous sedimentary rocks, altered pyroclastic deposits, or a combination of both. Author Affiliation: (a) Institut d'Astrophysique Spatiale, Universite Paris Sud, 91405 Orsay Cedex, France (b) Mars Space Flight Facility, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA Article History: Received 4 April 2008; Revised 3 August 2008; Accepted 15 August 2008

The Thermal Emission Spectrometer (TES) has observed a high-silica material in the dark regions of Mars that is spectrally similar to obsidian glass and may have a volcanic origin. An alternate interpretation is that the spectrally amorphous material consists of clay minerals or some other secondary material, formed by chemical alteration of surface rocks. The regions where this material is observed (e.g., Acidalia Planitia) have relatively high spectral contrast, suggesting that the high-silica material exists as coarse particulates, indurated soils or cements, within rocks, or as indurated coatings on rock surfaces. The geologic interpretation of this spectral result has major implications for understanding magmatic evolution and weathering processes on Mars. One of the complications in interpreting spectral observations of glasses and clay minerals is that both are structurally and compositionally complex. In this study, we perform a detailed spectroscopic analysis of indurated smectite clay minerals and relate their thermal emission spectral features to structural and crystal chemical properties. We examine the spectral similarities and differences between smectite clay minerals and obsidian glass from a structural-chemical perspective, and make further mineralogical interpretations from previous TES results. The results suggest that neither smectite clays nor any clay mineral with similar structural and chemical properties can adequately explain TES observations of high-silica materials in some martian dark regions. If the spectrally amorphous materials observed by TES do represent an alteration product, then these materials are likely to be poorly crystalline aluminosilicates. While all clay minerals have Si/O ratios [less than or equal to] 0.4, the position of the emissivity minimum at Mars suggests a Si/O ratio of 0.4-0.5. The spectral observation could be explained by the existence of a silica-rich alteration product, such as Al- or Fe-bearing opal, an intimate physical mixture of relatively pure silica and other aluminosilicates (such as clay minerals or clay precursors), or certain zeolites. The chemical alteration of basaltic rocks on Mars to phyllosilicate-poor, silica-rich alteration products provides a geologically reasonable and consistent explanation for the global TES surface mineralogical results. Keyword: Mars, surface; Mineralogy; Spectroscopy; Infrared observations

The article presents a research report on intense subaerial weathering of eolian sediments in Gale crater, Mars. Topics include early observations of a lacustrine environment, few of the subsequent deposits exhibit demonstrable lacustrine character; and discoveries by Curiosity provide a critical piece to Mars' global alteration puzzle.