P21A-01 INVITED
Mars Mineralogy from OMEGA Visible-Near Infrared Measurements
Data from the OMEGA imaging spectrometer on the Mars Express spacecraft have opened a new avenue in the exploration of Mars through remotely sensed mineralogy, complementing the results from the successful thermal emission and gamma ray experiments. OMEGA measures reflected visible-near infrared (VNIR) reflectance of the martian surface, from which absorptions due to iron-bearing and hydrated minerals are recognized. From these absorptions the minerals olivine, pyroxene, iron oxides, hydrated sulfates, and phyllosilicates have been identified. The mafic minerals show a global distribution that is broadly consistent with previous results, while extending the richness of iron-bearing mafic mineral identification. Phyllosilicate-bearing outcrops are apparently restricted largely to Noachian-aged terrains, while hydrated sulfate-bearing outcrops are present in late Noachian to Hesperian-aged terrains. Although hydrated minerals have been suggested by previous measurements, the VNIR data from OMEGA exhibit robust overtone and combination overtone bands that uniquely and definitively identify several sulfate and phyllosilicate minerals. Each OMEGA spectrum shows the presence of a strong hydration band near 3 microns, indicating the presence of hydration. The degree of hydration as measured by this absorption increases towards the poles, and is broadly consistent with the GRS neutron data. Nevertheless there are important differences that are the topic of ongoing investigations. While VNIR data are well suited for Fe- and hydrated minerals, thermal infrared measurements are well suited for a broad range of silicate and other minerals that do not show visible-near infrared absorptions (e.g. feldspar). Both wavelength regions are differentially affected by texture, coatings, and particle size. For example mineral identification in the TIR, can be hampered by fine particulate textures. Significant new insights are likely to be gained by combining these wavelength regions, extracting the most well resolved mineral identifications and estimating abundances that are consistent with the complete spectral domain. Examples include combining VNIR and TIR data over phyllosilicate-rich regions to fully resolve the silicate mineralogy, and full spectral range analyses of low albedo surfaces in the northern plains.
P21A-02 INVITED
Martian surface mineralogy from orbital spectroscopy: A single story from multiple wavelength regions
Spectroscopic remote sensing of Mars has made enormous advances over the past decade due to the Observatoire pour la Min�ralogie, l'Eau, les Glaces, et l'Activit� (OMEGA), Thermal Emission Imaging System (THEMIS), and the Thermal Emission Spectrometer (TES) investigations. These three instruments are highly complimentary in both spatial and spectral coverage and, when used together, provide a great deal of confidence in their derived science results. Although both visible and near-infrared (VNIR) and thermal infrared (TIR) spectroscopy are useful for identifying surface mineralogy, the two wavelength regions have variable sensitivities to different types of minerals and surface textures. Where the capabilities overlap, as in the case of pyroxenes and olivine, they bolster the individual results. Their capabilities can also be complimentary, as in the case of the association of hematite (identified by TES) with sulfate (identified by OMEGA) deposits. Though there has been debate regarding some of the mineralogical results from these investigations, a remarkably consistent picture of Martian global surface mineralogy is emerging. This includes basaltic highlands composed of plagioclase and variable pyroxene compositions. Minor olivine appears to be present except in localized regions such as Nili Fossae that can have much higher olivine concentrations. The basalts are absent from the northern lowlands except in isolated localities. The northern lowlands are dominated by plagioclase and high silica amorphous glass. Clear evidence of significant chemical weathering or alteration is present in localized regions where compositions such as phyllosilicates, hematite, and sulfates have been discovered. In addition, a number of isolated unique compositions have been identified, including orthopyroxenites and granitoids. It is important to consider the TES, THEMIS, and OMEGA instrument sensitivities when assessing these results. For example, pyroxene compositions derived from OMEGA data at first appear to be inconsistent with TES results. However, when the uncertainties of both instruments are taken into account, the results are actually in good agreement. It is a comparison of these types of details that are crucial for properly assessing the investigation results.
P21A-03
Martian Surface Mineralogy from OMEGA/MEx: Global mineral maps
After near three years of operation, the OMEGA imaging spectrometer on-board Mars Express has achieved a large coverage of the Martian surface with spatial resolution varying between 300 m and 4.8 km depending on the pericentre altitude of the spacecraft's highly elliptical orbit. We report the global surface distributions of some minerals based on the OMEGA observations in the Visible and Near Infrared wavelength domains (0.35-2.5 micrometer). Global maps at 32 pixels per degree of mafic minerals (pyroxenes, olivines), hydrated minerals, and ferric oxides have been derived from spectral parameters. The limits of detection in term of abundance for some minerals will be given. The distributions of these materials are in general agreement with the global mineral maps of previous telescopic and space observations. Terrains with water bearing minerals cover a very small fraction of the surface. The bright regions are spectrally well reproduced by anhydrous nanophase ferric oxides. It is demonstrated that this product, commonly referred to dust, could result from in situ alteration over large parts of northern regions. Olivine (preferentially Mg-rich composition and fine grain sizes) is detected in larger parts of the pyroxene-rich regions than previously considered. By contrast, olivine with higher iron content and/or larger grain size (larger than 100 micrometer) is not widely distributed and hardly visible at a global scale. The surface composition of the northern low albedo regions are discussed in the light of these mineral maps. Chemical alteration producing a coating or varnish of anhydrous ferric phases over a dark basaltic surface best explains the VNIR spectral properties of these regions. A glassy composition resulting from impact is also considered.
P21A-04
Constraints on Mineral-Phase Abundances and Compositions in the Low-Albedo Northern Plains of Mars using MGS-TES, OMEGA, and Laboratory Spectral Data.
The abundances and compositions of mineral-phases in the low-albedo northern plains of Mars have been a focus of considerable study and debate in recent years. Large expanses of Acidalia Planitia surface materials are characterized by the MGS-TES Surface Type 2 (ST2) spectral endmember [1]. The ST2 spectrum is distinguished by a rounded, slightly V-shaped 800 to 1200 wavenumber region of absorption and uniform absorption at low wavenumbers. The same areas are also characterized by an OMEGA spectral signature that is relatively featureless, but with a strong blue slope (decreasing reflectance as a function of wavelength) from 0.9 to 2.6 microns [2]. A central question with both observations is whether they represent the spectral signature of a high-silica primary volcanic lithology (andesite) or the effects of chemical alteration on basaltic surface materials. Ambiguity in classifying the ST2 lithology arises because a spectral component of this unit (20-30 vol %) can be interpreted as volcanic siliceous glass [1, 3] (an abundant phase in andesite) or a combination of secondary phases found in altered basalt (amorphous silica-rich coatings, palagonite, smectite, and zeolite) [4-8]. Similarly, the OMEGA spectrum lacks evidence of distinct mafic mineral bands (found in andesite) as well as molecular vibration absorptions due to H$_{2}$O and/or OH$^{-}$, which might indicate the presence of well-crystalline alteration products and phyllosilicates [2]. Constraining these compositions is significant for understanding the petrogenesis of the Martian crust and its subsequent alteration. Identification of widespread andesite may imply an early episode of plate tectonics on Mars while altered basalt would indicate extensive surface-volatile interactions. The objective of this study is to combine TES and OMEGA observations of the low-albedo northern plains for comparison to laboratory thermal infrared and visible/near-infrared measurements of primary volcanic lithologies (basalt to dacite) and chemically weathered basalts from different terrestrial environments. Thermal infrared emission and visible/near-infrared reflectance measurements will be performed on rock chips, sorted particle sizes, and soil samples. Emission spectra have been acquired at Arizona State University using a Nicolet Nexus 670 FTIR spectrometer that has been modified to measure emitted energy over the range of 5 to 50 microns at 2 wavenumber spectral sampling. Bidirectional reflectance measurements from 0.32 to 2.55 microns at 0.05 micron sampling will be collected at the Brown University RELAB facility. Work by [5] on palagonitic alteration rinds developed on basaltic rocks demonstrates the effectiveness of combining wavelength regions in laboratory studies and applying results to orbital observations. In our study, we further this type of work by examining both unaltered volcanics and chemically altered basalts in an effort to constrain interpretations of the igneous lithology and the degree of secondary mineral-phase production in Acidalia Planitia. [1] Bandfield et al. (2000) Science, 287, 1626�1630. [2] Mustard et al. (2005) Science, 1594-1597. [3] Hamilton et al. (2001) JGR, 106, 14,733-14,746. [4] Wyatt and McSween (2002) Nature, 417, 263-266. [5] Morris et al. (2003) 6 Int. Mars. Conf, Abstract 3111. [6] Kraft et al. (2003) GRL, 30, 24, 2288, doi 10.1029.2003GL018848. [7] Ruff (2004) Icarus, 108, 131-143. [8] Michalski et al. (2005) Icarus, 174, 161-177.
P21A-05
Recent Near-Neutral Chemical Weathering of Martian High-Latitude Surfaces
Recent scientific investigations of Mars, including those conducted by TES, OMEGA, and the MER lander missions, have expanded the discussion about aqueous alteration on Mars. Results from these missions indicate that the styles and/or intensity of water-rock interactions on Mars have changed over time, and they provide evidence for geographical differences in weathering typically associated with latitude. Work that we have done on the spectroscopy of terrestrial weathering rinds and rock coatings indicates that small volumes of weathering products mixed with primary minerals considerably change thermal emission spectra of volcanic rocks. Based on that work, we suggest that low-intensity chemical weathering leading to the formation of small volumes of weathering products can explain the global distribution of TES observations. Whereas MER results indicate acidic alteration at low latitudes since the late Noachian, we suggest that major surface-mineralogical differences observed by TES (and broadly corroborated by OMEGA) may be due to near-neutral pH chemical weathering, pedogenically driven by near-surface pore waters at mid-to-high latitudes.
P21A-06
Thermal Infrared Spectroscopy and Modeled Mineralogy of Fine-Grained Mineral Mixtures: Implications for Martian Surface Mineralogy
Spectral data suggest that the Martian surface may be chemically altered. However, TES data show evidence for abundant primary glass, and Mini-TES data from MER Spirit in the Columbia Hills identify primary basaltic glass in rocks that are believed to be altered (Haskin et al., 2005, Ming et al., 2006, Wang et al., 2006). Debate over whether the primary glass identified spectrally may be interpreted as alteration products, such as clay minerals and/or amorphous silica coatings (Wyatt and McSween, 2002, Kraft et al., 2003), has focused on their spectral similarities (Koeppen and Hamilton, 2005). We suggest that some of the putative primary glass may be due to nonlinear spectral mixing of primary and secondary phases. We created physical mixtures made up of a primary phase (augite, andesine, or a 50:50 weight percent mixture of augite and andesine) and a secondary phase (montmorillonite clay or amorphous silica in 2.5, 5, 10, and 20 weight percent abundances) to test how secondary phases affect primary mineral thermal infrared spectra and modeled mineralogies. We found that the presence of small to moderate amounts of secondary material strongly affect modeled mineralogies, cause the false identification of primary glass in abundances as high as 40 volume percent, and report modeled plagioclase to pyroxene ratios that differ from actual ratios in the mixtures. These results are important for the surface mineralogy of Mars because surface type two (ST2), which may be altered, has the highest modeled plagioclase to pyroxene ratio. The presence of alteration material on Mars may cause the false identification or overestimation of primary glass in TES and Mini-TES data and may cause incorrect modeling of primary phases on Mars.
P21A-07
The Hydration State of the Martian Surface as Seen by Mars Express OMEGA
All VIS-NIR spectra acquired by the Mars Express OMEGA spectrometer exhibit a 3 um absorption due to H2O. This feature varies in strength and shape over spatial and temporal scales, suggesting the hydration state of the near surface also varies over these dimensions. We use a laboratory-based model to relate the strength of this absorption to absolute water content (wt. % H2O) on Mars, finding that equatorial and mid-latitude zones have water contents of 2-5 wt. %, whereas hydration increases poleward of ~60� and can reach values upward of 12 %. The latter agrees qualitatively with estimates of H content from Mars Odyssey GRS data, though GRS is sensitive to a greater depth than OMEGA and observes buried ice, whereas OMEGA is observing the hydration of the uppermost fraction of the surface. The regolith at high latitudes likely holds more H2O because it seeks equilibrium between both ice at depth and water vapor in the overlying atmosphere, whereas equatorial zones do not contain buried ice. Obliquity-driven glacial cycles during the Amazonian may have formed ice-rich deposits at mid and high latitudes, a possible mechanism for increasing the H2O content of any hydrous phases present in the regolith or dust within these regions. Spectra for high latitudes and dusty regions lack spectral features indicative of specific hydrated minerals, but the presence of a 3 um feature in all OMEGA spectra and our estimates of H2O for bright regions, which are greater than the limit for surface-adsorbed H2O, suggest some fraction of hydrated material is present in the ubiquitous Mars dust. The GRS instruments also detect a large region of increased H abundance in Arabia Terra, but OMEGA spectra only detect an increase in hydration for small, relatively dust-free outcrops of high thermal inertia materials in this region. The high albedo and low thermal inertia of Arabia suggests a thin dust cover may commonly mask the presence of hydrated phases from OMEGA, thus they are only visible in small outcrops where this cover has been removed. Spectra of these outcrops do not exhibit absorptions directly attributable to OH, SO4, or CO3 groups, thus the composition of the hydrated phase is unknown. Nearby regions such as Terra Meridiani and Mawrth Vallis, however, contain hydrated sulfates and phyllosilicates. Our model estimates water contents of 6-9 wt. % for these materials, the highest observed in the equatorial regions to date. The OMEGA observations, in conjunction with GRS and MER results, suggest hydrated phases are present in the sedimentary record over vast regions of Mars, primarily within +/- $30\deg$ of the equator. Upcoming observations by Phoenix and the high-resolution CRISM spectrometer onboard MRO will likely provide additional insight into the nature of these hydrated phases.
P21A-08
Identification of sharp water-depth boundaries at +/-60 degrees latitudes on Mars using the Mars Odyssey Neutron Spectrometer
It has long been predicted that the transition zone from ice-free lower latitudes to ice-stable high latitudes is narrow. This transition zone has been observed using measurements of epithermal neutron currents by the Mars Odyssey Neutron Spectrometer (MONS). These boundaries correspond to a sharp increase in the depth of the stable water ice table from the surface (near both polar caps) to depths much larger than 1 m, where it cannot be detected from orbit using neutron spectroscopy. The depth below the surface of this ice table can be derived from thermal and epithermal neutron currents measured using MONS assuming a two-layer model. The macroscopic absorption cross section, given by the ratio of fast to thermal neutron currents, is a good proxy for this depth poleward of plus and minus 60 degrees latitude. Maps of both this cross section and the depth determined from thermal and epithermal neutrons reveal two latitudinal bands of maximum apparent depth at plus and minus 60 degrees that have FWHM widths of about 25 degrees, which corresponds to a physical boundary less than 1500 km. We interpret these bands to reflect a transition from water molecules held in hydrous minerals at lower latitudes to water in the forms of water ice and water of hydration at higher latitudes.