P22A-01 INVITED
Implications of Martian Surface Composition Determined by Mars Odyssey Gamma-Ray Spectrometry
The Mars Odyssey spacecraft carries a gamma-ray spectrometer (GRS) that allows measurement of several elements (K, Th, Fe, Si, Al, Ca, Cl, and H) on the surface of Mars. Although its spatial resolution is 500 km, it measures the composition in the upper few tens of centimeters (compared to microns to a few hundred microns for reflectance or emission spectral techniques). GRS provides the first global chemical perspective on the composition of the Martian surface, allowing us to assess the composition of the crust and, with judicious assumptions, the bulk silicate composition of the planet. The data also allow us to identify major geochemical provinces to understand the evolution of the crust and search for the effects of aqueous alteration. GRS data show that the Martian surface is chemically heterogeneous. Elemental concentrations vary across the surface, including variations within high-albedo areas that are presumably covered with dust. Fe concentrations are uniformly high, in accord with the compositions of Martian meteorites and most rock samples analyzed by landed instruments. K/Th is variable, but 95% of the surface has a weight ratio between 3000 and 7000. The mean (5300) is double that in terrestrial crustal rocks and in the bulk silicate Earth. These data indicate that bulk silicate Mars is enriched in moderately volatile elements and in FeO compared to Earth, suggesting that there was not extensive mixing throughout the inner solar system during planetary accretion. Cl varies substantially, with the highest values (0.8 wt%) in the region west of the Tharsis Montes. Surface Types (ST) 1 and 2 (identified from TES spectra) are indistinguishable in Si and Fe concentrations, but ST2 is enriched in K and Th by about 30% relative to ST1 (the K/Th ratio is similar in both regions). The combination of TES and GRS data suggests that ST2 is composed of a different suite of (probably) basaltic rocks than is ST1 and has been weathered. The H2O mass fraction (derived from the H content) in equatorial regions ranges from about 1.5 to 7 %, suggesting the presence of hydrous minerals. Variation in H2O and highly soluble Cl, coupled with some areas with anomalous K/Th, provide geochemical evidence for the role of aqueous alteration on Mars, although the generally uniform K/Th might indicate that wet periods were short.
P22A-02 INVITED
Martian Sulfates as Observed by the Mars Exploration Rover M\"{o}ssbauer and Alpha Particle X-ray Spectrometers
The M\"{o}ssbauer and Alpha Particle X-ray Spectrometers (APXS) onboard the Mars Exploration Rovers have returned data from over 200 distinct martian samples. Elemental correlations established using the APXS data provide clear evidence for Mg, Ca, and Fe sulfates. Indications of iron sulfates in the chemical data are shown by the M\"{o}ssbauer spectrometer to be present in ferric phases. Specific examples include the following: (1) Four localized samples of light-toned, subsurface soils have been imaged by Spirit at Gusev Crater, and two of these exposures have been analyzed in detail using the APXS and M\"{o}ssbauer Spectrometers. The compositions are highly variable, even over cm-scale distances, but are generally dominated by sulfates and silica. Ferric sulfate (Fe:S ~ 2:3) is the most significant reservoir of the sulfur, though Mg and occasionally Ca sulfates are also present. Associations with hematite, indications of trace element enhancements, and incorporation of chemical signatures of local rocks suggest that these deposits are precipitates from fluids and/or vapors (possibly hydrothermal/fumeraloic). (2) Ca-sulfate (Ca:S ~ 1:1) has been observed in weakly consolidated layered materials in close proximity to light-toned soils described above, providing additional support for an aqueous origin of these deposits. (3) Ca-sulfate is also observed as a cementing agent in a localized occurrence of magnetite-rich sandstones on the northwest flank of the Columbia Hills within Gusev Crater. Ca-sulfate cements may also be present in clastic rocks that dominate the West Spur of the Columbia Hills. (4) The outcrop rocks within Endurance Crater at Meridiani Planum show decreasing amounts of Mg and S with depth and an apparent Mg:S of 1:1, a strong indication of Mg-sulfates. (5) Also within Meridiani outcrop rocks, Ca-sulfates are likely to be present, as the Mg and ferric sulfate (determined to be jarosite by the M\"{o}ssbauer spectrometer) are inadequate to account for the >20 weight percent SO$_{3}$. (6) Over 50 analyses of soils by the two rovers show a well-defined correlation between sulfur and chlorine (S:Cl ~ 3.7:1), likely due to surface reactions with volcanic volatiles. Two excavations of subsurface soils on the Gusev plains, however, show an enhancement in S relative to this trend with a corresponding increase in Mg. These Mg-sulfate deposits may be a product of localized aqueous alteration.
P22A-03
Layered Deposits of Arabia Terra and Meridiani Planum: Keys to the Habitability of Ancient Mars
Understanding the habitability of ancient Mars is a key goal in the exploration of that planet. Evidence for conditions favorable to early life must be sought in ancient sedimentary rocks, such as those of Arabia Terra and Meridiani Planum. Arabia Terra, the northernmost extension of the ancient highlands, is dominated by cratered plains and minor ridged units. These plains extend south into the adjacent Meridiani Planum. The Opportunity rover landed in northern Meridiani, close to the border with Arabia. High resolution MOC images reveal extensive layered sequences across much of the Arabia and Meridiani region. These layers have been interpreted as eroded remnants of sedimentary rock deposits (Edgett, 2005). The layered sequences are concentrated in the SW quadrant of Arabia and in northern Meridiani. Preliminary mapping by Edgett (2005) distinguished four large scale layered sequences in the Arabia and Meridiani region. These have dimensions of hundreds to more than 1,000 km. MOLA altimetry shows that each of the sequences can attain a thickness of 200 to 400 m, with a total thickness greater than 1 km. The sequences are generally flat lying, with regional slopes of a few degrees. Much finer layering is evident within a number of craters. The plains and ridged units of the Arabia and Meridiani region were originally mapped as Noachian based on crater statistics, particularly the number of large craters (Scott and Carr, 1978). The layered sequences in the current study postdate many, but not all, of these large craters. The layered sequences have partially or totally filled a number of craters with diameters ranging from 20 to over 50 km. The topmost layered sequence, as well as the lower two sequences, have intermediate thermal inertia, as derived from THEMIS, indicative of moderate induration. The TES spectra from the lower sequences include features indicative of basalt. Some areas of the topmost sequence, which includes the Opportunity landing site, have TES spectra dominated by hematite. Just below this topmost sequence lies a sequence with higher thermal inertia, indicative of more indurated or coarser grained material. The TES spectra of this sequence lack distinctive mineral features, and the rocks may be obscured by a thin coating of dust. The layers have been extensively eroded. The uppermost sequences are characterized by deeply scalloped boundaries. Filled craters have been partially exhumed. Finely layered deposits within craters have been strongly dissected. Landforms uniquely attributable to wind erosion are rare, but erosive styles and geomorphology characteristic of water and possibly ice are present. The layered sequences in Arabia Terra and Meridiani Planum likely reflect an epoch when the planet was much more habitable than it is today. Several areas in these layered sequences are under intensive study as candidate landing sites for the 2009 Mars Science Laboratory.
P22A-04
Olivine and pyroxene-rich deposits in Holden Crater, Mars
Holden crater is a ~150 km diameter crater centered at 326 E, 26S breached in the south by Uzboi Vallis. Holden has been proposed as a possible landing site for the Mars Science Laboratory (MSL) rover and has been the subject of intense scrutiny due to its potential as a paleolake basin. Near the region where Uzboi Vallis breaches the southern Holden Crater wall are layered rocks that have been suggested to be aqeously deposited. Additionally, fan-shaped deposits originate in the southern and western crater walls that have been interpreted as alluvial fans and fan deltas. New analyses of THEMIS and TES data indicate that both the basement and layered rocksâ€â€�including those interpreted to have been deposited in an aqueous environmentâ€â€�near the breach point of Holden crater are olivine and pyoxene-rich. Based on THEMIS IR imagery, these units have a higher thermal inertia than the surrounding terrain. The higher-TI units correspond with lighter-toned layered units seen in THEMIS Vis and MOC imagery that also have a distinct mafic signature in THEMIS daytime IR false-color decorrelation stretch images. The fan deposit in western Holden Crater, which has been proposed as a specific landing site for MSL may also have an elevated olivine abundance based on TES data. High olivine and pyroxene abundances in the sedimentary units of Holden crater do not preclude the presence of water in the history of the crater. However, based on analysis of TES and THEMIS data over the region, there is no evidence for extended chemical weathering present at the surface. Olivine-rich bedrock is likely to be a regional property as it is also present in chaos terrain to the northeast of Holden which is the source of the Landon Vallis outflow channel. These regions should be considered high priority targets for future investigations by OMEGA and CRISM.
P22A-05
A Thick (>1500 m) Section of Clay-Bearing Clastic Rocks Near Mawrth Vallis: Evidence for Early Crustal Processing on Mars
Aluminous and ferruginous clay minerals detected in the Mawrth Vallis area [Poulet et al., 2005] correspond to ancient, light-toned (TES albedo $=$ ~0.22), layered bedrock. We conclude that the layered surface materials are indeed bedrock based on 1) their weathering style, they form scarps, steep cliffs, buttes, and mesas; and 2) their thermal inertia which, based on THEMIS data, is > 500 J K-1 m-2 s-1/2. Because the rocks are nearly flat-lying and morphologically unique, it is possible to make stratigraphic correlations throughout the Mawrth Vallis area. The clay mineral deposits all correspond to a complex section of rock present throughout an area similar in size to the Colorado Plateau. Through stratigraphic correlations and basic structural interpretations, we conclude that the layered clay mineral-bearing bedrock is >1500 m thick and contains 100s to 1000s of individual layers, indicating a dynamic environment during deposition. At least some of the light-toned layers are phyllosilicate-poor, making origin of the clay minerals by regional-scale, subsurface replacement an unlikely scenario. Geomorphic evidence shows that the light-toned rock unit was eroded and incised after lithification and it is disconformably overlain by a clay mineral-poor, dark-toned (TES albedo $=$ ~0.12), layered sedimentary rock that occurs at all elevations throughout the area. This dark toned unit is interpreted as sedimentary rock because it contains abundant evidence for inverted topography in the form of meandering streams that may have fed Mawrth Vallis. The light-toned, clay mineral-bearing unit ends abruptly at the present day erosional remnant of the dichotomy boundary, though remnants of it are observed in buttes within the northern plains. Therefore, either the light-toned, clay mineral-bearing unit is older than the dichotomy (possibly > 4 Ga), or the basin in which these units were deposited was previously bounded to the north by a topographic barrier that no longer exists. It is unclear whether the upper, dark-toned units could also be older than the dichotomy. It is impossible to assign a unique interpretation to the light-toned clay-bearing rock unit, as it could ultimately be sedimentary rock or altered pyroclastic rock. However, its preservation implies the former existence of a depositional basin in this area. If the layered rocks are interpreted as volcanic and erosion erased the evidence of associated volcanic structures, then the erosion must have generated sediments that would have been deposited along with volcanic materials in the basin. Regardless of the interpretation of volcanoclastic $+$ sedimentary versus purely sedimentary origins, these rock units are seemingly a window into early geologic processes in the Solar System, in a time period not observable on Earth. Indeed the evidence for ancient orogenic systems on Earth lies in ancient sedimentary sequences. This package of ancient clastic rocks may represent ancient fluvial, lacustrine, and/or marine deposits, and could have implications for early orogenic processes on Mars.
P22A-06
Multi-sensor, mapping and multispectral analysis of the Mawrth Vallis region of Mars
We have undertaken a study to examine the Mawrth Vallis region of Mars (centered roughly at 24� N and -20� W). This is an area under active examination as a Mars Science Laboratory (MSL) landing site due to the presence of clay minerals as observed by Mars Express OMEGA. We are utilizing THEMIS Visible (VIS) and Infrared day and night data in conjunction with MOC, MOLA, OMEGA, and HRSC data to better understand the geologic history of the Mawrth Vallis region. The OMEGA team has reported on the presence of Fe-OH bearing smectite clays, nominally nontronite, in the Noachian terrain carved by the Mawrth Vallis outflow. The OMEGA team has also observed the presence of an Al-OH bearing phase nominally the dioctahedral clay mineral montmorillonite. We have performed our own analysis of OMEGA data. We concur that the clay-bearing materials occur in the Noachian-aged highlands materials and that this material is dissected by the Mawrth Vallis channel, punctured by craters and covered by mantling material. Mapping of the Fe-OH and Al-OH phases indicates stronger concentrations of the latter on terrains west of the channel, especially outside the northeast rim of the large degraded, partially filled crater centered at 23.4� N, 20.0� W. The presence of Al-bearing clays is important because it suggests a more Earth-like, higher water activity than was extant even for the later sulfate deposition sampled by the Opportunity rover at Meridiani Planum. The light-toned outcrop that hosts the clay-rich material has higher thermal inertia values than the surrounding highlands mantle, but has lower thermal inertia values than putative basaltic sands filling the floor of the channel and the floors of large craters to the south. Subtle color differences are apparent in multispectral THEMIS VIS images of the light-toned outcrop which should be useful in constructing a stratigraphic column in the area. While deposition of the clay-bearing materials predates formation of the main channel, we find evidence for a long fluvial history in the region as evidenced by terraces on the walls of the channel, possible inverted channels on the outer flanks of the main channel, and tributary channels leading into the main channel.
P22A-07
Spectral Evidence for Silica in Eos Chasma, Mars
Thermal Emission Imaging System (THEMIS) data in Eos Chasma have revealed spatially small areas, typically mounds or knobs, with materials having significant (>~35%) fractions of silica in one or more as-of-yet unidentified phases [1]. Silica, SiO$_{2}$, occurs geologically in both crystalline (e.g., quartz) and amorphous (e.g., opal, glass) forms. The identification of associated minerals and the specific silica phase(s) observed in the thermal infrared data is critical to constraining the abundance estimate further. New results from THEMIS multispectral data show that if the silica is present as quartz or one of its polymorphs (e.g., tridymite, cristobalite, coesite), it is probably equal to or less than ~35% of the modal mineralogy. If the silica is present in an amorphous form with different spectral character, such as opal, this number could increase by several tens of percent. Cherts, which are quartz in rock form, exhibit a variety of microscopic textures (e.g., microcrystalline, fibrous, and "megaquartz") [2] and contain contaminating phases that produce variations in their spectra; we have identified several chert samples that also are candidate components and could be present at abundances of several tens of percent or greater. Primary and secondary silica phases are formed by a wide array of geologic processes, many of which include interactions with ambient or hydrothermal fluids and some of which are well- known preservers of biomarkers on Earth. Thus, silica enrichments on the Martian surface are likely to be important recorders of aqueous processes, and possibly biomarkers as well. As such, an area in Eos Chasma adjacent to silica-bearing deposits has been proposed as a landing site for NASA's 2009 Mars Science Laboratory rover [3]. The majority of silica-bearing deposits are a few hundred m$^{2}$ in size, and there is a paucity of high-resolution visible images with which they can be investigated. A 3-m/pixel Mars Orbiter Camera (MOC) image of a relatively large (~3 km$^{2}$) silica-bearing mound reveals that it is bright-toned with numerous boulders on the summit, and has a bright-toned debris apron (including apparently bright-toned boulders) at the base of the structure. The presence of boulders suggests that the material is not friable. At the southern end of the mound, lineaments are visible, which we tentatively interpret as the surface expression of layering. The geomorphology of these materials differs from that of the regional dark-toned materials, which include primarily basaltic, and orthopyroxene- and olivine-enriched materials, according to analyses of Thermal Emission Spectrometer (TES) data [4]. This presentation will focus on updated geomorphologic and mineralogic information from analyses of MOC and THEMIS visible images and THEMIS and TES spectral data, including an exploration of the various hypotheses for the origin of the materials. [1] Hamilton, V.E. (2005), {\it Eos Trans. AGU, 86 (52), Fall Meet. Suppl.,} Abstract \#P24A-08. [2] Knauth, L.P. (1994), in {\it Chert: Physical Behavior, Geochemistry, and Materials Applications, Rev. Min., 29,} 233-258. [3] Hamilton, V.E., S.L. Cady, and P.J. Boston (2006), 1st Mars Science Laboratory Landing Site Workshop, http://marsoweb.nas.nasa.gov. [3] Hamilton, V.E., et al. (2003), {\it Meteor. Planet. Sci., 38,} 871-885.
P22A-08
Mineralogy and Geologic Context of Bedrock Exposures in Mare Serpentis, Mars
Numerous exposures of bedrock (defined here as surfaces that exhibit thermal inertia values > 1200 J m$^{- 2}$K$^{-1}$s$^{-1/2}$) have been identified on Mars from THEMIS data [e.g., 1]. Relative to surfaces dominated by sands, these sites are likely to consist of locally-derived materials (ie, true bedrock or blocks derived from the underlying rock), with little compositional contamination from mobile surface layer materials. The ability to confidently place observed compositions in their original geographic and stratigraphic context provides insight into the mineralogic heterogeneity of the crust and degree of influence from aqueous processes on the surface. In addition, characterization of these rare sites provides a link to results from previous spectroscopic studies, most of which determined compositions on a much larger scale and were limited to measurements of somewhat mobile materials. Three exposures located in the intercrater plains of Mare Serpentis, Mars (17-$25\deg$S,45- $49\deg$E) were analyzed using MGS and Mars Odyssey datasets. The bedrock regions measure ~30-700 sq km in area and are separated by ~100-300 km. Visible images from MOC and THEMIS were used to examine the contacts between bedrock and surrounding lower inertia plains. Two of the bedrock surfaces appear to consist of a darker toned, competent unit that is superposed on the surrounding plains. The third region consists of a lighter toned unit that lies below a darker, competent unit. Although brightness differences are observed, TES albedo values vary by less than .01 between the plains and bedrock surfaces. No bedforms are observed in the available THEMIS visible images of the bedrock surfaces, consistent with the extremely high thermal inertia and indicating a dominance of blocky material. TES dust cover index values are >0.97, consistent with relatively dust-free surfaces. THEMIS multispectral images and TES spectra indicate that in general, the bedrock surfaces exhibit greater abundance of olivine and pyroxene, and lower abundance of plagioclase than the surrounding lower thermal inertia areas. However, preliminary analysis of TES spectra suggests that absolute abundances of plagioclase, orthopyroxene, clinopyroxene and/or high-silica phases may vary between the bedrock regions. Comparison of bedrock and surrounding plains spectra from individual TES orbits indicate that sulfates are modeled at abundances of 10-15% (right at the TES detection limit) for the bedrock surfaces, and well below the detection limit for the surrounding plains. Further analysis using additional techniques is needed to validate these potential differences. Finally, THEMIS infrared images show that two of the bedrock regions are superposed by a ~50 m thick, lower thermal inertia layer that exhibits higher plagioclase and total pyroxene abundance, and lower high-silica phase abundance, suggesting a similar sequence of processes/events may have occurred in both regions. A synthesis of these and additional geologic and mineralogic observations and their implications will be presented. [1] Edwards, C. S. et al., Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abs. P21C-0158.