P43B-1392
A new Hypothesis for the Origin and Redistribution of Sulfates in the Equatorial Region of Western Mars
The formation of sulfates on Mars has been under debate since they were identified by several Mars
missions starting from the 1970s. We propose that sulfates formed as evaporites in enclosed standing bodies
of water in the Valles Marineris area following the early alteration of Martian basaltic crust, were then
elevated by the Tharsis uplift, and transported together with rock materials to Meridiani Planum by periodic
outbursts of water, where they were deposited as sediments. The proposed model comprehensively
addresses all forms of sulfate occurrences near the equator in the western Martian hemisphere and relates it
to physiographic processes (volcanic, tectonic and sedimentary) affecting the Martian surface (Fan et al.
2008).
Fan, C., Schulze-Makuch, D., Wolff, J.A., and Fairen, A.G. (2008) A new hypothesis for the origin and
redistribution of sulfates in the equatorial region of Western Mars. Geophysical Research Letters 35,
L06201, doi:10.1029/2007GL033079
http://www.sees.wsu.edu/ABcenter/index.html
P43B-1393
Stratigraphy of Phyllosilicates and Sulfates in Northern Meridiani Planum, Mars
The Meridiani Planum region on Mars is currently being explored by the Mars Exploration Rover Opportunity, and multiple landing sites for the upcoming Mars Science Laboratory (MSL) have been proposed in the region. Northern Meridiani is characterized in part by expanses of exposed light-toned layered rock (formerly known as "etched terrain"). We have generated new regional mosaics of Meridiani Planum data from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) near-IR imaging spectrometer on the Mars Express orbiter and have used them to map spectral parameters that are indicative of hydrated minerals, sulfates, and phyllosilicates. Our maps are consistent with though not identical to previous OMEGA maps of this region. They reveal widespread (thousands of km2) evidence for hydration in the Meridiani light-toned layered rock (indicated by an absorption band at 1.9 microns). Sulfates, indicated by a decrease in reflectivity at 2.4 microns, are also apparent in our OMEGA mosaics, and are most clearly detected in a specific ~130 km × 30 km valley in the light-toned layered rock, near 2°N, 0.4°W. We also detect phyllosilicates in small (several km2) isolated patches of the light-toned layered rock. Of particular interest is a phyllosilicate detection in the light-toned layered rock roughly 60 km southwest of the sulfate-bearing valley. This phyllosilicate detection has not been reported in previous OMEGA maps of the region. The phyllosilicates occur ~300 m higher in elevation than the floor of the sulfate-bearing valley. If both the phyllosilicates and sulfate detections are representative of bulk bedrock, this may indicate that the phyllosilicates are stratigraphically younger than the sulfates. Alternatively, it is possible that the sulfates and/or the phyllosilicates were not deposited with the bedrock but are the result of subsequent alteration, or have been transported to their current location by aeolian or fluvial processes. Finally, it is possible that in the Meridiani region, phyllosilicates and sulfates were forming concurrently at some points in the past. To test the hypothesis that the phyllosilicates are stratigraphically younger than the sulfates in this region, we are examining high resolution images from the Mars Orbital Camera (MOC), the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) and determining the tilt and stratigraphy of the sulfate- bearing valley, the phyllosilicate-bearing regions, and the surrounding terrain. This investigation will be supplemented by Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations of the same region. Wherever possible, we will correlate composition with morphology to develop a more complete understanding of the stratigraphy in Northern Meridiani Planum.
P43B-1394
Sulfates and Other Hydrated Minerals in Ius Chasma, Valles Marineris and Implications for Water Geochemistry
Ius Chasma is a linear trough in western Valles Marineris containing horst and graben structures, multiple landslides, and light-toned floor deposits. Elsewhere is Valles Marineris, sulfate deposits identified by OMEGA and CRISM are restricted to Interior Layered Deposits or nearby autochthonous loose material (Gendrin et al., 2005; Murchie et al., 2007; Murchie et al., in revision). In Ius Chasma, however, sulfates are found in thin floor units in enclosed depressions at the lowest elevations. Kieserite is at the lowest elevation, with polyhydrated sulfate and an unidentified hydrated phase, at higher elevations. Some poorly defined layering is visible on the HiRISE scale, but is not diagnostic. The total exposed thickness of kieserite is 300 m, with a total range in elevation of -4175 to -4475 m. The kieserite deposit is covered in places by pyroxene-bearing dunes. The polyhydrated sulfate outcrops range from –3515 to -4000 m. The unidentified hydrated phase outcrops between -3770 to -4100 m. The polyhydrated sulfate and unidentified hydrated material occur at the same elevations and do not have a clear stratigraphic relationship. Elsewhere in Ius Chasma, the unidentified hydrated material clearly drapes chasma floor units. The unidentified hydrated material is characterized by absorptions near 1.4 and 1.9 μm, and a narrow doublet at 2.21 and 2.27 μm. The deep ~1.9 μm is due to the combination tone of the OH stretch and H2O bend and the ~1.4 μm absorption is due to the 1st overtone of the OH stretch. Sulfates or other minerals with 2 or more water molecules per unit cell in their structure are necessary to account for deep water and hydroxyl absorptions in many spectra we observe. The doublet doesn't match any known sulfate, phyllosilicate, chloride, hydrated silica library spectra. Possibly a mixture of hydrated phases could explain this phase. 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. However, neither sulfates' doublet matches the unidentified spectrum's minima. While there is no phyllosilicate that exactly matches the 2.21 and 2.27 μm doublet, a combination of smectite clays, such as nontronite and montmorillonite, produces a similar doublet at 2.21 and 2.29 μm. A third option that is spectrally close but not exact is hydrated silica. Hydrated silica has a wider absorption over 2.21- 2.25 μm and its hydration bands are shifted to ~1.38 and ~1.91 μm (Milliken et al., 2008). The location of kieserite in the lowest points of Ius Chasma, with polyhydrated sulfate exclusively found at higher elevations, suggests they were formed by nearly complete evaporation of a closed basin. Polyhydrated evaporites form first and then kieserite as the basin is drawn down. Originally, Ius Chasma may have been less connected to Melas Chasma than it is now (Peulvast and Masson, 1993). These sulfates may be evidence of an environment that supported isolated, evaporating basins. 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.
P43B-1395
Sulfates, Ferric Oxides and Al-OH Bearing Minerals in Aram Chaos
Aram Chaos is a 280 km wide Martian crater centered at 2.5N, 338.5E. This crater is filled by chaotic terrains, overlain by a presently dome-shaped layered, 900 m thick formation, displaying spectral signatures of ferric oxides and sulfates on TES and OMEGA data (Glotch et al. 2005, JGR 110, E09006). In a previous study (Masse et al. 2008, JGR in press), using OMEGA, MOLA, MOC, TES, THEMIS and CTX data, we proposed that the presently dome-shaped formation is composed of a bright material that contains both monohydrated sulfates and ferric oxides. After its emplacement, this formation has been grooved down to various depths by large aeolian erosion corridors. The borders of the corridors are steep linear cliffs where the bright, layered, sulfate-rich material crops out. These cliffs are also partially covered by dark debris fans, which originate from the bright formation itself and which feed dark sand sheets covering the lowest stratigraphic levels of the bright formation. We therefore infer that the dark ferric oxide sand sheets and debris fans are erosional products of the bright formation. We therefore infer that the dark ferric oxide sand sheets and debris fans are erosional products of the bright formation. Due to the relatively low spatial resolution of OMEGA, it is not possible to analyse the exact composition of the cliffs. The aim of the present work is to refine these results and to compare them with newly acquired, high resolution, hyperspectral data from the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). CRISM data confirm the mineralogical conclusions made with OMEGA data. Moreover, CRISM data allow the detection of a new layer, containing an Al-OH bearing mineral, at the bottom of this dome-shaped formation.
P43B-1396
The Origin of the Terra Meridiani Sediments: Volatile Transport and the Formation of Sulfate Bearing Layered Deposits on Mars
The chemistry, sedimentology, and geology of the Meridiani sedimentary deposits are best explained by eolian reworking of the sublimation residue of a large scale ice/dust deposit. This large ice deposit was located in close proximity to Terra Meridiani and incorporated large amounts of dust, sand, and SO2 aerosols generated by impacts and volcanism during early martian history. Sulfate formation and chemical weathering of the initial igneous material is hypothesized to have occurred inside of the ice when the darker mineral grains were heated by solar radiant energy. This created conditions in which small films of liquid water were created in and around the mineral grains. This water dissolved the SO2 and reacted with the mineral grains forming an acidic environment under low water/rock conditions. Subsequent sublimation of this ice deposit left behind large amounts of weathered sublimation residue which became the source material for the eolian process that deposited the Terra Meridiani deposit. The following features of the Meridiani sediments are best explained by this model: The large scale of the deposit, its mineralogic similarity across large distances, the cation-conservative nature of the weathering processes, the presence of acidic groundwaters on a basaltic planet, the accumulation of a thick sedimentary sequence outside of a topographic basin, and the low water/rock ratio needed to explain the presence of very soluble minerals and elements in the deposit. Remote sensing studies have linked the Meridiani deposits to a number of other martian surface features through mineralogic similarities, geomorphic similarities, and regional associations. These include layered deposits in Arabia Terra, interior layered deposits in the Valles Marineris system, southern Elysium/Aeolis, Amazonis Planitia, and the Hellas basin, Aram Chaos, Aureum Chaos, and Ioni Chaos. The common properties shared by these deposits suggest that all of these deposits share a common formation process which must have acted over a large area of Mars. The results of this study suggest a mechanism for volatile transport on Mars without invoking an early greenhouse. They also imply a common formation mechanism for most of the sulfate minerals and layered deposits on Mars, which explains their common occurrence.
P43B-1397
Coordinating CRISM Observations of Sulfates near Valles Marineris with the Subsurface Bright Salty Soils Exposed in Gusev Crater via Lab Experiments
CRISM has identified unique spectral signatures in inverted channels near Juventae Chasma and other chasmata in the greater Valles Marineris region [1] that are composed of light-toned layered sediments interpreted to be fluvial in origin [2]. These include a weak, broad feature near 1.45 μm, a strong, broad band centered near 1.94 μm, a sharp band at 2.23 μm, and a shoulder or band near 2.4 μm [3]. This signature is not characteristic of any single mineral; however, it is consistent with partially dehydrated ferricopiapite [original formula of Fe0.33Fe2(SO4)3(OH)·10H2O]. Our lab experiments show that incrementally heating this hydrated ferric sulfate to 300 °C at 1 atm changes the color, XRD pattern, and spectral properties of this mineral. The resulting spectral signature of our ferricopiapite heated to 300 °C matches well with the CRISM spectra of the material observed in inverted channels near Juventae Chasma. This result is of particular interest as ferricopiapite is the mineral thought to be present in the bright salty soils exposed by the Spirit rover at Paso Robles and other sites in combined analyses of the Pancam, Mössbauer, Mini-TES and APXS data [4, 5, 6]. Other sulfate minerals possibly present in lower abundance include butlerite, (para)coquimbite, fibroferrite, and metahohmanite [4]. Continued lab experiments are underway to characterize the spectral properties of partially dehydrated ferricopiapite and other OH and H2O-bearing sulfates. Opaline silica is also found in these inverted channels near Juventae Chasma [1] and both monohydrated (szomolnokite and kieserite) and polyhydrated (e.g. ferricopiapite) sulfates are observed in the bright mounds inside the chasma [7]. Identification of partially dehydrated ferricopiapite in these inverted channels provides a link between the aqueous processes occurring in the plains outside the chasma and those processes that formed the light-toned layered mounds inside the chasma. Sulfate deposits in the greater Valles Marineris region are consistent with lithification and diagenetic modification of eolian sediments by evaporation of near-surface groundwater [8], processes similar to those inferred at Meridiani [9,10]. [1] Milliken R. E. et al. (2008) Geology, in press. [2] Weitz C. M. et al. (2008) GRL, doi:10.1029/2008GL035317, in press. [3] Bishop J. L. et al. (2008) JGR, to be submitted. [4] Lane M. D. et al. (2008) Am. Miner., 93, 728-739. [5] Parente M. et al. (2008) Icarus, in review. [6] Johnson J. R. et al. (2007) GRL, 34, L13202, doi:10.1029/2007GL029894. [7] Bishop J. L. et al. (2008) LPSC, abs. 2334. [8] Murchie S. L. et al. (2008) Nature, in revision. [9] Squyres S. W. et al. (2006) Science, 313, 1403- 1407. [10] Andrews-Hanna J. C. et al. (2007) Nature, 446, 163-166 doi:10.1038/nature05594.
P43B-1398
Iron Sulfate and Sulfide Spectroscopy at Thermal Infrared Wavelengths for Application to Mars
Ferric sulfate minerals were identified in several light-toned subsurface soils in Gusev Crater, Mars, that were exposed by the Mars Exploration Rover (Spirit) wheels. Although the identified ferric sulfate minerals vary from site to site, several different ferric sulfates have been identified, including ferricopiapite (dominating the Paso Robles bright track soils), (para)coquimbite, fibroferrite, rhomboclase, and hydronium jarosite, and possibly minor (para)butlerite or metahohmanite. Recent work by the Mars Reconnaissance Orbiter (MRO) CRISM team suggests that polyhydrated sulfates such as ferricopiapite may be widely present on Mars, and that ferricopiapite may be found in a partially dehydrated state in inverted channels in Juventae Chasma and other nearby Chasma in the Valles Marineris region. A growing laboratory suite of ferric sulfate minerals (including the minerals mentioned above as well as many other ferric sulfates, some of which were intentionally dehydrated) have been analyzed using a thermal emission (midinfrared) spectroscopic technique to continue to provide well-defined and well-understood emissivity spectra of a large range of ferric sulfate minerals in order to aid and expedite the identification of these minerals using TES and Mini-TES thermal emission data from Mars. This work also includes thermal emission studies of a variety of sulfide minerals for a similar purpose of identifying them on Mars. Sulfides occur in terrestrial volcanic terrains similar to igneous terrains on Mars, and sulfides also are found in Martian meteorites. The sulfides are an important mineral class to understand because they are possible precursor minerals that can follow an oxidation pathway (with proper hydration and pH) to form ferric sulfate minerals.
P43B-1399
Quantitative Sulfur Analysis using Stand-off Laser-Induced Breakdown Spectroscopy
The laser-induced breakdown spectrometer (LIBS) in the ChemCam instrument on Mars Science Laboratory has the capability to produce robust, quantitative analyses not only for major elements, but also for a large range of light elements and trace elements that are of great interest to geochemists. However, sulfur presents a particular challenge because it reacts easily with oxygen in the plasma and because the brightest S emission lines lie outside ChemCam's spectral range. This work was undertaken within the context of our larger effort to identify and compensate for matrix effects, which are chemical properties of the material that influence the ratio of a given emission line to the abundance of the element producing that line. Samples for this study include two suites of rocks: a suite of 12 samples that are mixtures of sulfate minerals and host rocks, generally with high S contents (0.1-26.0 wt% S), and a large suite of 118 igneous rocks from varying parageneses with S contents in the 0-2 wt% range. These compositions provide several different types of œmatrices to challenge our calibration procedures. Samples were analyzed under ChemCam-like conditions: a Nd:YAG laser producing 17 mJ per 10ns pulse was directed onto samples positioned 5-9 m away from the laser and telescope. The samples were placed in a vacuum chamber filled with 7 Torr CO2 to replicate the Martian surface pressure as the atmospheric pressure influences the LIBS plasma. Some of the LIBS plasma emission is collected with a telescope and transmitted through a 1 m, 300 um, 0.22NA optical fiber connected to a commercial Ocean Optics spectrometer. We are testing and comparing three different strategies to evaluate sulfur contents. 1) We have calculated regression lines comparing the intensity at each channel to the S content. This analysis shows that there are dozens of S emission lines in the ChemCam wavelength range that are suitable for use in quantitative analysis, even in the presence of Fe. 2) Partial least-squares analyses of these data show that S can be predicted with better than 10% accuracy, even when present at levels <0.15 wt%. 3) When peaks in the spectra are fit, the resultant peak areas can be regressed against concentration using step-wise multiple regression analysis to determine which subset of S lines gives the most accurate concentrations. All three methods of calibration show that excellent S analyses can be produced under Mars conditions at stand-off distances of up to 9 m.
P43B-1400
Extended Jarosite Lifetimes in High Salinity Fluids
Particle lifetime calculations utilizing olivine (Olsen and Rimstidt, 2007; Stopar et al., 2006) and jarosite (Elwood Madden et al. 2008) dissolution rates have been used to constrain the duration of aqueous environments on the surface of Mars. Previous rate experiments have shown that jarosite dissolves relatively quickly in dilute aqueous solutions leading to short particle lifetimes. However, mineralogy and bulk chemistry of outcrops containing jarosite at Meridiani Planum suggest high salinity fluids were active in the region. The goal of this study is to determine the effects of high salinity (low activity of water) on jarosite dissolution rates. K-jarosite was synthesized using the methods of Baron and Palmer (1996) and characterized using powder X-ray diffraction, BET surface area analysis, transmission electron microscopy, and atomic force microcopy. Dissolution experiments were conducted by adding 0.5 g K- jarosite to 500 g ultrapure water at 293K. Samples were collected from the continuously-stirred batch reaction at predetermined intervals and filtered using 0.2 micron filters. K+ concentrations in the resulting supernatants were measured using atomic adsorption spectroscopy to determine the rate of jarosite dissolution. Jarosite dissolution experiments in halite saturated brine result in dissolution rates over one order of magnitude slower than similar experiments conducted in dilute solutions. Dissolution in ultrapure water proceeds at log k= -8.5. Jarosite dissolution in halite saturated brine is significantly slower: log k = -10. Using a shrinking sphere model to calculate particle lifetimes, the lifetime of a 10 micron diameter jarosite particle is extended from 1-2 years in dilute solutions to 100 years in high salinity brine. This suggests that while jarosite is an ephemeral phase in dilute solutions, it may persist for significantly longer time periods in high salinity waters, such as those interpreted at Meridiani Planum based on bulk chemistry and evaporite minerals present within the outcrops. However, even in high salinity fluids, jarosite would not be preserved in aqueous systems lasting thousands to millions of years.
P43B-1401
Acid Drainage Generation and Associated Ca-Fe-SO4 Minerals Near Eagle Plains, Northern Yukon: an Analogue for Low Temperature Sulfate Formation on Mars
Near Eagle Plains, northern Yukon, acidic Ca-Fe-Mg-SO4 waters are discharging year-long from disturbed permafrosted sandstone bedrock overlying pyritiferous shales. The acidic waters are also precipitating gypsum with minor amounts of jarosite-K (Na), schwertmannite and hematite, similar to the mineral sequence observed at Meridiani Planum, making this site a valuable analogue for low temperature sulfate geochemistry and mineral formations on Mars. Stable O-S isotope analysis of the acidic waters near Eagle Plains revealed that the oxygen in the dissolved sulfate was mostly derived from water, suggesting that the sulfide oxidation process could be in part biomediated (i.e., accelerated by Fe-oxidizing microorganisms). However, unlike the dissolved sulfate in the waters, the formation of the Ca-Fe sulfate minerals is abiotic. The stable O-S isotope composition of the sulfate minerals is well within the predicted equilibrium range, suggesting that they formed through physico-chemical processes (i.e., evaporation or freezing). Low temperature geochemical modeling with FREZCHEM and PHREEQC suggests that the mineral sequence at Eagle Plains formed through the freezing of Ca-Fe-Mg-SO4 waters, rather than through evaporation during the dry summer season. Although the later is still a valid process of sulfate mineral formation at Eagle Plains as the acidic stream nearly dries-up during the summer. Overall, the fact that acid drainage is presently active allows to directly observe the low temperature (bio)geochemical processes responsible for generating acid drainage and precipitation of gypsum, schwertmannite, jarosite-K, jarosite-Na, goethite and hematite.
P43B-1402
Thermochemical Sulfate Reduction (TSR) by Methane - in Situ Observation and Raman Characterization in Fused Silica Capsules at Temperatures up to 450°C
An experimental technique using optically transparent fused silica capsules has been developed for TSR studies. Reactions between sulfuric acid (5 m) and methane (~30 MPa) were observed and characterized by Raman spectroscopy at temperatures (T) up to 450°C. In situ Raman signals showed the transformation from sulfate (S6+) to sulfite (S4+), elemental sulfur (S0), and finally to sulfide (S2-) with the generation of CO2. The durations for each transformation range from a few minutes to a few tens of minutes depending on the T. The sample capsules were prepared by first loading sulfuric acid in a fused silica tube (0.3 mm OD, 0.1 mm ID, and ~6 cm long), which had one end sealed. The tube was attached to a vacuum line, methane was loaded cryogenically and the tube sealed with a hydrogen flame. (Chou et al., Geochim. Cosmochim. Acta, 2008, doi:10.1016/j.gca.2008.07.030). The sample was placed in a USGS-type heating-cooling stage and in situ Raman spectra were collected continuously during heating and cooling. In the aqueous phase (L), SO42- disappears at ~80°C and the transformation of HSO4- to SO2 in both aqueous and vapor (V) phases begins at ~350°C. Soon after L-V homogenization at ~410°C, CO2 was produced while HSO4- disappeared. Finally, SO2 transformed to S0, which was then reduced to H2S within 10 minutes. During cooling, L-V phase separation occurred at ~300°C (L-V homogenization T = 310°C), and only CO2, CH4, and H2S were detected in both L and V phases at room T. High concentrations of H2S in a number of deeply buried petroleum reservoirs (e.g., Orr, 1994, AAPG Bull., v. 50, p. 2295; Worden et al., 1995, AAPG Bull., v. 79, p. 854) are thought to be the product of TSR. However, reliable reaction kinetics as well as documented reaction mechanisms for TSR are still lacking. Our technique has great potential in examining TSR and also in the studies of ore forming processes in magmatic/hydrothermal (Rye, 2005, Chem. Geol., v. 215, p. 5) as well as Mississippi Valley type environments (Anderson, 1991, Econ. Geol., v. 86, p. 909).
P43B-1403
Using Crystal Structure Groups to Understand Mössbauer parameters of Ferric Sulfates
A Mössbauer doublet assigned to ferric sulfate (Fe3D2) was identified in Paso Robles, Mars, spectra by Morris et al. (2006), who noted that its parameters are not diagnostic of any specific mineral because a number of different sulfates with varying parageneses might be responsible for this doublet. Work by Lane et al. (2008) used a multi-instrument approach based on Fe3+ sulfate spectra acquired with VNIR and midinfrared reflectance, mid-infrared emission and Mössbauer spectrometers to narrow down the possible ferric sulfate phases present at Paso Robles to ferricopiapite possibly mixed with other ferric sulfates such as butlerite, parabutlerite, fibroferrite, or metahomanite. Thus, we explore here the crystal-chemical rationale behind these interpretations of the Mössbauer results, using similarities and difference among mineral structures to explore which phases might have similar coordination polyhedra around the Fe atoms in sulfates. Work by Hawthorne et al. (2000) organizes the sulfate minerals into groups with analogous crystal structures. Mössbauer doublets assigned to ferric sulfates ubiquitously have isomer shifts (IS) of 0.40-53 mm/s so that IS is non-diagnostic. However, quadrupole splitting of doublets in these mineral groups has a wide range (0-1.4 mm/s) and the variation can be systematically correlated with different structure types. Members of the hydration series Fe2(SO4)3 · n H2O, which includes quenstedtite, coquimbite, paracoquimbite, kornelite, and lausenite have Mössbauer spectra that closely resemble singlets because of their near-zero QS. These minerals share structures involving finite clusters of sulfate tetrahedral and Fe octahedral or chains of depolymerized clusters, and all mineral species with these structures share similar Mössbauer parameters. At the other extreme, ferric sulfates with structures based on infinite sheets (hydrotalcite, alunite, jarosite), tend to have large electric field gradients at the nucleus of the Fe3+ cation, resulting in larger QS values (1-1.4 mm/s). Between these extremes of QS are two populations of structures based on finite clusters of polyhedra with QS = 0.36-0.80 mm/s (coquimbite, römerite, halotrichite, rozenite) and infinite chains with QS = 0.70-0.97 mm/s (chalcanthite, butlerite, fibroferrite, metahomanite). Our fits to the Paso Robles sol 429A data show two ferric doublets, both with IS = 0.42-0.43 mm/s but with differing QS = 0.36 and 0.93 mm/s; these parameters rule out mineral structures that have spectra with very high or very low QS. Ferric sulfates with structures composed of finite clusters and infinite chains thus provide the closest matches to the Paso Robles Mössbauer doublets, as well as spectra of other bright soils. Further constraints provided by other types of spectroscopy are then needed to determine which species within these structure groups are present. As additional sulfate structures are characterized, it will be possible to better understand the interrelationships among sulfate crystal structures and their spectral characteristics may provide additional constraints on mineral identification from ferric materials of all types. Morris et al. (2006) JGR, 111, doi: 10.1029/2005JE002584. Lane et al. (2008) Amer. Mineral., 93, 738-739. Hawthorne et al. (2000) Revs. Mineral., 40, 1-112.
P43B-1404
Sulfur Concentrations on Martian Surface Derived by In-Situ APXS Measurements: Evidence for Sulfates
During the last four and a half years, the two NASA Mars Exploration Rovers (MER), Spirit and Opportunity, have been making scientific measurements on the Martian surface. One of the instruments is the Alpha Particle X-Ray Spectrometer (APXS) located on the rover arm. Most rock samples were abraded and their fresh surfaces were measured by several instruments. Once a target spot of a soil or rock surface was selected, the APXS was properly positioned to accumulate X-ray spectra. From these data, the concentration of all major and many minor and trace elements are derived. Sulfur is an element that could be detected in all samples measured, so far, which makes the surface of Mars a unique place, as its surface is somewhat dominated by sulfur chemistry. Whenever some moisture or even larger quantities of liquid water were present, the surface of soils and rocks were effected by sulfuric acid reactions. In addition, sulfur-rich deposits could be observed with the APXS. At Gusev crater, soils have been deeply disturbed by rover wheels andthe exposed subsurface samples were investigated that had high S contents, such as Paso Robles (Sol 401), Arad Samra (Sol 723), Berkner Island (Sol 1013), and Mount Darwin (Sol 1098). In these soils, S concentrations between 9 to 14 weight-percent were determined by the APXS. As the APXS method only allows determining elemental concentrations, the speciation of S, such as elemental sulfur, sulfate, pyrite, or sulfide, cannot be derived, directly. Mass balance considerations and mineralogical results by the Mössbauer Spectrometer (MB) strongly support the assumption of sulfate being the predominant oxide of S. The high S content of Paso Robles is accompanied by a high Fe content, consistent with the identification of ferric sulfate by MB as the main iron-bearing mineral at Paso Robles. At Meridiani Planum, varying S contents were observed by the APXS: basaltic soils that contained a few percent S as found for Gusev soils and sedimentary outcrops that revealed very high amounts of S (up to 11 wt.-%). When the rover Opportunity drove inside the crater Endurance, a stratigraphical chemical profile was obtained by frequent APXS measurements of abraded rocks. These interior rocks show a very good correlation of Mg and S, which suggests that MgSO4 is a major component of the outcrop. Since S concentrations are markedly high in all measured outcrops, the concentration of major elements can provide constraints on their sulfur compounds. In a two-component mixing model it can be shown that certain elements form a sulfur-bearing phase, while others not: Silicon and Al do not occur as S-bearing phases, while, Mg, Ca, and Fe form S-rich phases. Assuming all SO3 is bound to Mg and Ca sulfates, and ferric sulfates, according to MB data, these outcrops contain large amounts of sulfates. The ingress of rover Opportunity into the Victoria crater also revealed a correlation of S and Mg contents with depth as was observed previously. Assuming that sample Cha (Sol 962), found at the rim of the crater, is part of the deeper ejecta, its S and Mg contents fit the general trend of the interior samples. Maybe, we have evidence at two locations that in deeper layers Mg-sulfates are somewhat lower than in upper ones pointing to a possible change in the Martian environment during the formation of the sediments.
P43B-1405
Sulfur Concentrations Along the Traverse of Rover Opportunity at Meridiani Planum, Mars
The Alpha-Particle-X-ray Spectrometer (APXS) is part of the in-situ payload of the Mars Exploration Rovers (MER). It determined so far the elemental composition of ~300 soil and rock samples on both landing sites along the traverse. The composition is determined by x-ray spectroscopy and usually 16 elements from Sodium to Bromine are detected with detection limits down to ~20 ppm for Bromine. One of the most amazing findings at Meridiani Planum is the high sulfur content of the bedrock. It contains typically about 10% sulfur by weight, or rather 25% SO3 taking overwhelming mineralogical evidence into account that the sulfur is present as sulfate. The best samples to follow the overall sulfate content along the meanwhile 10 kilometer traverse are abraded samples, cleaned of loose soil, dust and possible altered rinds with the Rock Abrasion Tool (RAT). So far, 35 abraded samples were measured beginning in the ~10 meter impact crater Eagle, the 100 meter crater Endurance and further 5 km south to the 800 meter crater Victoria. The overall composition of these bulk samples is astonishingly constant, with certain important exceptions. Besides the high Sulfur, mainly Magnesium, Iron and to lesser extend Calcium are enriched in the remaining matrix, while Si and Al and most other elements are diluted compared to typical Martian materials like the soil. However, during the ingress into Endurance, and recently also into Victoria a remarkably similar parallel decrease of Mg and S was detected. This decrease by approximately 25% with a very good molar match for MgSO4 - with unknown hydration degree - is accompanied in both craters by an increase of Chlorine by a factor of ~3. The observed layering of likely very soluble minerals, Magnesium Sulfates and Chlorides - with so far no obvious accompanying Cation - observed in two impact craters about 5 km apart is evidence for a long ranging leveling interaction with water during the formation of Meridiani - or for shorter periods of time after these craters were created by impact. The parallel decrease of Mg and S and the nearly parallel, but steeper increase of Cl are very smooth over 11 abraded samples during the ingress into Endurance. In Victoria only 5 samples could be abraded. However, after a statistical significant trend in the first samples was already visible, the last and deepest reachable spot (Gilbert) in Victoria shows the clearest resemblance of the trends observed in Endurance. Bromine inversely correlates with Chlorine during the ingress, although the trend is more variable. The abundance of Sulfur, Chlorine and Bromine, is readily detectable by the APXS with very good precision and repeatability along the traverse on both MER rover landing sites. It reveals the different past environments in Gusev Crater and Meridiani Planum and provided ground truth for orbiter measurements. While in Meridiani the high Sulfur content of bedrock is very constant over kilometers but dipping in deeper layers, in Gusev Crater localized subsurface soil spots of highly enriched ferric sulfates and accompanying near pure Silica deposits are identified. The basaltic soil at both landing sites is very similar and contains about 6 % SO3, very variable and correlated with Chlorine.
P43B-1406
Gypsum, jarosite, and hydrous iron-phosphate in Martian meteorite Roberts Massif 04262: Implications for sulfate geochemistry on Mars.
Gypsum has been identified on Mars by MEX OMEGA [1] and jarosite identified via MER-B lander [2] and both minerals are examples of the importance of calcium and iron sulfates in Martian weathering processes. The weathering of Martian basalt to form Ca and iron sulfates should be an important process on Mars. Martian jarosite has been identified in MIL 03346 [3] and Ca-sulfate has been identified in EETA 79001 [4], but both phases have yet to be identified in the same Martian sample. In Roberts Massif 04262, an olivine-phyric shergottite, iron-sulfide and calcium-phosphate minerals are undergoing reaction (dissolution and reprecipitation?) to form gypsum, jarosite, and an iron-phosphate phase, presumably during the meteorite's residence in Antarctica. If true, then an acidic and oxidizing fluid was present in this meteorite, due to the formation of jarosite which requires fluid of this type to form [5]. The weathering of iron-sulfides on Earth to form acidic and oxidizing fluids is common, thus this can be reconciled with the formation of an acidic fluid in a basic rock. Presumably, under more extensive weathering of silicate minerals in Martian basalt, the pH would be raised to values where jarosite would not be stable. While the weathering of RBT 04262 is likely occurring in Antarctica, a similar susceptibility of the apatite and pyrrhotite to incipient weathering on Mars may be expected. Oxidizing crustal fluids on Mars may attack iron- sulfides first in Martian basalts. The weathering of iron-sulfides leads to increasing acidity of fluids, which would enhance the dissolution of the calcium-phosphate minerals [6]. The formation of jarosite, gypsum, and iron-phosphate minerals during the early stages of weathering of Martian basalts may be an important process on Mars globally. [1] Gendrin, A. et al. (2005) Science, 307, 1587–1591. [2] Klingelhöfer et al. (2004) Science, 306, 1740- 1745. [3] Vicenzi E. P. et al. (2007) LPSC XXXVIII, Abstract 2335. [4] Gooding J. L. et al. (1988) GCA, 52, 909-915. [5] Greenwood J. P. et al. (2005) LPSC XXXVI, Abstract 2348. [6] Greenwood J. P. and Blake R. E. (2006) Geology, 34, 953-956.
P43B-1407
Experimental Reproduction of Martian Soil by Alteration of the Synthetic Martian Basalt Under Hydrothermal Conditions with Sulfuric acid and CO2
The formation process of the Martian soil is one of the most essential problems to understand the surface environment of the Mars. Especially, iron minerals in the Martian soil should be the key component to characterize the red planet. The major Martian volcanoes consist of iron-rich basaltic rocks. Volcanic activities of the Martian volcanoes should involve fluid rich in sulfuric components and CO2. CO2 may have more essential role in the alteration processes related to fluids in Martian volcanic activities than that in the terrestrial volcanoes. In this study, hydrothermal alteration experiments are conducted to elucidate the soil formation processes on the Martian surface. We carried out alteration experiments of the synthetic iron-rich basaltic material with sulfuric acid and CO2-bearing hydrothermal fluid. Experimental temperatures are 100 ~ 300°C. Acidities of the solutions are pH1.0, 3.0 or 7.0. Run durations are 4, 8 or 16 weeks (100°C) or 3, 6 or 12 weeks (150 ~ 300°C). CO2 are introduced to the experimental vessels by appropriate mass of dry ice (100 ~ 150°C) or silver oxalate (200 ~ 300°C) for approximately 1 MPa of CO2. We reported preliminary results of alteration experiments of mafic minerals with sulfuric acid-bearing hydrothermal fluid without CO2 (Yoshizawa and Isobe, 2007). In the run products of the preliminary experiments, we found characteristic hematite fine particles which may bring about reddish color of the Martian soil. Morphology of the hematite produced at 100 ~ 150°C was granular to spheroidal with diameters of 0.5 to 3 micron meters. Major run products of the experiments are clay minerals and iron oxide/hydroxide minerals. Run products of the alteration experiments show characteristic reddish to brown color depending on the acidity and temperature. Iron mineral species have distinctive color. SEM/EDS and XRD observations also revealed representative iron mineral species in the run products. Alteration products by CO2-bearing hydrothermal fluid are more oxidized than those by hydrothermal fluid without CO2. CO2 in acidic hydrothermal fluid may accelerate oxidative alteration of basaltic rocks. Acidic hydrothermal alteration may have essential role to form the Martian soil which is rich in iron oxide. Especially, iron mineral species and morphology strongly depend on temperatures and acidities of the hydrothermal fluid. Direct observation of the Martian soil may provide us information on the conditions of hydrothermal alteration related to the Martian volcanic activities.