P12A-01
Petrography of Rocks Examined by the Mars Exploration Rover Spirit within the Gusev Plains and Columbia Hills, Mars
Petrographic and megascopic textures of rocks encountered along the traverse of the Spirit Mars Exploration Rover within the Gusev plains (landing site to hills) and within the Columbia Hills (West Spur to lower Husband Hill), together with chemical data based on in situ measurements with the Athena spectrometers, provide information about the mechanisms and conditions of rock emplacement. Primary igneous and low-grade alteration textures are identified within the plains whereas clastic and alteration textures dominate within the Columbia Hills. Plains rocks include primary igneous petrographic textures common to xenocrystic and phyric rocks, megascopic textures similar to that known to occur within differing vertical positions of terrestrial basaltic lavas, and uniform chemical compositions typical of batch melted mantle rocks. Apparent vesicularity (ratio of summed vesicle volume from area over rock unit volume) of rocks in the plains lava surface varied with host rock dimension and angularity. Vesicle distributions in the smaller clasts in the Gusev plains tend to be exponential, typical of upper sections, whereas, dis-tributions in larger, more angular Gusev plains blocks are characterized by hybrid distri-butions, typical of flow interiors. Since small vesicular clasts are a small fraction of the observed clast population, it is inferred that the upper vesicular zones within the Gusev plains lava flows were rela-tively thin (low vesicularity at shallow depths) compared with that predicted for lava flows on Earth. A thin upper vesicular zone in the Gusev plains lavas could be indirect evidence for atmospheric pressure close to current values at the time of Gusev plains basalt emplacement during the Hesperian. Rocks with volcaniclastic or impactite textures occur in outcrops visited within the Columbia Hills. Megascopic and microscopic (Microscopic Imager) textures were examined along apparent lamination planes at several outcrops of moderately consolidated, fine-grained rocks. Some rocks contain distinct matrix-supported, sub-round lithic or crystal fragments yet little evidence of vertical gradation or sorting. Texturally simple and bearing textures indicative of energetic emplacements, these rocks are complex chemically, suggesting derivation from multiple protoliths or variable alteration of a limited range of lithologies.
P12A-02
Mineralogical Diversity in the Columbia Hills of Gusev Crater, Mars as Observed by Mini-TES
After nearly 400 sols, the Spirit rover has completed its ~82 m ascent of Husband Hill, the highest of the 7 named Columbia Hills in Gusev Crater. The Miniature Thermal Emission Spectrometer (Mini-TES) onboard Spirit measures the terrain in 167 channels from ~2000 to 340 cm-1 (~5 to 29 microns). Spectra of rocks encountered along the traverse from the West Spur to the summit of Husband Hill have revealed a remarkable diversity of rock types that was notably absent on the adjacent plains. At least 8 distinct spectral classes have been observed, each representing multiple rocks or outcrops comprising 3 clastic rock types, 3 kinds of basalt, and 2 from a possible impact melt/breccia. Additional unique spectra from single rocks or outcrops have been observed. While spectral contributions from dust in the atmosphere and accumulated dust on the Mini-TES pointing mirror have in some cases hampered efforts to fully analyze the spectra, these issues are being addressed and a generalized picture has emerged for the mineralogy of the portions of the Columbia Hills encountered by the rover. The West Spur is dominated by clastic rocks known as Clovis Class in which basalt glass appears to be the dominant component with significant contributions from sulfates, lesser goethite, and possible secondary silicates indicating some amount of aqueous alteration. Immediately upon exiting the West Spur and arriving on the lower flank of Husband Hill, Mini-TES spectra revealed a completely different mineralogy for the float rocks that we now recognize as the dominant lithology all the way up to the summit. Known as Wishstone Class, these clastic rocks have up to 50% plagioclase content of intermediate composition and less obvious indications of alteration. A third clastic rock type called Watchtower Class was observed only in two adjacent outcrops along the Cumberland Ridge. Although we have not yet completed a full deconvolution analysis of this spectral class, it is clear that primary igneous minerals are not dominant. Fifteen float rocks have been observed with the spectral character of plains basalt (Adirondack Class) scattered across the flank of Husband Hill. Nine of a second basalt type known as Backstay Class have been observed starting roughly midway up the hill. A third type of basalt called Cherry Bomb Class has now been observed in two distinct locations: an isolated float rock on the north side of Husband Hill and on the south side in a linear arrangement of float rocks that may be an eroded dike although other possibilities are being investigated.
P12A-03 INVITED
Aqueous Alteration in the Columbia Hills of Gusev Crater, Mars: Geochemical and Mineralogical Properties
Water played a major role in the formation and alteration of rocks, outcrops, and soils in the Columbia Hills. The extent of alteration ranges from moderately to extensively altered materials. Six distinct rock compositional classes have been identified in the Columbia Hills (Clovis, Wishstone, Peace, Watchtower, Backstay, and Independence) and the rover wheels uncovered one unusual soil (Paso Robles). Clovis class rocks have elemental compositions similar to Gusev plains soil but with higher Mg, Cl, and Br and lower Ca and Zn; Wishstone and Watchtower classes have high Al, Ti and P and low Cr, Ni and Br; Peace has high Mg and S and low Al, Na, and K; Backstay basalts have high Na and K compared to Adirondack basalts on the plains; Independence has high Si; and the Paso Robles soil has high S and P. Some rocks are corundum-Normative indicating that their primary compositions were changed by loss and/or gain of "rock-forming" elements. The changes in primary igneous compositions likely result from aqueous alteration under open hydrologic systems. Mineralogical compositions were determined directly from Moessbauer and thermal emission spectra and inferred from inter-element correlations. Clovis materials consist of Fe-oxides/oxyhydroxides (magnetite, nanophase ferric oxides (np-Ox), hematite, goethite), Ca-phosphates, Ca-sulfates, pyroxene-like phases and secondary aluminosilicates. Wishstone and Watchtower rocks consist of Fe-oxides/oxyhydroxides, ilmenite, Ca-phosphates, pyroxene, feldspar, Mg-sulfates, and secondary aluminosilicates. Peace outcrop consists of magnetite, np-Ox, Mg- and Ca-sulfates, pyroxene, olivine, feldspar, apatite, chlorides, and secondary aluminosilicates. Independence class rocks consist mostly of secondary aluminosilicates along with Ca- and Mg-sulfates, np-Ox, ilmenite, and Ca-phosphates. Backstay is relatively unaltered basalt with a composition near the trachybasalt-basaltic trachyandesite boundary, and is best classified as a martian equivalent of hawaiite-mugearite. The Paso Robles soil consists of Fe(III)-, Mg-, Ca- and minor-element sulfates, Ca-phosphates, hematite, halite, allophane, and amorphous Si. Columbia Hills outcrops and rocks may have formed by the alteration of basaltic rocks, volcaniclastic materials, and/or impact ejecta by solutions that were rich in acid-volatile elements. However, it is not clear whether aqueous alteration occurred by metasomatism, hydrothermal solutions associated with volcanic or impact processes, aqueous vapors from volcanic emanations (i.e., acid fog weathering), and/or by low-temperature solutions.
P12A-04
First Low-Iron Materials on Mars and Possibility of a Major Montmorillonite Component
During exploration of Columbia Hills at Gusev crater, the Spirit rover of the MER mission has discovered several separate occurrences of material with a unique elemental signature. As measured by x-ray fluorescence emission using the APXS instrument, these samples stand out for their low Fe content, accompanied by corresponding increases in Al and Si but without high concentrations of mineralogically important cations such as Ca, Mg, Na, or K. No previous martian samples, from five landed missions and numerous martian meteorites, have such low iron content. Chemical trends implicate Mg sulfates and Ca phosphates are important but minor accessory minerals. Moessbauer analysis indicates some or all Ti to be present as ilmenite. The remaining component has high Al and Si abundances in proportions within the range of classical montmorillonite compositions found at various locations on Earth, including their correspondingly low concentrations of major cations. Typically the result of weathering of basaltic ash and often associated with more arid environments, the formation of montmorillonite or its alteration-product precursor implies that significant aqueous activity to facilitate geochemical separations occurred. In addition, various of these samples contain trace element anomalies that are unique for martian materials, including enrichments in Cu, Ni, Y, Ga, Cr, and possibly Pb, Co, Sr, and Zn. Adsorption by high cation exchange minerals such as montmorillonite clays or other alteration materials (allophane, silica, imogolite) often bear similar fingerprints due to their high-area charged surfaces which confer affinities for multivalent metal ions in solution. Samples of this "Independence Class" of materials have been found at three separated sites in the upper portion of Husband Hill. The samples are disparate in form, including clods (or peds), an outcrop, and a "rock". Their lighter color and rugged morphology are a common feature. The latter may indicate susceptibility to fragmentation, with possible derivation from a deeper-lying layer of source material.
P12A-05 INVITED
Recent Results from the Mars Exploration Rover Opportunity Mission
The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference. Since leaving Endurance crater, Opportunity has traversed southward approximately 3 km over undulating plains characterized by basaltic sand, a thin surface lag of hematite-rich concretions, and subparallel eolian ripples that have increased in size toward the south. Mottled terrain to the south has now been revealed to consist largely of flat-lying expanses of sulfate-rich bedrock similar to the materials found in Endurance crater. The distribution of hematite in these southern exposures is different, however, with concretions that show a wider diversity of sizes and shapes than at Endurance. Dark 'rinds' on outcrop rock show a distinctive chemistry that is similar to the outcrop in many respects but with changes in the concentrations of some elements likely to be present as salts. Notably, Na and Cl are enhanced in rind material in an approximate 1:1 molar ratio. Some small cobbles are found to be martian rather than meteoritic in origin, with a unique composition that represents a material not yet found as bedrock.
P12A-06
One Martian Year of in Situ Chemistry by the APXS on Board the Mars Exploration Rover Opportunity at Meridiani Planum
Two in-situ instruments, the Alpha Particle X-ray Spectrometer (APXS) and the Moessbauer Spectrometer (MB), gathered geochemical and mineralogical data along the traverse of the Mars Exploration Rover Opportunity at Meridiani Planum. Eagle crater, the landing site of the rover, contains undisturbed soils that resemble those at Gusev crater; however, the Fe, Ni, and Cr contents and Fe/Si ratios of Meridiani soils are higher than those of Gusev soils. The enrichment of Fe results from an admixture of the mineral hematite as determined by MB. This mineral occurs as a finely disseminated component of the outcrop rocks as well as in mm- to several mm-sized spherules, nicknamed blueberries, which are spread at the landing site and along the several kilometers traverse to the Erebus crater. The formation of hematite is typically an indicator for aqueous activity under oxidizing conditions. Light-toned layered outcrops were discovered in Eagle crater and later in other craters, as well as along the rover's traverse. Most of these undisturbed rock surfaces have a factor of 2 to 3 higher S concentrations compared to the soils. In Eagle crater, ground rock surfaces (exposed by the Rock Abrasion Tool, or RAT) showed even higher S contents of up to 9.5 weight percent. Assuming all SO3 is bound to Mg and Ca sulfates and, according to MB data, to ferric sulfates, mainly jarosite, these rocks contain about 40 weight percent sulfates. High concentrations of Br were discovered in some soils excavated with the rover wheels and rocks ground with the RAT. The high abundances of S and Br in these rocks point to ancient occurrence of acidified water and the formation of brines, which could have been occasionally evaporated. Small quantities of the hematite-rich spherules (ca. 2 volume percent) were found in the rocks of Eagle crater. The acidic conditions during the formation of the hematitic spherules in the rocks as concretions allowed co-precipitation of Fe2O3 and NiO but no MnO. When the rover was climbing into Endurance crater, a full stratigraphic sequence was measured with APXS and MB. Large enrichments of Cl were not accompanied by Br and S. The major elements varied within small ranges except for lower layers, where Mg is depleted together with S but Si and Al are enriched. Two rocks on the plains, analyzed by APXS and MB, are related to known meteorite classes: 'Bounce Rock' is similar in chemistry and mineralogy to basaltic shergottites, a subgroup of martian meteorites, whereas 'Heat Shield Rock' with high Fe and Ni concentrations is an iron meteorite. On the rover's journey from Endurance crater to Erebus crater, light-toned rocks were encountered whose chemical compositions resemble those of the well-known rocks of Eagle and Endurance craters, indicating that the sulfur-rich deposits occur on a scale exceeding several kilometers, consistent with evidence from orbital spacecraft.
P12A-07 INVITED
Evaporites at Meridiani Planum and Implications for Surficial Processes on Mars
Results obtained from the Opportunity Rover have significantly changed the current views of surficial processes on Mars. After one martian year of exploration, it is clear that the sediments which continue to be characterized preserve an intricate record of depositional and geochemical processes at Meridiani Planum. The saline mineral assemblages identified from Opportunity data imply a distinct sulfate-rich and carbonate-poor acidic environment which produced jarosite, Mg- and Ca-sulfates, and other phases. The major processes controlling mineralogy and chemistry at Meridiani Planum were chemical weathering of basaltic materials, evaporation, sedimentary processing, and diagenesis. In addition, new data returned from the OMEGA instrument aboard Mars Express have revealed localized regions of sulfate-rich lithologies with a lack of carbonates. However, in the broader context of chemical weathering and evaporation processes at the martian surface, the saline mineral assemblages inferred from MER and OMEGA results differ considerably in character from those identified in SNC-type meteorites. For example, Nakhlite-type evaporite assemblages include carbonates such as siderite, gypsum (+ anhydrite), Mg-sulfates and halite. Here, we discuss saline mineral production at the martian surface by re-visiting the well established "chemical divide" concept, which has been used successfully to predict evaporite mineralogy and brine evolution on Earth. The chemical divide concept states that the precipitation of a salt mineral causes fractionation of chemical components in solution, depending on the ratio of the components in solution compared to that of the saline mineral. A new system of chemical divides is built for martian evaporative systems which shows that the uniqueness of evaporite mineralogy at the martian surface is controlled by at least three factors fundamental to surficial processes on Mars: (1) acidic environments controlled largely by SO$_{4}$, HCO3 and Cl input, (2) increased mobility and concentration of Fe in aqueous systems, and (3) dilute water chemistry controlled by the weathering of basalt. The chemical divide system shows that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among martian evaporite assemblages evident from MER, OMEGA, and SNC results.
P12A-08
A Volcanic Environment for Bedrock Diagenesis at Meridiani Planum, Mars
Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water. Based on the observed bedding morphology, crystal vugs, varying Br:Cl ratios, as well as high abundances of hematite and sulfate minerals, the rocks have been interpreted by the Mars Exploration Rover (MER) team as sedimentary/evaporite deposits. However, the composition of the Meridiani bedrocks indicates that the formation model advocated by the MER team is not plausible. If the sulfates were attributable to precipitation of salts from an evaporating brine the rocks would be enriched in a balancing cation (e.g., Ca, Mg, or Fe) but this is not observed. Ratios of cations including Fe, Mg, Ca, and Na to (Si + Al) in the rocks are nearly identical to the basaltic martian meteorite Shergotty, and also very similar to unweathered basaltic rocks at Meridiani and Gusev Crater. The compositional data strongly suggest that the Meridiani rocks represent typical martian basalt with a sulfur component added. Here, we propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed SO2- and H2O-bearing vapors in a solfatara-like setting. We suggest that volcanic deposition may account for the textures observed in Meridiani rocks since morphologic bedding features including sorted grains, planar bedding, low-angle cross-stratification, festoon bedding, and ripple lamination are common in base surge deposits of ash on Earth. The lath-shaped vugs may be the remains of ferrous sulfates (or mixed Fe-Mg-Ca sulfates) formed in the early stages of alteration that became unstable during oxidation. Widely varying Br:Cl ratios are also common in solfatara environments on Earth. In this scenario, most of the alteration of the rocks likely occurred at high temperatures (~100° C or higher), possibly soon after the volcaniclastic deposits were emplaced and still retained their original heat. Extended periods of time, a standing body of water, and clement conditions would not have been required for completion of the hypothesized diagenetic sequence.