P13B-01 INVITED 13:40h
Aqueous Processes on Mars: Results From the Mars Exploration Rover Mission
The Mars Exploration Rovers Spirit and Opportunity have both yielded evidence for aqueous processes at their landing sites. In Gusev crater, Spirit found only trace evidence for the action of water on the basalt-covered plains of the crater floor. This water action left thin salt-rich deposits on the surfaces of rocks and in fractures within rocks, and what appear to be magnesium sulfate salt concentrations in soils. The older rocks in the Columbia Hills, however, show evidence for much more substantial aqueous alteration. At Meridiani Planum, Opportunity has found layered sedimentary rocks that we interpret to be "dirty" evaporites. Environmental conditions that they record include episodic inundation by shallow surface water, evaporation and desiccation. After deposition, these rocks underwent a complex diagenetic history that resulted in recrystallization, vug formation, and growth of hematite-rich concretions. A stratigraphic section obtained within Endurance crater shows significant variation of rock chemistry and texture as a function of depth, indicating changes in depositional and/or diagenetic processes with time.
P13B-02 14:00h
High-Resolution Textures of Soils and Rocks at Gusev Crater and Meridiani Planum From the Mars Exploration Rover Microscopic Imagers
The Microscopic Imagers on the Spirit and Opportunity rovers have returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Gusev and Meridiani landing sites. The Microscopic Imager (MI) is a fixed-focus camera mounted on the robotic arm of each Mars Exploration Rover (MER). The MI was designed to function like a geologist's hand lens, acquiring images at a scale of 31 microns/pixel over a broad spectral range (400-700 nm). The MI provides critical documentation of the constitution and texture of targets analyzed by the other MER in-situ instruments. The MI on the Spirit rover found weakly bound agglomerates of dust in the soil near the Columbia Memorial Station. Bedforms have coarser particles at their crests and finer grains in the troughs, like eolian ripples on Earth. Some of the brushed or abraded rock surfaces show igneous textures and evidence for alteration rinds, coatings, and veins consistent with secondary mineralization. The textures of rocks on the floor of Gusev crater are consistent with a volcanic origin and subsequent alteration and/or weathering by impact events, wind and possibly water. MI observations in the ''Columbia Hills'' are consistent with interpretations, based on syntheses of various MER data, that the rocks there are more altered than those on the floor of Gusev. The MI on the Opportunity rover has returned images of the Meridiani landing site that include evidence for both surface and ground water activity in Mars' ancient past. Soil particles imaged by the MI show constituents typical of windblown materials. The uppermost millimeter of some soils is weakly cemented, probably by salts precipitated from evaporating brines. Rock outcrops are laminated on a millimeter scale; image mosaics of cross-stratification suggest that some sediments were deposited by flowing water. Vugs in some outcrop faces are probably molds formed by dissolution of relatively soluble minerals during diagenesis. MI data support the hypothesis that hematite-rich spherules (typically 3-5 mm diameter) observed in outcrops and soils also formed diagenetically as concretions.
P13B-03 INVITED 14:15h
Chemical Composition of Rocks and Soils at Gusev and Meridiani Landing Sites of the Mars Exploration Rovers: APXS Results and Implications.
The Alpha-Particle X-ray Spectrometers (APXS) are part of the instrument suites of both Mars Exploration Rovers, Spirit and Opportunity, which landed on Mars at Gusev crater and Meridiani Planum in the beginning of 2004. They are in-situ instruments for the determination of major and minor elements of soils, rocks, and outcrops. Soils at these landing sites are chemically characterized by high sulfur and chlorine contents, similar to soils at previous landing sites. Abundances of major and minor elements of all soils are very similar, strongly supporting the concept of global distribution and thorough mixing of soils on Mars. Locally, minor deviations from average soil composition are observed. These are attributed to the addition of local components to "global soil". In one trench at Gusev crater magnesium and sulfur concentrations increase with depth and give direct evidence for magnesium sulfate, possibly formed by weathering of olivine under acidic conditions, and local redistribution processes. Rocks at Gusev crater plains are primitive magnesium-rich basaltic rocks with normative olivine. They are coated to varying degrees with soil/dust and alteration rinds. Highly mobile elements are enriched in these outer layers. Outcrop materials at the base of the Colombia Hill site are possibly basaltic or volcaniclastic rocks. They are chemically highly altered as reflected by very high concentrations of water soluble elements (S, Cl, and Br), observed even after removal of a more than 8 mm thick surface layer. Apparently, the alteration extends to much greater depth. Outcrops at the Opportunity landing site were analyzed in Eagle crater and Endurance crater. These are light-toned sedimentary rocks of siliciclastic materials with up to 40 weight percent of sulfates. Based on mass balance calculations, in addition to Mg-sulfate and jarosite, other sulfates, e.g., Ca- and Al-bearing sulfates must be present. Outcrop rocks in Eagle crater are enriched in bromine relative to chlorine to varying degrees. Rocks in Endurance crater are stratigraphically layered sediments, possibly deposited under aqueous and aeolian conditions. The silicate to sulfate ratio of these units increases with depth. Vein fillings have high bromine concentrations. The chemical composition of rocks, soils and outcrops analyzed at both landing sites provides clear evidence for water-rock interaction and the presence of water over an extended period of time.
P13B-04 14:30h
Hematite at Meridiani Planum and Gusev Crater as identified by the Moessbauer Spectrometer MIMOS II
The Moessbauer (MB) spectrometers on the MER rovers Opportunity and Spirit, which landed on Mars in January 2004, have identified the iron-containing mineral hematite (a-Fe2O3) at both landing sites. On Earth, hematite can occur either by itself or with other iron oxides as massive deposits, in veins , and as particles dispersed through a silicate or other matrix material. Hematite particle size can range from nanophase (superparamagnetic) to multidomain and particle shape ranges from equant to acicular to platy. Fine-grained hematite is red in color and is a pigmenting agent. Coarse-grained hematite can be spectrally neutral (gray) at visible wavelengths. Substitutional impurities, particularly Al, are common in hematite. Chemically pure, coarse-grained, and well-crystalline hematite has a magnetic transition (the Morin transition) at ~260 K. Moessbauer spectra, recorded as a function of temperature, provide a way to characterize Martian hematite with respect to some of the physical and chemical characteristics. At Meridiani Planum besides the iron-sulfate mineral jarosite also the Fe-oxide hematite has been identified by the Moessbauer spectrometer, mainly in three distinct types of reservoir: - outcrop matrix material dominated by the mineral jarosite in the MB spectrum, certain basaltic soils, and mm-sized spherules dubbed blueberries. Moessbauer spectra of each reservoir yield a distinct set of hyperfine parameters for hematite, suggesting different degrees of crystallinity and particle size. The hematite found by MB instrument MIMOS II in the outcrop material shows the Morin transition at relatively high temperatures (ca. 250 K) which is an indication of pure and well-crystallized hematite. The source of the hematite in the `Blueberries' as identified by Moessbauer spectroscopy, and also by MiniTES, is not known. These spherules, covering nearly the whole landing site area (Eagle crater, plains, Endurance crater), may be concretions formed in the outcrop involving aqueous processes. They are dispersed throughout the hematite containing jarositic outcrop material. According to Moessbauer analysis the dominating iron-bearing mineral in the spherules is hematite, and jarosite not been detected so far in significant amounts. But there are some places with hematite showing characteristics different from the hematite described above: (1) the soil at B023-HematiteSlope-Hema2 has some blueberries, but the MB did not seem to intercept any. The temperature dependence of its MB parameteres are different from those of blueberries. (2) B049-RasberryNewton-Filling shows a unique Hm MB signature, as well as (3) B051-RealSharksTooth-Enamel1. At Gusev Crater no widespread occurrence of hematite has been detected on the Gusev plains, although minor amounts of hematite were detected in an alteration rind on the rock Mazatzal at the rim of Bonneville crater. At the Columbia Hills, however, the MB instrument did find hematite in significant amounts in highly altered rocks, showing at the same time a significant decrease in the amount of olivine compared to typical basaltic rocks at Gusev crater.
P13B-05 14:45h
Visible to Near-IR Spectral Properties of Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Pancams
Mars Exploration Rover Pancam multispectral images of a wide range of rock and soil targets were acquired using as many as 11 narrowband filters with central wavelengths from 432 to 1009 nm. These images were acquired in order to characterize the overall color properties of materials at the Gusev and Meridiani landing sites, to constrain the iron-bearing mineralogy of these materials based on the nature of crystalline and/or nanophase $Fe^{3+}$ (ferric) absorptions in the visible to near-IR in altered materials and $Fe^{2+}$ (ferrous) absorptions in the near-IR from less-altered volcanic materials, and to aid in the selection of specific targets for detailed chemical and mineralogic investigations using the rovers' arm instruments. At Gusev during the plains traverse to the Columbia Hills, most bright soil and rock surfaces appear covered or coated by optically thick fine-grained ferric-iron rich dust. Spectra of some darker rock surfaces, including regions that were brushed or drilled by the RAT, show near-IR signatures consistent with ferrous silicates like pyroxene or olivine. Since Spirit's arrival in the Columbia Hills, Pancam images have revealed evidence for some intrinsically less dusty or less altered rock surfaces, as well as isolated occurrences of more crystalline ferric signatures that may be indicative of enhanced weathering or alteration relative to materials in the plains. At Meridiani during the exploration of Eagle crater and during the traverse to Endurance crater, a wide range of visible to near-IR spectral properties was identified among small rock clasts and spherules. For example, spectra of dark sand, some dark rock clasts, and one larger dark rock found on the plains show spectra consistent with the presence of pyroxene or olivine. Spectra of dark spherules are consistent with the presence of crystalline ferric oxides/oxyhydroxides. Bright materials, including windblown dust, bright spherules, and the sulfur-rich laminated outcrop deposits, have Pancam spectra that indicate the presence of poorly-crystalline or nanophase ferric-iron rich minerals. Within Endurance crater, Pancam spectra reveal evidence for outcrop and spherule materials similar to those found in Eagle and during the plains traverse, plus additional color/spectral units associated with cm-scale layering within the crater walls, a morphologically-different class of spherules, isolated blocks of wall material and/or crater ejecta, and coatings and fractures/veins within rocks in the lower part of the stratigraphic sequence investigated by Opportunity.
P13B-06 15:00h
Mineral Composition and Abundance of the Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Mini-TES Instruments: Implications for Aqueous Processes
The Mini-TES instruments on Spirit and Opportunity have studied the mineral composition and abundance of the outcrops, rocks, spherules, and soils at Gusev Crater and the Meridiani Plains. At Gusev undisturbed soil spectra closely match MGS TES bright-region dust spectra, with features interpreted to be due to minor carbonates and bound water. Dark-toned soils observed on rover-disturbed surfaces are likely derived from rocks and has a derived mineralogy, with uncertainties of 5-10 percent, of 45 percent pyroxene (20 percent Ca-rich pyroxene, 25 percent pigeonite), 40 percent sodic/intermediate plagioclase, and 15 percent olivine (Fo45 ñ~10). Rocks have complex spectra that are influenced by coatings and atmospheric downwelling radiance, as these high-thermal-inertia rocks are typically colder during the day than the atmosphere. Their Mini-TES spectra are consistent with olivine-rich basalts with varying degrees of dust and other coatings. Aeolian drift material has a unique spectral character with higher oxide abundances than disturbed soil. One (or possibly two) spectrally distinct coatings are observed on rocks, a possible indicator of the interaction of water, rock, and airfall dust. At Meridiani, the Mini-TES has identified coarse crystalline hematite and olivine basalt sands as predicted from orbital TES spectroscopy. Light-toned outcrops of aqueous origin exposed in crater walls are composed of 20 to 40 percent Mg and Ca sulfates, a high-silica component that is modeled as glass/feldspar/sheet silicates (~20-30 percent), and hematite. The Fe sulfate jarosite is not reliably identified in Mini-TES spectra. The mm-sized spherules appear from analysis of Mini-TES spectra to be dominated by hematite, with no other components detected. Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped by TES from orbit. Bounce Rock is dominated by clinopyroxene and is closer in inferred mineral composition to the basaltic SNC meteorites. Bright wind streak material downwind of Eagle crater closely matches the IR spectrum of global dust, suggesting that these materials were deposited as fallout from global dust storms as predicted by Veverka and colleagues. The occurrence of waterlain rocks covered by olivine, pyroxene, and feldspar in basaltic sands suggests a significant change from an aqueous environment at the time the rocks were deposited to one dominated by physical weathering. The occurrence of basalts and olivine basalts throughout much of the equatorial and mid-latitude regions suggests that chemical weathering may have been a relatively minor process, at least in low- to mid-latitudes, throughout much of martian history. Thus, the presence of a body of water may represent a relatively brief, localized phenomenon early in Mars history.
P13B-07 15:15h
Rock Rinds and Coatings: Indicators of Aqueous Processes
The Rock Abrasion Tools (RAT) onboard the Mars Exploration Rovers enable chemical and mineralogical comparisons of undisturbed surfaces, brushed surfaces, and interiors of rocks at the Gusev Crater and Meridiani Planum landing sites. The variability of Alpha-Particle X-ray Spectrometer (APXS) and M\"{o}ssbauer Spectrometer data with depth into rocks is an effective indicator of the extent of aqueous alteration. On the plains within Gusev crater, three rocks were brushed and abraded by the RAT. The interiors of these rocks ($\sim$2 to 5 mm from the surface) are nearly indistinguishable in elemental chemistry and iron mineralogy, indicating a common source region. As evidenced by the consistency of brushed and abraded measurements, the relatively small interior ferric-to-total iron ratio (0.10 to 0.20), and the low bromine content ($ < $50 PPM), two of these rocks are minimally altered basalts with a thin ($ < $500 microns) surface coating of dust/soil. The third rock, referred to as ``Mazatzal,'' has a brushed surface with moderate amounts of Br ($ > $100 PPM), ferric-to-total iron ratios of $\sim$0.4 and greater, and indications that interior magnetite may have weathered into other oxide phases such as hematite. ``Mazatzal'' is the most extensively weathered of the abraded Gusev plains rocks. The rocks abraded and analyzed in the Columbia Hills, however, are far more weathered than even ``Mazatzal.'' A rock referred to as ``Clovis'' was abraded to a depth of 8.9 mm, and its interior remains dominated by ferric iron. The surface has minimal differences in elemental chemistry relative to the interior, suggesting an alteration thickness in excess of the RAT depth. The high surface concentration of bromine ($\sim$1000 PPM) is indicative of aqueous interaction, and its variability over centimeter-scales suggests redistribution with minor amounts of water. On the basis of elemental differences observed after application of the RAT and the ferrous iron signature in the M\"{o}ssbauer data, the other fully abraded rocks in the Columbia Hills exhibit lower levels of alteration in comparison to ``Clovis.'' At the Opportunity landing site, the RAT enabled interior analyses of outcrop material as well as rock coatings. In one example of a coating, the surface bromine concentration was approximately 800 PPM and decreased as the layer was brushed and abraded. Associated decreases of potassium, sodium, and chlorine in the abraded target suggest a redistribution of salts through aqueous transport at some point after emplacement of the underlying rock.
P13B-08 15:30h
Climate Change on Mars: From Wet in the Noachian at Meridiani to Dry and Desiccating in the Hesperian/Amazonian Plains of Gusev
Sedimentary dirty evaporites in Meridiani Planum were deposited in salt-water playas or sabkhas in the Noachian, roughly coeval with a variety of geomorphic indicators (valley networks, degraded craters and highly eroded terrain) of a possible early warmer and wetter environment. In contrast, the cratered plains of Gusev that Spirit has traversed (exclusive of the Columbia Hills) have been dominated by impact and eolian processes and a gradation history that argues for a dry and desiccating environment since the Late Hesperian. The Late Hesperian/Early Amazonian cratered plains of Gusev crater are generally low relief moderately rocky plains dominated by hollows, which appear to be craters filled with soil. Rocks are generally angular basalt fragments in an unconsolidated $ > $10 m thick regolith of likely impact origin. Eolian bedforms appear to be presently inactive ripples and no active sand dunes have been identified. Moderate localized surface deflation of 5 to 60 cm is indicated by two-toned rocks with a redder patination along the base, ventifacts that originate from a common horizon above the soil (suggesting that the lower part of the rock was shielded), rocks that appear to be perched on top of other rocks, and some undercut rocks, in which the soil has been removed from their bases. The observed gradation and deflation of ejected fines and deposition in craters to form hollows thus provides a measure of the rate of erosion (average vertical removal of material per unit time), which yields extremely slow erosion rates of order 0.1 nm/yr comparable to those estimated at the Mars Pathfinder ($\sim$0.01 nm/yr) and Viking Lander 1 ($\sim$1 nm/yr) sites and argues that a dry and desiccating environment similar to today's has been active throughout the Hesperian and Amazonian (since $\sim$3.7 Ga). By comparison, erosion rates estimated from changes in Noachian age crater distributions and shapes are 3-5 orders of magnitude higher and comparable to slow denudation rates on the Earth ($ > $5 micron/yr) that are dominated by liquid water. The erosion rates from Gusev as well as those from Viking 1 and Pathfinder strongly limit this warmer and wetter period (possibly recorded in the Meridiani evaporites) to the Noachian, pre-3.7 Ga and a dry and desiccating climate since.