One Year on Mars: MER at Gusev and Meridiani
Presiding: J R Zimbelman, Smithsonian Institution; R E Arvidson, Department of Earth and Planetary Sciences, Washington University in St. Louis
P31A-01 08:31h
Recent Results from the Mars Exploration Rover Spirit Mission
Since arriving at the Columbia Hills, the Spirit rover's primary area of geologic investigation has been the West Spur of Husband Hill. Pancam images of West Spur rocks show morphology ranging from massive to finely layered. Microscopic Imager images show the rocks to be clastic in nature, with a substantial range in grain sizes. Grains vary from rounded to angular. Mini-TES data show little variability from one rock to the next, and the best fit to the IR spectral signature of the rocks is dominated by basaltic glass. The chemistry revealed by the APXS is broadly basaltic in nature, but substantially enhanced in P, S, Cl, and Br relative to plains rocks. Moessbauer data show that olivine is absent in West Spur rocks, and pyroxene signatures are weak. Fe oxides and oxyhydroxides are present. We interpret the rocks of the West Spur to be aqueously altered basaltic materials of volcaniclastic or impact origin. Since leaving the West Spur, Spirit has explored toward the northeast, higher onto Husband Hill. Loose rocks ("float") on the north flank of the hill are dominated by another poorly sorted clastic lithology that contains olivine and that has strikingly high abundances of Ti and P. Only a few bedrock outcrops have been identified on the main body of Husband Hill. All of these examined to date consist of a coarse-grained clastic rock dominated by basaltic chemistry and cemented by sulfate salts. Grain sizes range up to several mm, and sub-cm layering is present. Moessbauer data show pyroxene, olivine, and a high abundance of magnetite in the basaltic component. APXS data are consistent with the rock being up to 20 percent magnesium sulfate salts by mass, and microscopic images show a high degree of cementation by these salts.
P31A-02 08:51h
Recent Results from the Mars Exploration Rover Opportunity Mission
After egress from Endurance crater on sol 315 the Opportunity rover traversed to the heat shield impact site identified from post-landing Mars Global Surveyor Mars Orbital Camera images. An extensive campaign was undertaken at this site to understand how well the heat shield system performed on entry and descent through the atmosphere, focusing on characterizing the charred to preserved cross section through the thermal protection system. A second purpose was to explore the "divot" where the shield made initial contact with the surface to understand the mechanical properties of the shallow subsurface and to make measurements of the materials excavated during the impact. The campaign focused on a set of traverses around the shield site, spiraling inward closer to the main shield and flank piece as the debris was inventoried and traverses could be planned to avoid roving near strewn shield materials. A 25 cm wide rock near the shield impact site was also investigated. Based on the very low spectral emissivity, coupled with high iron and nickel content and the presence of an iron-nickel alloy, it is clear that this "heat shield rock" is a relatively fresh metallic meteorite. Opportunity is traversing south to exposures of etched terrain surfaces to characterize sedimentary rocks in plains outcrops and in crater walls. The primary intent of the new traverse and measurement campaign is to determine if the aqueous environments associated with deposition and alteration of the sedimentary rocks found in Eagle and Endurance craters are characteristic of processes that happened on a regional-scale in the Meridiani Planum area. Understanding the past aqueous environments and implications for habitability of Mars will be the prime legacy of the Opportunity Mission. During traverses south Opportunity will also search for and characterize selected cobbles and boulders strewn on the plains, including additional meteorites, and make measurements that will document the surface and shallow subsurface characteristics of the unique surfaces typical of the plains of Meridiani.
P31A-03 09:11h
Albedo and Multispectral Properties of Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Pancam Imaging Systems
The Mars Exploration Rover Pancam multispectral imaging systems have been operating for over an Earth year on the martian surface, acquiring more than 48,000 high resolution 400 to 1100 nm images at hundreds of locations along more than 6 km of total rover traverse odometry as of early February 2005. Multispectral images at these wavelengths can be used to assess the geologic context and topography in Gusev crater and Meridiani Planum and to provide constraints on the mineralogy and physical properties of iron-bearing rocks and fines. The use of color information (near-UV to near-IR) has been particularly important in deciding where to make more detailed elemental and mineralogic measurements using the rovers' in situ instruments. The Pancam multispectral properties of Gusev plains rocks and soils are dominated by a strong ferric absorption edge indicating the presence of significant nanophase dust coverings or coatings on all surfaces. Soils exposed by wheel actions or rock interiors exposed by RAT abrasion have weaker (or no) ferric bands and near-IR reflectances consistent with the presence of relatively pristine ferrous phases like pyroxene or olivine. Columbia Hills materials display more geomorphologic evidence for alteration than plains materials, but their Pancam spectra are generally similar to plains materials, even in areas where in situ measurements have detected low abundances of alteration phases like hematite and goethite. The average albedo of Gusev materials is 0.25+/-0.05, varying between 0.20 for dark wind streaks and 0.30 for brighter hollows. The Pancam multispectral properties of rocks and soils at Meridiani Planum display a wider range of diversity. Dark soils exhibit near-IR spectral slopes consistent with the presence of ferrous minerals, and similar to disturbed subsurface soils at Gusev. Rarer bright soils (some of which underly darker soils) are ferric rich and spectrally similar to homogeneous global dust. Some of the ubiquitous spherules are bluer than typical soils, some are gray and consistent with the spectral properties of coarse-grained hematite at Pancam wavelengths, and some exhibit near-IR spectra indicating the presence of a different ferric oxide (possibly an oxyhydroxide or oxyhydroxysulfate). Some small dark cobbles exhibit relatively gray spectra and occasional glints, suggesting either that they are also carriers of coarse-grained hematite, have glassy coatings, or have polished surfaces. Bright outcrop rocks have a strong ferric absorption edge and weak 900 nm band that probably indicates the presence of an aeolian dust component and/or a nanophase ferric oxyhydroxide within the materials identified as sulfur- and chlorine-rich by in situ measurements. Two large dark rocks have been encountered so far in the plains. One, Bounce Rock, has a Pancam spectrum that exhibits one of the strongest "1 micron" absorption features ever measured on the surface, indicating a significant relatively pristine ferrous component, subsequently identified as pyroxene from Moessbauer and Mini-TES measurements. The other, Heat Shield Rock, has Pancam spectra consistent with a dusty Fe-rich meteorite. The average albedo of Meridiani plains materials is 0.12+/-0.01. Bright, dusty ripples and wind streaks exhibit albedos ranging from 0.19 to 0.29, and bright reddish outcrop rocks exhibit an average albedo around 0.25. This presentation will summarize the observations used to identify these different spectral classes of materials, their occurrence and distribution, and their implications for the origin and evolution of materials at both landing sites.
http://marswatch.astro.cornell.edu/pancam_instrument/
P31A-04 09:26h
Mineral Composition and Abundance of the Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Mini-TES Instruments
The miniature Thermal Emission Spectrometer (Mini-TES) has provided remote measurements of the mineralogy and thermophysical properties of outcrops, rocks, spherules, and soils surrounding the Spirit and Opportunity Rovers [1, 2]. The composition of surface materials provides insight into the origin and evolution of the martian crust and the nature of igneous and sedimentary processes. At Gusev, undisturbed soil spectra closely match MGS TES bright-regions 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 have a derived mineralogy, with uncertainties of 5-10 vol.%, of 45% pyroxene (20% high-Ca pyroxene, 25% pigeonite), 40% sodic/intermediate plagioclase, and 15% olivine (Fo35-Fo55). Aeolian drift material has a unique spectral character with higher oxide abundances than disturbed soil. Along the rover's traverse from the lander into the Columbia Hills, at least three distinct rock types have been recognized: olivine rich basalt, a volcaniclastic rock dominated by an amorphous component (possibly basaltic glass and/or shocked plagioclase), and a second volcaniclastic rock dominated by plagioclase of intermediate composition with lesser pyroxene and olivine components. 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 basaltic sands as predicted from orbital TES spectroscopy [3, 4]. Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped by TES from orbit. Light-toned outcrops of aqueous origin exposed in crater walls are composed of 20 to 40% Mg and Ca sulfates, a high-silica component that is modeled as glass/feldspar/sheet silicates (~20-30%), and hematite. The Fe-bearing sulfate, jarosite, that was identified by the Mossbauer spectrometer [5], is detected in deconvolutions of several Mini-TES outcrop spectra, but never in concentrations >5%. The dominance of Mg and Ca sulfates in Mini-TES spectra is consistent with the Alpha Particle X-ray Spectrometer (APXS) results, which show that Mg and Ca are present, and that there is significantly more S and too little Fe for the sulfates to be jarosite alone [6]. Two unique surface rocks have been identified during the rover's traverse. Bounce Rock is dominated by clinopyroxene relative to basaltic sands and is closer in inferred mineral composition to the basaltic SNC meteorites. Heat-Shield Rock has a Mini-TES spectral signature that closely resembles a typical spectrum of the sky. The initial Mini-TES interpretation that Heat-Shield Rock was a metallic object with a near reflective surface, and possibly an iron bearing meteorite, was confirmed by APXS and MB observations. 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. 1)Christensen, P.R., et al., Science, 2004. 305: p. 837-842. 2)Christensen, P.R., et al., Science, 2004. 306: p. 1733-1739. 3)Christensen, P.R., et al., J. Geophys. Res., 2000. 105: p. 9623-9642. 4)Christensen, P.R., et al., J. Geophys. Res., 2001. 106: p. 23873-23885. 5)Klingelhofer, G., et al., Science, 2004. 306: p. 1740-1745. 6)Rieder, R., et al., Science, 2004. 306: p. 1746-1749.
P31A-05 09:41h
Moessbauer Mineralogical Evidence for Water- and Sulfate-Bearing Phases at Gusev Crater and Meridiani Planum: The View After One Year
A major science goal of the Mars Exploration Rover (MER) mission is to search for mineralogical evidence of water activity in the form of water- and sulfate-bearing phases and other phases that form in aqueous environments. The Moessbauer spectrometers on the MER rovers at Gusev Crater (Spirit rover) and Meridiani Planum (Opportunity rover) have measured the relative abundances of iron with respect to both oxidation state and iron-bearing phase. The assemblage of phases identified by Moessbauer spectroscopy indicates aqueous alteration processes at both landing sites. Although the rock and soil of the Gusev Crater plains are dominated by Fe(2+) in olivine-bearing basalt, a Fe(3+)-rich component (nanophase ferric oxide, np-Ox) has significant abundance in surface soils and in the surface coatings/rinds of certain rocks (up to about 40% of total Fe) but not in rock interiors exposed by grinding (less than 6%). The mode of occurrence of np-Ox implies that it is the product of oxidative alteration of Fe(2+) silicate and oxide phases in the presence of H2O. Using both Moessbauer data and elemental concentrations from the APXS, the concentration of Fe associated with np-Ox has a positive correlation with S (Fe to S ratio of about 0.6), implying a sulfate or formation in direct proportion to sulfates. This ratio is low compared to values for the acid sulfate weathering product schwertmannite (Fe to S ratio of about 4.6 to 8) but is comparable to that for rhomboclase ((H3O)Fe(SO4)2(H2O)2. There is currently insufficient evidence, however, to refine the mineralogical identification beyond np-Ox. A possible weathering agent is volcanic emanations rich in H2O and SO2. Depending on how np-Ox is modeled, the basaltic soils in the Gusev plains have the equivalent of up to approx. about 1 wt% H2O. Generally, rocks in the Columbia Hills are significantly more altered than those in the Gusev plains, with a higher proportion of Fe(3+)-bearing phases compared to Fe(2+)-bearing phases. The oxyhydroxide mineral goethite (FeOOH) has been identified in at least one rock, imparting the equivalent of about 1 wt% H2O. Because its structure contains the hydroxide anion, the Moessbauer detection of the hydroxide sulfate jarosite (K,Na)Fe3(SO4)2(OH)6 (about 28% of total Fe) in outcrops at Meridiani Planum is direct mineralogical evidence for oxidative aqueous alteration of basaltic precursors under acid sulfate conditions. We calculate that average outcrop has the equivalent of about 1-2 wt% H2O contained in the jarosite. Another Fe(3+) doublet component is also present in outcrop materials, and it can be associated with the equivalent of an additional about 0.5 wt% H2O. Modeling using APXS elemental abundances and hydrated phases suggests total H2O contents of about 10 wt% are possible. Acid-sulfate conditions could arise through different pathways, including oxidation of sulfide minerals by aqueous fluids and volcanic emanations with a high SO2/H2O ratio. Hematite is pervasive and is found within the outcrop matrix (about 35% of total Fe) and in spherules (interpreted as concretions) dispersed throughout the outcrop. Presumably, the process of spherule formation involved aqueous transport processes. As at Gusev Crater, basaltic surface soils at Meridiani Planum have significant concentrations of nanophase ferric oxide and the equivalent of up to about 1 wt% H2O.