P24A-01 16:00h
Stratification, Sediment Transport, and the Early Wet Surface of Meridiani Planum
Several stratification styles are present in the outcrops investigated by the Opportunity rover at Meridiani Planum. These include planar lamination, low-angle cross-stratification, cross-bedding, ripple cross-lamination, and crinkly and undulatory lamination. Planar lamination and low angle stratification are well-developed in several locations, particularly at Slickrock and Shoemakers Patio in Eagle crater and at Karatepe in Endurance crater. MI images at Slickrock show "pinstripe" lamination where individual laminae can be single-grain thick layers, suggestive of eolian sedimentation. At Shoemaker's Patio, a single cross-bed set with thickness of 5-7 cm is preserved. At Burn's Cliff, in Endurance crater, a single cross-bed set of up to several meters thickness is preserved. In contrast, ripple cross-lamination with festoon geometry is present at several locations within Eagle crater including Last Chance, the Dells, and in an isolated rock (Scoop) on the southwest rim of the crater. Ripple cross-laminae sets are 0.8 to 1.7 cm thick. In the case of Scoop, the sets are possibly climbing. Grain size appears to range from 0.1 to 0.8 mm in diameter. The cross-bedding preserved at Eagle crater suggests both eolian and subaqeous environments. The set at Shoemaker's patio represents deposition from either subaerial or subaqeous dunes. The thicker, meter-scale set at Burns Cliff is most consistent with transport by eolian dunes. In contrast, the small-scale festoon cross-laminae at Eagle crater are indicative of sediment transport in subaqueous ripples. The reconstructed size of former bedforms is inferred to be only a few cm, and therefore inconsistent with eolian dunes which commonly have significantly larger minimum amplitudes. Yet the cross-lamination is significantly larger and of differing geometry from the climbing translatent cross-strata produced by wind ripples. Thus, the size of bedforms that produced the cm-scale cross-laminae preserved at Eagle crater occupy the spectral gap in the size distribution of eolian dunes and ripples. A further clue to the likely subaqueous origin for Eagle cross-laminae is provided by their festoon geometry which requires that the reconstructed ripples have three-dimensional geometry defined by highly sinuous crestlines. In terrestrial settings such bedforms are known only to develop in subaqueous, subcritical flows, with velocities of less than one meter per second. On Mars, the initiation of subaqueous sediment movement is expected to occur at velocities lower than on earth, by a factor of 1.34. With respect to length scales, such as bedform dimensions, the scaling factor turns out to have the opposite sense so that, given dynamically similar flows, ripples on Mars might be larger by a factor of 1.34. Neither of these rescaling effects influences the interpretation that cm-scale, festoon cross-lamination at Eagle crater most probably requires sediment transport in subaqueous, low-velocity flows across the Meridiani plains. The assemblage of primary facies recorded at Eagle crater is most consistent with an environment characterized by episodic inundation by surface water to shallow depths, followed by evaporation, and exposure and desiccation. Terrestrial analogs for such a suite of facies and surface processes include small interdune depressions, playa lakes, and sabkhas adjacent to marginal seaways. All rover observations indicate that the evaporite-bearing cross-bedded unit extends at least on the order of 1 km though MOC and THEMIS data suggest it extends much further.
P24A-02 16:15h
The Opportunity Landing Site in Meridiani Planum: Lacustrine or Marine?
The landing sites for Spirit and Opportunity were selected to "follow the water" to the best of our abilities, given the knowledge gained from previous orbiter instruments and the limitations of the landing system to deliver the rovers safely to the surface. Even with these limitations, it was still possible to select landing sites with high potential to deliver on this requirement. Prior to the Opportunity landing, the regional, hematite-bearing deposit discovered by the Thermal Emission Spectrometer on Mars Global Surveyor was interpreted either as a subaqueous precipitate or a volcanic ash or other deposit subjected to hydrothermal alteration by groundwater. The landing site fortuitously placed the rover in the 20 meter diameter Eagle Crater with immediate access to sulfate-rich sediments. This outcrop exhibits clearly recognizable subaqueous depositional structures such as water-formed current ripples. To date, Opportunity has found this sedimentary deposit to be remarkably uniform in composition and stratigraphic expression over the 750 meters traversed between Eagle Crater and Endurance Crater. The evaporite composition and finely laminated stratigraphy over this region implies low-energy subaqueous deposition in a large body of shallow water. The regional extent and topography of the Meridiani Planum deposit can be interpreted in at least two ways. First, the long-wavelength hummocky topography of the deposit suggests a somewhat eroded sequence laid down in either a very large lake or playa environment, for which the northwest confining topography is now absent. Second, the hummocky form of the regional deposit could be primary, indicating evaporite platform deposition at the margin of a northern plains ocean.
P24A-03 16:30h
VNIR spectral features observed by the Mars Exploration Rover Opportunity in hematite-bearing materials at Meridiani Planum
The Mars Exploration Rover Opportunity was sent to Meridiani Planum based largely on MGS TES spectroscopic evidence of a large surface exposure of coarse grained gray hematite. The presence of hematite at Meridiani Planum has been confirmed through thermal infrared spectroscopy by the rover's Mini-TES instrument and by in-situ measurements by its Moessbauer (MB) spectrometer. Several types of hematite, as expressed by differences in MB spectral parameters, have been associated with various rocks and soils examined in Eagle crater and on the surrounding plains. The host materials include the small spherules (informally known as "blueberries") littering the floor of Eagle crater and the plains of Meridiani, the outcrop rock itself, specific types of soils, and two measurements on unique rocks in the Shoemaker's Patio area of Eagle crater. At the visible to near infrared (VNIR) wavelengths covered by the rover's multispectral Panoramic camera (Pancam), gray hematite is spectrally neutral. However, multispectral observations by Pancam of some of these hematite-bearing materials show discernable spectral features. Specifically, portions of the outcrop visible in the walls of Eagle crater display a strong 535 nm absorption feature. This feature resembles a similar feature in laboratory spectra of red hematite, but the characteristic 860 nm absorption of red hematite is either absent or is instead replaced by a longer wavelength absorption centered on Pancam's 900 nm channel. The blueberries display a deep and broad absorption centered on 900 nm and as well as an increase in reflectance in the 1009 nm band. The shape of the absorption feature in the blueberries is consistent with that seen in red hematite, but again the band minimum is displaced to a longer wavelength than would be expected for red hematite. The blueberries also lack the prominent absorption at the shortest wavelengths that would be expected of red hematite. The unique hematite-bearing (or coated) rocks at Shoemaker's Patio lack the very strong 535 nm band depth of other portions of the outcrop but still have a stronger 535 nm feature than most of the outcrop. Interestingly, VNIR spectra more consistent with that expected for red hematite have been found in cuttings released by grinding into outcrop by the rover's Rock Abrasion Tool. The cause of the observed spectral features in the portions of outcrop with strong 535 nm band depths and of the reddish rocks in the Shoemaker's Patio area is believed to be attributable either to red hematite mixed with other Fe3+ - bearing phases (such as jarosite and/or schwertmannite) or, at the longer wavelengths, with Fe2+ - bearing phases (such as pyroxenes). Determination of the nature of these iron-bearing materials will further elucidate the geologic, aqueous and diagenetic history of the rocks at Meridiani Planum.
P24A-04 16:45h
Mini-TES Derived Chemical Abundances at Gusev Crater and Meridiani Planum: Implications for Global Surface Compositions
The Miniature Thermal Emission Spectrometer (Mini-TES) experiments provide remote measurements of mineral abundances and compositions at the Spirit and Opportunity landing sites of Gusev Crater and Meridiani Planum. Olivine basaltic sands and rocks are identified at Gusev Crater, along with variable amounts of fine-grained dust and other possible coatings. Olivine basaltic sands and coarse crystalline hematite, a clinopyroxene-rich basaltic rock, fine-grained dust, and outcrops composed of sulfates, hematite, and sheet silicates/glass are identified at Meridiani Planum. The occurrence of olivine basalt was predicted at both landing sites by observations from the orbiting Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and Mini-TES spectra of basaltic sands are very similar to a global average TES basalt spectrum in the overall spectral shape and positions of spectral features. Ground-truth observations of the mineralogy and chemistry of olivine basalt from the Mini-TES, Alpha Particle X-ray Spectrometer (APXS), and Mossbauer Spectrometer (MB) are significant because of the extensive distribution and high-abundance of olivine basalt on Mars and the inferred petrogenesis and evolution of a basaltic crust. In this study, we calculate bulk chemical oxide abundances of surface materials from Mini-TES derived mineral abundances and compositions and compare results to APXS and MB observations to examine the accuracy of thermal emission derived chemistry. We also examine methods to constrain APXS derived normative minerals using multivariate regression with Mini-TES and MB mineral abundances and compositions. We focus on comparisons of Mini-TES and TES derived chemistries of basalt to determine the limits to which we can constrain bulk compositions and the depth and degree of partial melting of their source regions. Local ground-truth observations of the bulk mineralogy and chemistry of a global basaltic unit provide insight to the degree of differentiation of the crust and mantle.
P24A-05 17:00h
Formation of Jarosite-bearing Deposits Through Aqueous Oxidation of Pyrite at the Meridiani Planum, Mars
The discovery of sulfate-rich layered deposits with hematite spherules at the landing site of the Opportunity rover is consistent with mineral deposition in an aqueous environment. We evaluated conditions responsible for the formation of the jarosite-gypsum-goethite assemblage in an ancient martian lake with speciation and reaction-path models that include concentrations and activities of solutes, and amounts of precipitated minerals. Variations in the ranges of water to rock (W/R) ratio, pH, K content, and oxygen fugacity were explored in models of aqueous reactions involving jarosite, jarosite-gypsum-goethite assemblages, pyrite, and compositions of martian meteorites. Our results show that formation of jarosite requires the pH to be about 1-3, depending on the W/R ratio. Lower W/R ratios make jarosite stable over a broader pH range. Goethite forms together with jarosite, except at very low pH, and the jarosite/goethite ratio decreases as the pH and the W/R ratio increase. Increases in the W/R ratio also lead to dissolution of gypsum, while raising the pH favors its precipitation. Formation of goethite from decomposing jarosite and dissolution of gypsum (suggested from microscopic images) is consistent with an increased W/R ratio at the end of the aqueous reactions. Dissolution of gypsum caused by a decrease in the pH should cause dissolution of goethite and precipitation of jarosite, and is less consistent with observations of hematite spherules. Neither volcanic emanations nor aqueous weathering of igneous rocks can account for low-pH conditions in a surface water reservoir several hundreds of km across. However, the jarosite-gypsum-goethite assemblage could have precipitated from acidic solutions formed through near-surface oxidation of pyrite-rich deposits. By analogy to Earth, such deposits may have formed through regional hydrothermal activity and/or biologic sulfate reduction in organic-rich sediments. Our hypothesis is that regional heating of the ice-bearing crust in the Meridiani Planum caused a release of subsurface waters, formation of pyrite-rich deposits, their oxidation by atmospheric oxygen, and precipitation of sulfates and goethite followed by dehydroxylation of goethite to hematite.
P24A-06 17:15h
Coordinated Mars Exploration Rover and Mars Express OMEGA Observations over Meridiani Planum
The OMEGA hyperspectral imager (0.35 to 5.1 micrometers) covered the hematite-bearing plains and underlying etched terrains of Meridiani Planum during orbits 171, 314, 430, 485, 518, and 529, with spatial resolutions ranging from several hundred meters to approximately a kilometer. In addition, the Opportunity rover acquired Pancam and Mini-TES observations of the surface and atmosphere during the times when OMEGA acquired its data. Using a radiative transfer modeling approach (DISORT) that combines surface and orbital observations we extracted surface reflectance spectra from OMEGA for the approximately 864,000 square kilometers surrounding the Opportunity site. OMEGA spectra show significant variation within the hematite-bearing plains from the northwest to the southeast, in part associated with an increasingly deeper and asymmetrical band centered at 1 micrometer, assigned to ferrous silicates. The etched terrains that underlie the hematite bearing plains show significant spectral variability that is associated with stratigraphic position within the 300 m thick stack of layered materials. Significant features include a prominent 1.92 micrometer absorption in some deposits (associated with hydrated phases and/or molecular water) and absorptions in the 2.2 to 2.5 micrometer region associated with cation-hydroxl vibrations. Finally, the 3 micrometer water band also shows high absorption values for the etched terrain materials (as compared to the underlying cratered terrain), consistent with an abundance of hydrated phases and/or molecular in the layered deposits. OMEGA data, calibrated using ground truth observations from Opportunity, will allow us to extend to rover-based observations to regional scales and thus provide the overall geologic context and environments of deposition for the sedimentary sections explored by the rover since January 2004.
P24A-07 17:30h
Global Spectral and Compositional Diversity of Mars: A Test of CRISM Global Mapping with Mars Express OMEGA Data
A new chapter in understanding the surface composition of Mars has begun with OMEGA, a visible-near infrared imaging spectrometer on the European Space Agency's Mars Express spacecraft. OMEGA acquires reflected solar spectra of Mars between 0.35 and 5.1 $\mu$m in 352 contiguous spectral channels. Thermal emission is also measured for wavelengths longer than $\sim$3.5 $\mu$m. Spatial resolution varies between 300 m and 4.8 km depending on pericenter altitude. Over its first 7 months of operation OMEGA has sampled all major geologic and geographic areas and is approaching complete coverage of the northern hemisphere of Mars. OMEGA data have increased our understanding of the types and distributions of the mafic minerals olivine and pyroxene, and have revealed large deposits of hydrated minerals, including sulfates and hydrated silicates. In August 2005, the CRISM spectrometer will be launched on NASA's Mars Reconnaissance Orbiter. CRISM covers a similar wavelength range to OMEGA, but will sample down to 18 m/pixel spatial resolution. CRISM will cover up to 5% of the surface at full spectral and spatial resolution, and 100% of the surface in 60 spectral bands and 200 m/pixel spatial resolution. In anticipation of CRISM data, we have been analyzing OMEGA data to test the CRISM strategy of evaluating information obtained by mapping spectral parameters, which are derived from the multispectral bands. OMEGA spectra are sampled to approximate CRISM multispectral band passes. For parameters requiring atmospheric correction, the data are only corrected for atmospheric transmission. A total of 36 parameters related to atmospheric and surface properties have been calculated from the multispectral bands using OMEGA data. Detailed analysis of these parameters reveals that much of the spectral diversity of Mars is captured using this multispectral approach. However, several of the parameters do not show relevant variations on Mars as anticipated, while variations observed at the full OMEGA spectral resolution are not captured by any of the current parameters. We are refining the choice of spectral parameters based on this work and will present global mapping of spectral parameters related to mafic minerals (pyroxene, olivine), sulfates, and hydrated minerals. The current suite of parameters highlight regional sulfate deposits in Terra Meridiani, Aram Chaos, and northwestern Arabia Terra, as well as numerous small-scale sulfate deposits in Valles Marineris. Mafic minerals are well discriminated by the parameters, showing pyroxene-rich volcanic rocks in expected regions such as Syrtis Major, but also revealing outcrops of pyroxene-rich bedrock in crater rims. Olivine-rich regions are observed in north-eastern Syrtis Major as well as on the floors of many craters in the southern hemisphere. These results demonstrate that the CRISM strategy for multispectral mapping will achieve its goals for a) global mapping of the spectral diversity of Mars, b) provide critical inputs for targeting high spectral and spatial resolution observations, and c) provide high value products for integrated global science investigations.
P24A-08 INVITED 17:45h
A View from the Cheap Seats: MER and its Implications for Future Mars Exploration
The spectacular success of the Mars Exploration Rover mission is reflected by the exciting scientific discoveries that Spirit and Opportunity have made concerning habitability potential and geologic history of two sites on Mars. The apparent discovery of strong evidence for the presence of liquid water at the Opportunity site is of historic importance (although at the time of this writing we must note that there is not yet any peer-reviewed publication of this result). But perhaps even more importantly, the success of MER provides key validation of a strategy and approach to Mars exploration. Here, we highlight some of the strategic successes, as well as lessons learned for refining our approach to Mars exploration, and for future exploration endeavors across the solar system and beyond. Almost more than imaginable, MER has validated both the "follow the water" strategy and the "seek-in situ-sample" approach of the Mars Exploration Program developed by a broad, inclusive group of Mars explorers. Seeking interesting, water-relevant sites with remote observations from orbit resulted in one clear success at Meridiani, and in perhaps what is most optimistically termed "delayed success" at Gusev. Three lessons from this are clear. First, it's always better to fly two spacecraft! Second, significant mobility enables greatly enhanced science return. Third, mineralogical evidence appears more reliable than geomorphic evidence if the goal is to seek preserved evidence of aqueous processes (Pathfinder results also support this observation), although the combination of mineralogical information with high-resolution imagery is almost certainly the best way to optimize for success. The 2005 Mars Reconnaissance Orbiter's spectral mapping and imaging capability will thus be critical to future landing site selection. And yet, it is important to note that in spite of mineralogical evidence pointing to Meridiani as a site of "aqueous interest", and the detailed regional geological observations leading to multiple hypotheses for the formation environment of this site, virtually all hypotheses were incorrect or of insufficient detail to predict the extraordinary discoveries in the sedimentary strata at Meridiani. In other words, "in situ" exploration resulted in significant revision of ALL previously proposed hypotheses, which validates the strategy of performing in situ exploration. It is highly likely that "sampling," returning samples to Earth-based labs from sites that have been previously explored in situ, will result in a similar revolution in thinking about the history of a site. Thus, sample return missions should remain a high priority in the exploration strategy. Finally, the mission success of MER is the direct result of the hard work and dedication of one of the most well-integrated science-engineering teams in space exploration history. The value of team integration to maximizing the success of MER is perhaps the most enduring legacy of the mission to the exploration community. As we move forward returning humans to the Moon, then on to Mars and beyond, we should continue to strive for the integration of scientists and engineers into unified exploration teams.