P23B-01 INVITED 13:40h
Geochemical and Mineralogical Indicators for Aqueous Processes in Gusev Crater and on Meridiani Planum
The Athena Science Instrument Payload is providing geochemical and mineralogical information for determining the properties of rocks, soils, and outcrops at the Mars Exploration Rovers landing sites. These measurements indicate that a variety of aqueous processes as well as various degrees of alteration occurred at the two landing sites. Light-toned rocks around the Spirit landing site in Gusev crater appear to have coatings or alteration rinds that may have resulted from limited aqueous alteration on the surfaces of basaltic rocks. Hematite and high Fe(III)/Fe(total) occur at the surfaces of these rocks. High concentrations of elements highly mobile in water (i.e., S, Cl, and Br) occur in rock veins, vugs, and coatings and at the bottom of soil trenches in the "intercrater plains." One scenario for the formation of rock coatings or rinds and translocation of mobile elements is that water might have occurred briefly at the Martian surface during periods of high obliquity and thin films of water may have mobilized elements and altered the surfaces of rocks. Outcrops on the slopes of the Columbia Hills appear to be extensively altered as suggested by their relative "softness" (measured as resistance to abrasion) as compared to basalts on the adjacent plains, high Fe(III)/Fe(total), iron mineralogy dominated by nanophase Fe(III) oxides and hematite, and high Br and Cl concentrations beneath outcrop surfaces. These outcrops may have formed by the alteration of basaltic rocks and/or volcaniclastic materials by solutions that were rich in volatile elements (e.g., Br, Cl, S). However, it is not clear whether aqueous alteration occurred at depth (e.g., metasomatism), by hydrothermal solutions (e.g., associated with volcanic or impact processes), by vapors rich in volcanic gases, or by low-temperature solutions. The occurrence of jarosite, hematite, and other sulfates (e.g., Mg sulfates) in Eagle and Endurance crater outcrops are strong indicators of aqueous processes at Meridiani Planum. These phases occur with siliciclastic materials in outcrops. Jarosite can only form by aqueous processes under very acidic conditions; e.g., acid sulfate weathering conditions resulting from the oxidation of Fe sulfides or by sulfuric acid alteration of basalts by solutions associated with SO$_{2}$-rich volcanic gases. It is plausible that acidic solutions rich in sulfur (and Fe(II)) reacted with basaltic sediments (which provided a host of soluble cations) under oxidizing conditions and then, through evaporation, formed sediments rich in jarosite and other sulfates along with siliciclastic materials. Hematite-rich spherules in outcrops may have formed by aqueous processes within the sedimentary layers, which promoted transport of Fe(II) solutions to nucleation sites where oxidation and precipitation occurred to form hematite-rich spherules.
P23B-02 13:55h
The First Plains Geologic Transect on Mars: Spirit Rover Traverse of the Plains in Gusev Crater
The location and near-field geologic setting of Spirit were mapped on regional image and remote sensing data during the approximately 2.3 km traverse from the landing site within the Gusev Crater floor plains to the Columbia Hills. Differences in surficial geology together with boundaries between geologic units identified from ground observation were recorded and mapped along a 40 m-wide corridor along the traverse. The observations yield the first systematic geologic transect of a plains surface and multiple impact crater ejecta sheets on another planet. Several concepts were examined or tested: (1) the regional morphologic variability of an Hesperian plains surface, (2) the distribution of small craters on the plains, (3) the relative degree of chemical weathering and importance of impact gardening in modifying basaltic plains, (4) surface characteristics in regions of differing themophysical properties, (5) the systematic structure of crater ejecta, and (6) the ability to locate and identify contacts defined from orbital and remote sensing data during surface traverse. The resulting geologic transect map also provides a base for the systematic measurements made by Athena science instruments. Geologic contacts crossed included ejecta blanket boundaries, crater rims, a two-fold geologic and topographic division of inner and outer ejecta, and the regional contact between the lava plains and the Columbia Hills. A comparison of contacts in geologic maps prepared from orbital data sets prior to the traverse and contacts observed at the surface along the transect confirm that surface traverses on Mars can identify the contact relations defined from orbital image data. Maps prepared from surface traverse and remote sensing differ mainly in the precision of location of boundaries, such as the outer edge of ejecta sheets, and distribution of surficial materials. Apparent vesicularity (ratio of summed vesicle volume from area over rock unit volume) of rocks in the plains lava surface decreased with increasing host rock size and angularity. It is interpreted that this population of vesicular clasts is a result of (1) impact gardening of an upper vesicular zone, and (2) that the vesicular zone is thin compared with that predicted for lava flows on Earth. If supported by additional on-going analysis, the presence of 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.
P23B-03 14:10h
The Columbia Hills: Origin and Evidence for Aqueous Alteration
The second phase of the MER Spirit mission, in Gusev crater, began when the rover entered the province of the Columbia Hills. The rover traveled over 3km to arrive at this destination. The Columbia Hills rise up to 90m above the basaltic plains that Spirit spent the first 5 months investigating. These plains appear to be impact gardened lava flows that have subsequently undergone aeolian reworking. The Columbia Hills are embayed by the plains and are therefore older materials. The origin of the Columbia Hills, as of this writing, is still unknown. Several possible origins have been considered and include: eroded and partially buried impact crater rim, ejecta deposit remnant at the junction of several ancient crater rims, central peak of an ancient crater, volcanic construct, eroded wrinkle ridge, or remnant of a formerly extensive crater fill deposit. The slopes of the Hills are covered with boulders and outcroppings of rock are present along the flanks. Some of these rocks appear to be layered. Heretofore unseen deep cavernous weathering (case hardening and core softening) of rocks was also observed upon crossing the contact with the plains. Hollowed out or cavernous rocks are the most noticeable manifestation of salt weathering on earth. This type of weathering typically involves groundwater and salts. Several of these rocks have unusual morphologies (pedestals or fingers projecting away from rock's center with terminal clumps). These features are most likely the result of differential weathering by the wind. These morphologic observations combined with mineralogic data indicate that the rocks of the Columbia Hills have experienced a very different geologic history than the rocks located on the plains. The rocks of the Columbia Hills are considerably more highly altered and weathered by the interaction of aqueous processes than the plains forming rocks.
P23B-04 14:25h
Mineralogic Variations from the Plains of Gusev to the Columbia Hills: Results from Mini-TES
The Miniature Thermal Emission Spectrometer (Mini-TES) onboard the Spirit rover in Gusev Crater has documented significant thermal-infrared (TIR) spectral differences between the rocks on the plains surrounding the lander and those of the hills to the east informally known as the "Columbia Hills". These spectral variations are related to differences in the bulk mineralogy of the rocks. The darkest, least dusty rocks on the plains are notably similar in their spectral characteristics. A strong emissivity peak at ~440 cm-1 is attributable to olivine of intermediate composition. Surprisingly, plagioclase and pyroxene components are not apparent from the deconvolution of these spectra. However, the combination of very cold temperatures of the rocks relative to the warmer, dusty atmosphere early in the mission suggests that atmospheric downwelling radiance may have contributed spurious spectral features that have obscured some of the mineral features. The low wavenumber range of Mini-TES spectra is least affected by atmospheric contributions due to the relatively low opacity in this range. Dark-toned rocks in the location known as the west spur of the Columbia Hills were measured under much less dusty atmospheric conditions, a fact that may partially explain some of the spectral differences. The strong olivine feature that is so obvious in the plains rocks is absent in the west spur rocks suggesting that olivine is either absent or only a very minor component. Spectral deconvolution reveals evidence for plagioclase and more notably, a significant basaltic glass component. The apparent difference in olivine content between the west spur and plains rocks may imply a difference in the composition of the primary source material. Alternatively, it may represent a weathering or alteration process that involves water. Refinement of the spectral deconvolution results along with the incorporation of additional datasets likely will resolve this question.
P23B-05 14:40h
Moessbauer Mineralogical Evidence for Aqueous Processes at Gusev Crater and Meridiani Planum
The Moessbauer spectrometers on the MER rovers have measured the relative abundances of iron with respect to both oxidation state and iron-bearing phase at Gusev Crater (Spirit rover) and Meridiani Planum (Opportunity rover). The assemblage of phases 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 (~Fo60), a Fe(3+)-rich component (nanophase ferric oxide, np-Ox) has significant abundance in surface soils (13-28% of total Fe) and in the surface coatings (rinds) of certain rocks (39%) but not in rock interiors exposed by grinding (5-6%). 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. The ubiquitous presence of sulfur in soil and in rock coatings, as determined by the MER-A APXS instrument, suggests that the alteration occurred under acid-sulfate conditions, so that both hydrolytic and sulfatic reactions are viable. A possible source for the weathering agents is volcanic emanations rich in H2O and SO2. Generally, rocks in the Columbia Hills are significantly more altered than those in the Gusev plains, with a higher proportion of Fe(3+) oxide phases compared to Fe(2+) silicate phases. This mineralogical dichotomy implies a difference in the timing, rate, duration, and/or mechanism of alteration for basaltic material in the Gusev plains compared to basaltic material in the Columbia Hills. It is possible, for example, that the basaltic material in the Columbia Hills underwent aqueous alteration in a paleoclimate that favored nearly complete alteration and that the basaltic material of the Gusev plains will not achieve the degree of alteration exhibited by the Columbia Hills under current martian surface conditions. Because its structure contains the hydroxide anion, the Moessbauer detection of the hydroxide sulfate jarosite (K,Na)Fe3(SO4)2(OH)6 in outcrops (~28% of total Fe) at Meridiani Planum is direct mineralogical evidence for oxidative aqueous alteration of basaltic precursors. We calculate that average outcrop has the equivalent of ~2 wt. % H2O contained in the jarosite. By analogy with terrestrial jarosite occurrences, acid-sulfate conditions at low pH are required. 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 (~35% of total Fe) and in spherules (interpreted as concretions) dispersed throughout the outcrop. Whole and fragmented spherules occur on outcrop surfaces and are concentrated in the crests of ripples (lag deposits). The process for spherule formation is incompletely understood, but presumably involved aqueous transport processes. As at Gusev Crater, basaltic surface soils at Meridiani Planum have significant concentrations of nanophase ferric oxide (12-37%). In particular, a high-albedo aeolian deposit (np-Ox ~ 29%) on the rim of Eagle Crater suggests that martian dust is basaltic in bulk composition and is a product of oxidative, aqueous alteration and subsequent disintegration of coarser basaltic materials.
P23B-06 14:55h
Chemical Alteration of Soils on Earth as a Function of Precipitation: Insights Into Weathering Processes Relevant to Mars
Soils lie at the interface of the atmosphere and lithosphere, and the rates of chemical and physical processes that form them hinge on the availability of water. Here we quantify the effect of these processes on soil volume and mass in different rainfall regimes. We then use the results of this synthesis to compare with the growing chemical dataset for soils on Mars in order to identify moisture regimes on Earth that may provide crude analogues for past Martian weathering conditions. In this synthesis, the rates of elemental gains/losses, and corresponding volumetric changes, were compared for soils in nine soil chronosequences (sequences of soils of differing ages) - sequences formed in climates ranging from ~1 to ~4500 mm mean annual precipitation (MAP). Total elemental chemistry of soils and parent materials were determined via XRF, ICP-MS, and/or ICP-OES, and the absolute elemental gains or losses (and volume changes) were determined by normalizing data to an immobile index element. For the chronosequences examined, the initial stages of soil formation (10$^{3}^$ to 10$^{4}^$ yr), regardless of climate, generally show volumetric expansion due to (1) reduction in bulk density by biological/physical turbation, (2) addition of organic matter, (3) accumulation of water during clay mineral synthesis, and/or (4) accumulation of atmospheric salts and dust. Despite large differences in parent materials (basalt, sandstone, granitic alluvium), there was a systematic relationship between long-term (10$^{5}^$ to 10$^{6}^$ yr) volumetric change and rainfall, with an approximate cross-over point between net expansion (and accumulation of atmospheric solutes and dust) and net collapse (net losses of Si, Al, and alkaline earths and alkali metals) between approximately 20 and 100 mm MAP. Recently published geochemical data of soils at Gusev Crater (Gellert et al. 2004. Science 305:829), when normalized to Ti, show apparent net losses of Si and Al that range between 5 and 50% of values relative to adjacent rocks. However the chemical impact of globally distributed dust on Mars greatly affects the interpretation of these apparent elemental losses. From the available soil data, no Earth-based soil geochemical signature perfectly matches the reported Martian data, though arid soils in the Atacama Desert and elsewhere exhibit certain similarities (losses of Si, Al and gains of S). For both Earth and Mars, an understanding of the chemical signature of atmospherically derived elements is critical for calculating accurate measures of chemical weathering in soils.
P23B-07 15:10h
Did Habitable Environments Once Exist in Gusev Crater? Astrobiology and the Mars Exploration Rover Spirit
Spirit's observations address whether the Gusev crater site provided the ingredients ("biogenic" elements: C, O, H, S, N, P and key metals) and conditions (useful energy, liquid water, etc.) necessary for life as we know it. In concert with earlier observations of martian meteorites and the martian atmosphere, Spirit observed that virtually all of the major biogenic elements appear to be readily available. An assessment of energy sources for life must consider whether liquid water and favorable environments were simultaneously present, thus enabling life to utilize that energy. For example, the solar energy flux at the surface could sustain photosynthesis, but it is uncertain whether liquid water was also present at the surface at a chemical potential sufficient for life. However, aqueous alteration of the mineral olivine releases H2, which, together with oxidants as mild as CO2, provides chemical energy. Olivine rich basalts dominate the local rock population. Normative olivine abundances in the rocks Adirondack, Humphrey and Mazatzal lie in the range 21 to 36 wt. percent. Low rock strengths (shown by the RAT), textures (Microscopic Imager) and mineralogy (Moessbauer) and elemental abundances (APXS) indicate that these rocks have been altered by water to varying degrees. Thus subsurface basaltic environments had the capacity to provide energy to "chemoautotrophs," which are microorganisms that utilize chemical redox reactions to obtain energy and to convert CO2 to biomass. The distribution and duration of potentially habitable conditions in the near-subsurface environment remain uncertain, but Spirit has shown that a broad range of scenarios existed. Laguna hollow is a relatively recent secondary impact crater filled with dust and drift material. A trench dug in the hollow revealed that soluble salts and insoluble constituents were uniformly physically mixed together and thus were deposited and stored under very dry conditions. In contrast, Spirit trenched the much older, more mature "intercrater plains material" (e.g., "The Boroughs" trench) and found substantially elevated Mg and S abundances at depth, consistent with their mobilization by water. Weathering of olivine basalts in this subsurface environment might have provided chemical energy for life, however it is uncertain whether the ionic strength of the aqueous solution was sufficiently low to sustain Earth-like metabolism. However, the extensively altered rocks at West Spur, Columbia Hills indicate that, sometime in the past, conditions probably had the capacity to sustain chemoautotrophic life.
P23B-08 15:25h
Environmental History and Stresses on Mars and Their Effect on Microbial Populations
We simulated the major environmental stresses on and near the Martian surface and recorded their effects on microbial populations of E. coli and D. radiodurans. Viability rates were determined for the populations under the single and combined stresses of sub-zero temperature, low-pressure, and UV radiation in salt-water soil and fresh-water soil at variable depths, and in sea water. Our results demonstrate that E. coli and D. radiodurans can remain viable under these exposure conditions, but are most sensitive to low pressure. The effect of low pressure is lessened under sub-zero temperatures as microbial populations from both species appear to respond synergistically to these combined stresses. However, the low-pressure effects are so dramatic (much more so than from UV radiation), that microbial life near the surface of Mars appears unlikely if it shares the characteristics of the model organisms in our study. This finding is consistent with our earlier projection that single cell life would, if extant near the surface, most likely be dormant. Results from the Mars Exploration Rovers and orbiters confirm a picture of Martian environmental history as consisting of long periods of quiescence with extremely slow surface erosion punctuated by short-duration episodes of quasi-stable conditions (e.g. triggered by magmatic driven activity at Tharsis) of a considerably wetter and warmer environment, which likely included hydrothermal activity and ponded surface water bodies in the northern plains (e.g. from cataclysmic floods). This environmental history would have promoted the (1) origin of life as chemoautotrophs, some of which may have evolved into heterotrophs, (2) possible origin of phototrophs, (3) strong directional selection for enhancing chemoautotrophy and the development of alternative energy sources, (4) strong directional selection for life cycles alternating between dormant and proliferative forms, and (5) possible persistence of some chemotrophs and organisms using alternative energy sources below the Martian surface to the present time. Putative microbes may thrive in ground water, which we would expect to differ substantially from ancestral forms.