P44A-01 INVITED
The role of groundwater in the origin of the indurated layered deposits of Arabia Terra, Mars
Indurated layered deposits of likely sedimentary origin are widely distributed across the Arabia Terra region of Mars. In situ observations by the MER Opportunity rover of one such deposit in Meridiani Planum have been interpreted to represent grains composed of dirty evaporites that have been extensively reworked by fluvial and aeolian processes in a playa environment and diagenetically modified by a fluctuating water table. Stratigraphic relationships, morphological similarities, and spectral evidence suggest a related origin for the many layered deposits throughout Arabia Terra. Isolated intra-crater deposits, erosional outliers, and pedestal craters suggest that the Arabia Terra deposits were once thicker and more widespread than their current extent. We investigate the origin of these sedimentary deposits using global and regional hydrological models, in which groundwater flow is driven by evaporation where the water table intersects the surface and redistribution of that water as low-latitude precipitation. These models predict focused groundwater upwelling and evaporation in Arabia Terra during the Late Noachian to Early Hesperian, driven by its unique topography relative to the adjacent southern highlands and northern lowlands. This hydrological cycle would have brought a steady flux of groundwater to the surface, which upon evaporation, would concentrate any dissolved solutes as a cementing salt that would indurate aeolian material and allow buildup of thick sedimentary deposits. Groundwater upwelling would first be limited to the large craters in the region, resulting in rapid sedimentary infilling by a combination of evaporites and evaporite-cemented clastic material. As the craters were filled, groundwater upwelling would spread out over broad regions of Arabia Terra, producing widespread deposits covering much of the inter-crater plains. The observed distribution and thickness of the deposits agrees with the predictions from the hydrological models. This work suggests that the extensive sedimentary deposits of Arabia Terra preserve the record of a Late Noachian-Early Hesperian global hydrological system, and a climate in which surface temperatures in the low latitudes were largely above the freezing point of water, allowing liquid precipitation to infiltrate the surface to recharge aquifers and drive continued groundwater flow.
P44A-02
New Evidence for the Origin of Layered Deposits in Valles Marineris
Results from CRISM, HiRISE, and CTX on MRO provide new insights into the origin of interior layered deposits (ILDs) in Valles Marineris. A well-exposed, thick sequence in western Candor Chasma has spectral properties consistent with basaltic sand mixed with nanophase iron oxide-rich dust, with the addition of sulfates and crystalline ferric oxides. Most of the deposit is dominated spectrally by the dust component. Monohydrated and polyhydrated sulfates are concentrated in separate, interbedded layers, which in some cases are traceable over tens of kilometers. Monhydrated sulfates dominate the lower part of the deposits whereas polyhydrated sulfates are more common in upper strata. The deposits are partially mantled by low- albedo eolian ripples that contain pyroxenes similar in composition to what is found on the surrounding plateau, plus sulfates predominantly in monohydrated form. The dark ripples originate from discrete, friable layers. Similar dark, erodible layers elsewhere on Mars have been interpreted as buried eolian sand. Crystalline ferric oxides are concentrated in the sulfate-rich layers, and mass wasting has accumulated them at the base of steep slopes to form the deposits of gray hematite detected by TES. The persistence of monohydrated sulfates in debris shows that alteration of monohydrated to polyhydrated sulfates, proposed as an important weathering process, takes long compared to formation of the thin layer that dominates reflectance properties. The observed stratification of sulfate compositions implies differences in the abundance of liquid water or brine chemistry during deposition or early chemical modification of sediments. Inferred mineralogy and compositional stratification are similar to what is observed in sulfate-rich sediments in the Meridiani and Aram Chaos regions. The Meridiani deposits were proposed to accumulate where evaporites formed in areas of groundwater discharge and cemented eolian sediments, in which coarse- grained hematite formed by diagenetic alteration. Modeling of the history of groundwater discharge in Valles Marineris shows that thick evaporite sequences are also expected within the chasmata, and could have similarly trapped eolian sediments. Areas with predicted thick accumulations enclose the major eroded remnants of the ILDs. Formation of the ILDs by lithification of eolian sediment by evaporites in areas of groundwater discharge links the spectrally and morphologically similar, sulfate- and ferric-oxide bearing deposits in Valles Marineris, Aram Chaos, and Meridiani to a common regional process.
P44A-03
Local Sulfate-rich Layered Deposits in Noctis Labyrinthus, Mars, and Their Chronological Consequences
The theiikian period has been postulated to end the primitive "wet" period of Mars history at a time when the liquid water was already scarce at the surface of Mars. The basis for such a chronology is the presence of sulfates formed after the Noachian period, such as those deposited inside Valles Marineris canyons, which opened in the Early Hesperian epoch. Nevertheless, the end of this period is poorly constrained because the ages of layered deposits, and the associated alteration, are difficult to know precisely. Secondly, sulfates form by aqueous alteration of basaltic materials, and the resulting fluids can move far from their point of genesis. The main interior deposits of Valles Marineris miss the context of deposition that could have allowed us a better understanding of the sulfates' origin. In this context, regions such as the canyons of Noctis Labyrinthus studied here are key regions to solve these two questions. Indeed, these crosscutting canyons of deep and narrow shape display fresh scarps without spur and gully erosion and cut plains of Late Hesperian age, indicating a younger age than the main canyons of Valles Marineris. Several flat areas inside Noctis canyons display strong signatures of pyroxenes according to OMEGA spectroscopic data over high thermal inertia outcrops. This suggests the existence of young lava flows (see Mangold et al., LPSC, 2008). Additionally, several layered deposits exist locally which do not display features at the OMEGA scale. At this location, CRISM data allow us a close-up into the layered deposits at a spatial sampling of about 20 meters. Over these light toned layers, we detect sulfates and a collection of hydration signatures that might be related to other hydrated minerals. This material does not represent a major volume of alteration material compared to the main canyons accumulation, but it corresponds to some of the latest signatures of alteration found yet, perhaps of similar age as the alteration found on Valles Marineris plateau (see Milliken et al., LPSC, 2008). ). Our study will ultimately try to discriminate the origin of this local alteration: (1) is it related to the heat transfer and fluid circulation created by the volcanic event observed in the surroundings? In that case, this would deny any global consequence. Or, (2) is it one of the last "oasis" of the theiikian period characterized by episodic standing bodies of water?
P44A-04
Discovery of the Acid-Sulfate Mineral Alunite in Terra Sirenum, Mars, Using MRO CRISM: Possible Evidence for Acid-Saline Lacustrine Deposits?
CRISM spectral data collected over an unnamed 70-km-diameter impact crater in the Noachian-age southern highlands of Terra Sirenum (30°S, 158°W), Mars, indicate the presence of the acid-sulfate mineral alunite [KAl3(SO4)2(OH)6], based on diagnostic absorptions at NIR wavelengths. The USGS Tetracorder spectral shape-matching system was used to create color-coded maps of the distribution of IR-active mineral phases on the Martian surface. These maps indicate that a discontinuous, 1-km-wide, several-km-long, SE-trending band of alunite-rich material grades laterally into materials spectrally dominated by kaolinite or halloysite, and montmorillonite or partially hydrated silica. These minerals are found in layered terrain near the SW crater wall. Lava flows cover portions of the crater floor and partially embay this layered terrain. Montmorillonite or partially hydrated silica is also present in ridged material located several kilometers east of the alunite zone. Meter-scale HiRISE data indicate that the alunite occurs in a high albedo layer that is tens of meters thick and is capped in places by a spectrally neutral material. Kaolinite or halloysite occurs in polygonally cracked materials reminiscent of desiccated sediments. The observed association of minerals is consistent with advanced-argillic alteration associated with relict high-temperature hydrothermal systems, but is also compatible with the mineralogy of low- temperature, acid-saline, evaporative lacustrine deposits. Recent research indicates that alunite's NIR spectral features can be used to determine the approximate temperature at which alunite forms. CRISM alunite spectra closely match spectra of low-temp. terrestrial lacustrine alunites, based on relatively weak vibrationally-coupled absorptions at 1.43, 1.76, 2.21, and 2.32 μm. Given the observed lack of inflow channels, it is likely that the alunite was deposited in a spring-fed lake that once covered the bottom of the crater. Alunite may have formed as an alteration product of basaltic material in contact with H2SO4-rich lake water. Although oxidation of sulfides or hydrothermal H2S requires oxygen to form H2SO4, the disporportionation of SO2 that condenses in water above volcanic vents does not require an external source of oxygen and is a more likely source of aqueous sulfate for the formation of the alunite. Exploration for evidence of past aquatic life should be considered.
P44A-05
Isotopic Zonation Within Sulfate Evaporite Mineral Crystals Reveal Quantitative Paleoenvironment Details
Isotopic variations measured within a single crystal of hydrated magnesium sulfate are greater than 30 permil for delta 2-H, almost 10 permil for δ18O in water of hydration; and greater than 3 permil in sulfate oxygen. These results are interpreted to indicate the relative humidity of the system during evaporation (15 to 20 percent in this test case) and constrain the volume of water involved. The theoretical basis of this system is the isotopic fractionation between the species in solution and those precipitated as evaporite salts. Precipitation preferentially accumulates more of the heavy isotopes of sulfur and oxygen in mineral sulfate, relative to sulfate in solution. During the course of mineral growth this leads to successive depletion of the respective heavier isotopes in the residual brine reflected in a parallel trend in successive precipitates or even in successive zones within a single crystal. The change in isotopic composition at any one time during the process, relative to the initial value, can be described by an isotopic version of the Rayleigh Fractionation equation, depending only on the extent of the completion of the process and the relevant fractionation factor. Evaporation preferentially removes isotopically lighter hydrogen and oxygen leading to successive extents of enrichment in the respective heavier isotopes in the residual water. However, the relative effects on hydrogen and oxygen isotopes differs as function of relative humidity [1]. ALL OF THESE CHANGES ARE PRESERVED IN THE MINERAL ISOTOPE COMPOSITIONS. We precipitated barium sulfate from epsomite or gypsum samples, which was reduced at 1450°C in the presence of graphite and glassy carbon in a Finnigan TC/EA to produce CO for O isotopic analysis in a Finnigan 253 mass spectrometer, while a separate subsample was oxidized to SO2 in a Costech Elemental Analyzer. However, to make progress with this approach we needed to make a large number of measurements of hydration water and so we developed a new analytical method [2]. We use a modification of the standard TC/EA continuous-flow protocol to measure both hydrogen and oxygen of water of hydration from the same small sample. We have proved the concept of this new approach by analyzing zones within crystals and individual grains, growing epsomite (magnesium sulfate heptahydrate) in the laboratory and by analysis of natural gypsum evaporites. We are now exploring the effects of varying the controlling parameters. Eventual application to Martian sulfates will reveal amount of water involved in sulfate formation, its isotopic composition(s) and details of the paleo-atmospheric humidity. [1] Gat JR and Gonfiantini R, (Eds) (1981) IAEA Technical Report Series. [2] Rohrssen MK, Brunner B Mielke RE and Coleman M (2008) Analyt. Chem. (in press).
P44A-06
Using Australian Acidic Playa Lakes as Analogs for Phyllosilicate and Sulfate Depositional Environments on Mars
Recent work on the origin of martian sulfates and their relationship to phyllosilicate deposits suggest that these deposits formed in different eras of Mars' history, under distinct environmental conditions. In southwestern Meridiani Planum phyllosilicates exist in close proximity to sulfate deposits. One possible explanation for this relationship is that it is an unconformable stratigraphic sequence, representing a significant change in aqueous geochemical conditions over time. Specifically, it may be interpreted to record a change in environment from neutral pH aqueous alteration in the Noachian to an acidic evaporitic system in the late Noachian to the Hesperian. On Earth, two different geochemical systems need not be evoked to explain such chemical variation. Acidic playa lakes in Western Australia have large pH differences separated by only a few tens of meters and demonstrate how highly variable chemistries can coexist over short distances in natural environments. Playa lakes on Earth tend to be dominated by lateral flow of water and salts leading to lateral chemical variation. Heterogeneity of playa mineralogy in Australia is due to the varied source rocks of brines and the mixing of dilute oxidizing brines and freshwater with the saturated evaporitic brines. This is evidenced by the ferricretes in the near-shore environment and more soluble phases in basin interiors. Playa lakes on Mars would be much larger than their terrestrial counterparts, leading to the prevalence of large-scale surface and crustal advection of water and salt rather than short-distance lateral flow, except at lake boundaries. Little or no influx of freshwater would preclude the formation of playa rim (e.g., crater rim) ferricretes and silcretes. Instead, we expect to see mainly vertical facies changes, and any diachronous lateral facies changes are expected to occur on very large spatial scales. Comparison of high spatial resolution, hyperspectral airborne data for Australian playa lakes with the OMEGA and CRISM data for regions on Mars that contain both phyllosilicates and sulfates will aid in determining if these deposits on Mars could be formed in a single depositional system. Western Australian ferricretes are spectrally similar to phyllosilicates. We therefore suggest that these analogs point to a single depositional system for Mars if phyllosilicates represent near-shore facies and sulfates interior lake deposits. The identification of ferricretes on Mars would provide an important paleoenvironmental indicator and might reveal sites of dilute brine influx.
P44A-07 INVITED
Sulfate Hydration States in Interpretation of Martian Mineral Assemblages
Remote spectral data and surface-measured chemical associations with S indicate widespread distribution of Mg-, Ca-, and Fe-sulfate salts on Mars. These salts are identified at least in part as hydrates, but spectral data and the low temperatures and low pH2O of Mars suggest that hydration states vary with origin, latitude, and exposure history. An understanding of stability limits and dehydration/rehydration rates is vital to understanding occurrences that may be interpreted variously as lacustrine, alteration via groundwater or discharge with evaporation, surface weathering, thermal brine systems, eolian recycling, or others. Different sulfates on Mars have varied susceptibility to desiccation at relatively warm, low-RH conditions or to hydration at cold, high-RH conditions. This variability provides a potent tool for interpreting exposure history. Among Ca-sulfates, gypsum and insoluble anhydrite should be stable and remain, respectively, fully hydrated or water-free at most latitudes and through diurnal and seasonal cycles, but bassanite is more sensitive to transient hydration. Mg-sulfates may have various values of n in the formula MgSO4.nH2O, and rehydration of desiccated forms often produces metastable phases. At low pH2O, unlike Ca- sulfates, amorphous forms appear with low values of n dependent, in part, on temperature. Kieserite resists dehydration but may hydrate in conditions where ice is stable at the surface. Fe-sulfates have more complex dehydration and rehydration properties. Jarosite is very resilient because of the lack of H2O molecules and presence of OH. Other Fe-sulfates are not so durable, e.g., coquimbite (Fe2 (SO4)3.9H2O) has independent H2O and dehydration on heating to 30 °C produces an amorphous product that does not rehydrate. Copiapite is similarly susceptible to dehydration. Modest heating of many H2O-bearing ferric sulfates can be destructive, and degradation can produce both cemented solids and viscous liquids. Sulfate salt associations on Mars provide mineral tools to interpret depositional origins, paleohydrology, and paleoclimatology, but these tools require attention to environments of formation, stability relations, and kinetics of hydration and dehydration.
P44A-08
Ferric sulfates on Mars: Surface Explorations and Laboratory Experiments
Recent results from missions to Mars have reinforced the importance of sulfates for Mars science. They are the hosts of water, the sinks of acidity, and maybe the most active species in the past and current surface/near-surface processes on Mars. Fe-sulfate was found frequently by Spirit and Opportunity rovers: jarosite in Meridiani Planum outcrops and a less specific "ferric sulfate" in the salty soils excavated by Spirit at Gusev Crater. Pancam spectral analysis suggests a variety of ferric sulfates in these soils, i.e. ferricopiapite, jarosite, fibroferrite, and rhomboclase. A change in the Pancam spectral features occurred in Tyrone soils after ~ 190 sols of exposure to surface conditions. Dehydration of ferric sulfate is a possible cause. We synthesized eight ferric sulfates and conducted a series of hydration/dehydration experiments. Our goal was to establish the stability fields and phase transition pathways of these ferric sulfates. In our experiments, water activity, temperature, and starting structure are the variables. No redox state change was observed. Acidic, neutral, and basic salts were used. Ferric sulfate sample containers were placed into relative humidity buffer solutions that maintain static relative humidity levels at three temperatures. The five starting phases were ferricopiapite (Fe4.67(SO4)6(OH)2.20H2O), kornelite (Fe2(SO4)3.7H2O), rhomboclase (FeH(SO4)2.4H2O), pentahydrite (Fe2(SO4)3.5H2O), and an amorphous phase (Fe2(SO4)3.5H2O). A total of one hundred fifty experiments have been running for nearly ten months. Thousands of coupled Raman and gravimetric measurements were made at intermediate steps to monitor the phase transitions. The first order discovery from these experiments is the extremely large stability field of ferricopiapite. Ferricopiapite is the major ferric sulfate to precipitate from a Fe3+-S-rich aqueous solution at mid-low temperature, and it has the highest H2O/Fe ratio (~ 4.3). However, unlike the Mg-sulfate with highest hydration state (epsomite, at mid-low temperature), which would dehydrate readily at low relative humidity, ferricopiapite remains unchanged over ten months under extremely dry conditions. On the other hand, amorphous ferric sulfate which forms easily from solutions at dry conditions, is similar to the amorphous magnesium sulfate in stability field, thus can potentially be a very important phase in the phase transition pathways of ferric sulfates on Mars.
P44A-09
Mg- and Fe-Sulfate Layers in Aram Chaos, Mars
The post-chaos layered deposits in Aram Chaos extend laterally approximately 10,000 km2 with a present-day height of ~800 meters above the basement. The deposits experienced significant erosion, in some places down to the basement, indicating a more extensive depositional coverage than evident today. Crystalline hematite, mono- and poly-hydrated sulfate, and pyroxene have been identified in the layered deposits with OMEGA, CRISM, and TES data. Analysis of CRISM targeted observations and CTX images in conjunction with previous work in this region shows a series of layers, listed here stratigraphically from top to bottom: (1) a 250-500m thick, erosionally-resistant cap unit dominated by nano-phase iron oxides, (2) a <500m thick unit with polyhydrated sulfate and hematite signatures, (3) a <50m thick bright-toned unit with 2.1 and 2.4 μm absorptions (best fit by kieserite), and (4) a ~10m thick medium-toned unit with spectra dominated by a 2.23 μm absorption feature, interpreted to be due to a hydrated iron sulfate. The kieserite and candidate iron sulfate layers are visible around the edge of the polyhydrated sulfate/hematite layers, with the iron sulfate occupying the lowest stratigraphic level observable. Both kieserite and candidate iron sulfate layers are unconformably draped over the basement topography, making estimations of actual layer thicknesses difficult; those reported here should be taken as maximum thicknesses. The presence of hematite and hydrated sulfates is evidence that water was involved in the formation of the depositional unit, and variations in the layers indicate that there were temporal changes in the depositional environment. We believe the layered deposits were formed by evaporation of rising groundwater, which would have filtered through and been altered by the chemistry of the basement chaos unit. The sequence of sulfates and hematite overlain by an iron-oxide cap unit mimics the stratigraphy to the southwest in Meridiani Planum, implying similar aqueous histories for the deposits.
P44A-10
Aqueous Alteration on Mars: Estimating the Duration of Chemical Weathering of the Wishstone-Watchtower Weathering Sequence
Mineralogical abundance of primary minerals versus secondary minerals, chemical mixing relationships, and elemental ratios have been used assess aqueous alteration at Gusev Crater and Meridiani Planum. However, limited work has used chemical data to quantify the duration of aqueous alteration on Mars. The objectives of this work are to combine laboratory dissolution rates with Ti-normalized mass-balance analysis of APXS and Mossbauer data to estimate aqueous alteration times on Mars. Wishstone rocks are candidate parent materials for the Watchtower materials. Mass-balance analysis of the Wishstone-Watchtower sequence indicated that chemical alteration caused 37% loss of Na from Wishstone in forming the Watchtower materials. Mineralogy assumed from the APXS indicated Na loss was attributed to oligoclase dissolution. Laboratory dissolution rates of oligoclase under arguably Mars relevant conditions (25 C pH 4) and assuming a particle size of 1mm were used to calculate an aqueous alteration time of 2200 years. The Mossbauer and APXS data were combined to calculate Fe losses from olivine and pyroxene dissolution in the same Wishstone-Watchtower sequence. Lower aqueous alteration times of 150 and 800 years were calculated for olivine and pyroxene, respectively. If all three minerals were exposed to similar aqueous conditions, then similar dissolution times would be expected for all minerals. A possible explanation for the variation of dissolution times between the three minerals will be provided below. Calculated aqueous alteration times are minimum times because laboratory rates are measured under high water to rock ratios, low ionic strength, and do not consider the formation of surface precipitates. Field conditions can have low water:rock ratios, higher ionic strengths, and surface precipitates that inhibit mineral dissolution. The high concentration of nano-phase iron oxides (npFeOx) in Watchtower could be derived from Fe release from olivine and pyroxene. The formation of surface npFeOx could inhibit dissolution and extend the aqueous alteration times of the olivine and pyroxene particles. Secondary precipitates derived from oligoclase dissolution at pH 4 are not likely for oligoclase surfaces. This hypothesis suggests that the calculated olivine and pyroxene dissolution times would then increase and possibly approach the calculated oligoclase time. Despite the disparities in the dissolution times, this work demonstrates the value of combining Ti-normalized mass-balance with laboratory dissolution rates in assessing duration of aqueous activity on Mars. Results from this work will serve as the foundation for developing more sophisticated kinetic dissolution calculations that will provide improved estimates of aqueous alteration times on Mars.