Polyhydrated Sulfates Research Articles

Possible formation pathways for zeolites in closed-basin lakes on noachian Mars: Insights from geochemical modeling

Zeolites are among the most common and widespread authigenic silicate minerals found in saline-alkaline paleolacustrine environments on Earth. Analcime, a Na-zeolite, has been detected on crustal outcrops on Mars using orbital spectral image data. Identifying other zeolite species is complicated by the lack of diagnostic absorption features in spectral data and spectral similarity to some polyhydrated sulfates. Therefore, analcime is the only zeolite species so far unambiguously identified as present on Mars. The presence of certain zeolite mineral species or assemblages in a closed basin paleolake can be used to track hydrological changes in that basin. This paper discusses the application of geochemical modeling to identify zeolite phases that could have formed in closed lake basins of late Noachian Mars. The composition of the high-silica Buckskin sample from Gale crater is used as a starting material because 1) most zeolite-rich assemblages in closed hydrologic systems on Earth originate from the decomposition of high-silica volcanic glass, and 2) geochemical modeling studies are already available for the more common lower silica basalts on Mars. This study, along with previous studies involving basaltic starting materials, allow a comparison between terrestrial and Martian data and an exploration of the differences in secondary mineral precipitation with substrates of different silica contents. The EQ3/6 geochemical modeling code was adopted, and Late Noachian Mars was assumed to be warm and semi-arid, with an atmosphere with 1.3 bars of PCO2 and current Mars PO2 (14.7 μbar). The Buckskin sample was reacted with three different starting solutions including two different rainfall chemistries and an acidic H2SO4 − HCl solution, which could have formed through volcanic degassing. The mineral formation and re-dissolution patterns under different Water/Rock (W/R) ratios are also explored.The results show that near neutral and acidic solutions in closed-basin lakes filled with high-silica volcaniclastic materials could eventually produce zeolite minerals like those in saline-alkaline lakes on Earth. Analcime forms only in water-limited environments (low W/R ratio), while chabazite is present even at relatively high W/R ratio. Clinoptilolite transforms to analcime with an increase in pH, with decreased W/R ratio, and with time. Analcime was found to be the most stable zeolite mineral over the greatest range of conditions in this environment, associated with increased pH, consistent with its presence as a very common constituent of saline-alkaline lake deposits on Earth. The study shows that the mineral precipitation patterns associated with the dissolution of high-silica materials through acid weathering are comparable to the mineral precipitation patterns following the dissolution of basaltic rocks by acid weathering under 0 − 25oC conditions or hydrothermal conditions as described in previous laboratory, field, and geochemical modeling studies.

Testing Correspondence between Areas with Hydrated Minerals, as Observed by CRISM/MRO, and Spots of Enhanced Subsurface Water Content, as Found by DAN along the Traverse of Curiosity

Possible correlation is studied between Water Equivalent Hydrogen (WEH) in the Martian subsurface, as measured by the DAN (Dynamic Albedo of Neutrons) instrument along the Curiosity traverse, and the presence of hydrated minerals on the surface, as seen from the orbit by CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument onboard MRO (Mars Reconnaissance Orbiter). Cross-analysis of the subsurface WEH values from DAN passive measurements with the distribution of hydrated minerals over the surface of Gale crater according to Specialized Browse Product Mosaics is performed for the initial 20 km part of traverse. As a result, we found an increase up to 0.4 wt% of the mean WEH value for the surface areas with the spectral signatures of polyhydrated sulfates. The increase is shown to be higher with the more prominent spectral signature on the surface. Similar WEH increase for the two other types of hydrated minerals, such as monohydrated sulfates and phyllosilicates, was not found for the tested part of the traverse. Polyhydrated sulfates being a part of the sedimentary deposits composing the surface of Gale crater should have considerable thickness that is necessary for the subsurface neutron sensing by DAN measurements.

Investigation of mineral assemblages in a newly identified endorheic playa near Huygens basin on Mars and their astrobiological implications

The present study investigated an inter-crater depression, which is hypothesized to have been hydrologically active in the past. Mineralogical analysis of the putative lake was carried out to examine the presence of secondary minerals and decipher its aqueous alteration history. The basin was observed to be predominant in mono- and polyhydrated sulfates and hydrated silica with intermittent exposures of Al-phyllosilicates such as montmorillonite and beidellite. Later, mineral profiling of the surrounding terrain was also carried out to infer the origin of these minerals. We recorded signatures of Mg-rich smectite (saponite) and Fe/Ca‑carbonate in crater rims and ejecta here, in addition to the minerals previously identified within the basin. Spectro-morphological examination of the entire region helped in understanding the emplacement of the minerals and construction of the life cycle of the lake. The contrasting environmental conditions required for the formation of these minerals suggest that the basin witnessed multiple hydrological cycles and that it was active for a long period of time. We propose the basin to be an endorheic playa with geologically complex terrain which may hold good biosignature preservation potential for future exploration missions. Finally, our preliminary exploration of the area highlights the importance of inter-crater depressions on Mars and their significance in water budgeting, even if a small percentage of them were hydrologically active.

Investigating the origin of gypsum in Olympia Undae: Characterizing the mineralogy of the basal unit

The Olympia Undae sand sea contains the largest known deposit of gypsum discovered on the surface of Mars. The origin of this gypsum, a hydrated sulfate mineral requiring liquid water for its formation, remains largely unconstrained. We examine the hypothesis that gypsum was derived from the early-Amazonian aged Basal Unit, which is suggested to contain hydrated sulfates. Previous attempts to detect hydrated sulfates in the Basal Unit using CRISM and OMEGA data have been largely inconclusive. In this paper, we characterize the hydrated sulfate mineralogy of the Basal Unit using the Guided Endmember Extraction (GEEn) method which can detect target mineral spectra in mixed environments that obscure absorptions characteristic of certain minerals. In this paper, we outline a novel workflow for the application of GEEn to a set of CRISM images from the Olympia Cavi region and present spectral evidence for the presence of polyhydrated sulfates in the Basal Unit. We validate the applied GEEn workflow using CRISM data from various regions on Mars where sulfates have previously been detected. Non-linear mixture modeling is used to determine that spectra of the Basal Unit are best modeled as a spectral mixture of water-ice, sand/dust, mafic dune material, gypsum, and polyhydrated magnesium sulfate⁎. These sulfate detections could indicate the presence of liquid water in the polar region during the Amazonian.11*Polyhydrated magnesium sulfate here refers to the MICA library spectrum named “Polyhydrated Sulfate”, which according to Vivano-Beck et al., could be representative of a variety of polyhydrated sulfates including epsomite, hexahydrite, amorphous Mg-sulfate, and starkeyite (Chipera and Vaniman, 2007; Wang et al., 2009).

Spatial distributions and origin of hydrated sulfate minerals at the mineral bowl in Ophir Chasma, Mars

Numerous occurrences of hydrated minerals (e.g., phyllosilicate, sulfate) have been found on the surface of Mars by remote sensing and in-situ exploration missions, providing strong evidence of the aqueous environment in Martian history. However, the detailed origin of these hydrated minerals is often controversial, reducing their usefulness in constraining the aqueous history of Mars. For instance, in Ophir Chasma, the origins of hydrated sulfates are debatable among hypotheses of precipitating from groundwater, paleolake, or meteoric water. In this study, we performed detailed mineralogy, morphology and stratigraphy analyses of the hydrated phases in the mineral bowl, a depression between the southern wall and mensa of Ophir Chasma, using datasets from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), High Resolution Imaging Science Experiment (HiRISE) and High Resolution Stereo Camera (HRSC). A large number of monohydrate sulfates (MHS), polyhydrated sulfates (PHS) and some jarosite were identified in this region. Elevation of PHS-rich regions (mean elevation of −1130 ​m) is higher than those of MHS (mean value of −1401 ​m). Spectra of poly- and monohydrated sulfates mixtures were investigated in details in laboratory. Spectral features (i.e., band depth of ~1.9 ​μm, 2.1 ​μm, convexity at 2.1 ​μm and 2.4 ​μm) of sulfates mixtures changed regularly with the abundance of endmember. The linear function between the ratio of BD2100_2 to SINDEX2 and kieserite component was set up to estimate hydrated sulfates abundance. We employ the function onto CRISM data and find a coexistence of poly- and monohydrated sulfates for the formerly recognized PHS endmember, which may indicate a local dehydration/rehydration process for the sulfated minerals. Comprehensive mineralogy, topography, and stratigraphy analyses of hydrated sulfates outcrops suggest a complex formation scenario which could include surface runoff, paleolake, even aeolian erosion.

The Deposition and Alteration History of the Northeast Syrtis Major Layered Sulfates

AbstractAncient stratigraphy on Isidis Basin's western margin records the history of water on early Mars. Noachian units are overlain by layered, basaltic composition sedimentary rocks that are enriched in polyhydrated sulfates and capped by more resistant units. The layered sulfates—uniquely exposed at northeast Syrtis Major—comprise a sedimentary sequence up to 600 m thick that has undergone a multistage history of deposition, alteration, and erosion. Siliciclastic sediments enriched in polyhydrated sulfates are bedded at meter scale and were deposited on slopes up to 10°, embaying and thinning against preexisting Noachian highlands around the Isidis Basin rim. The layered sulfates were modified by volume loss fracturing during diagenesis. Resultant fractures hosted channelized flow and jarosite mineral precipitation to form resistant ridges upon erosion. The structural form of the layered sulfates is consistent with packages of sediment fallen from either atmospheric or aqueous suspension; coupling with substantial diagenetic volume loss may favor deepwater basin sedimentation. After formation, the layered sulfates were capped by a “smooth capping unit” and then eroded to form paleovalleys. Hesperian Syrtis Major lavas were channelized by this paleotopography, capping it in some places and filling it in others. Later fluvial features and phyllosilicate‐bearing lacustrine deposits, sharing a regional base level of ∼−2,300 m, were superimposed on the sulfate‐lava stratigraphy. The layered sulfates suggest surface bodies of water and active groundwater upwelling during the Noachian‐Hesperian transition. The northeast Syrtis Major stratigraphy records at least four distinct phases of surface and subsurface aqueous activity spanning from late Noachian to early Amazonian time.

Geology of Hebes Chasma, Mars: 1. Structure, Stratigraphy, and Mineralogy of the Interior Layered Deposits

AbstractHebes Chasma is an 8‐km deep, 126 by 314 km, isolated basin that is partially filled with massive deposits of water‐altered strata called interior layered deposits (ILDs). By analyzing the ILD's structure, stratigraphy, and mineralogy, a depositional history of Hebes Chasma is interpreted. Three distinct ILD units were found and are informally referred to as the Lower, Upper, and Late ILD. These units are distinguished by their layer thicknesses, layer attitudes, mineralogies, and erosional landforms. The Lower and Upper ILDs comprise the chasma's 7.5‐km tall, 120 by 43 km, central mound, and the Late ILD is located in the valley between the central mound and the chasma's northern wall. A horizontal unconformity separates the Lower and Upper ILDs, and layer attitudes revealed large‐scale shallow folding within the Lower ILD. All ILDs are characterized by both monohydrated and polyhydrated sulfates signatures. Erosional landforms such as hummocks, polygons, and debris flows suggest past glacial activity within the chasma. A scenario involving several ash fall events during various stages of chasma formation is proposed as the dominant setting throughout Hebes' geologic history.

Spectral and stratigraphic mapping of hydrated minerals associated with interior layered deposits near the southern wall of Melas Chasma, Mars

Orbital remote sensing data acquired from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard Mars Reconnaissance Orbiter (MRO), in conjunction with other datasets, are used to perform detailed spectral and stratigraphic analyses over a portion of south Melas Chasma, Mars. The Discrete Ordinate Radiative Transfer (DISORT) model is used to retrieve atmospherically corrected single scattering albedos from CRISM I/F data for mineral identification. A sequence of interbedded poly- and monohydrated sulfates associated with interior layered deposits (ILDs) is identified and mapped. Analyses from laboratory experiments and spectral unmixing of CRISM hyperspectral data support the hypothesis of precipitation and dehydration of multiple inputs of complex Mg-Ca-Fe-SO4-Cl brines. In this scenario, the early precipitated Mg sulfates could dehydrate into monohydrated sulfate due to catalytic effects, and the later-precipitated Mg sulfates from the late-stage “clean” brine could terminate their dehydration at mid-degree of hydration to form a polyhydrated sulfate layer due to depletion of the catalytic species (e.g., Ca, Fe, and Cl). Distinct jarosite-bearing units are identified stratigraphically above the hydrated sulfate deposits. These are hypothesized to have formed either by oxidation of a fluid containing Fe(II) and SO4, or by leaching of soluble phases from precursor intermixed jarosite-Mg sulfate units that may have formed during the later stages of deposition of the hydrated sulfate sequence. Results from stratigraphic analysis of the ILDs show that the layers have a consistent northward dip towards the interior of the Melas Chasma basin, a mean dip angle of ∼6°, and neighboring strata that are approximately parallel. These strata are interpreted as initially sub-horizontal layers of a subaqueous, sedimentary evaporite deposits that underwent post-depositional tilting from slumping into the Melas Chasma basin. The interbedded hydrated sulfate units and jarosite-bearing units, which have distinct stratigraphic relationships, are indicative of a complex sedimentary and aqueous history in south Melas Chasma.

The structural, stratigraphic, and paleoenvironmental record exposed on the rim and walls of Iazu Crater, Mars

AbstractIazu is a 6.8 km diameter crater located ~25 km south of Endeavour Crater in Meridiani Planum, Mars. The asymmetrical ejecta deposit and crater shape demonstrate that Iazu was formed by the impact of a projectile moving from west to east at 20 to 30° above the horizontal. The crater rim and walls expose an ~115 m thick section of intact banded bright and dark Burns formation polyhydrated sulfate deposits that overlie a lower section of basaltic crust of unknown thickness that has been slightly altered to Fe3+─Mg2+ smectites. The lower section also exhibits several basaltic outcrops that are overlain by banded bright and dark layers similar in appearance to the bright and dark bands evident in the overlying Burns formation. We interpret the lower section as indicative of a transition from regional‐scale fluvial activity to accumulation of sulfate‐rich deposits, culminating in a thick overlying section of Burns formation strata. Banding in the Burns formation and underlying strata is inferred to be a consequence of periodic variations in sulfate‐forming depositional environments.

End‐member identification and spectral mixture analysis of CRISM hyperspectral data: A case study on southwest Melas Chasma, Mars

AbstractWe present spectral unmixing results over the southwest Melas Chasma region, where a variety of hydrated minerals were identified. We use the Discrete Ordinate Radiative Transfer radiative transfer model to simultaneously model Mars atmospheric gases, aerosols, and surface scattering and retrieve the single‐scattering albedos (SSAs) modeled by the Hapke bidirectional scattering function from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data. We employ a spectral unmixing algorithm to quantitatively analyze the mineral abundances by modeling the atmospherically corrected CRISM SSAs using a nonnegative least squares linear deconvolution algorithm. To build the spectral library used for spectral unmixing, we use the factor analysis and target transformation technique to recover spectral end‐members within the CRISM scenes. We investigate several distinct geologic units, including an interbedded polyhydrated and monohydrated sulfate unit (interbedded unit 1) and an interbedded phyllosilicate‐sulfate unit (interbedded unit 2). Our spectral unmixing results indicate that polyhydrated sulfates in the interbedded unit 1 have a much lower abundance (~10%) than that of the surrounding unit (~20%) and thus may have been partially dehydrated into kieserite to form the interbedded strata, supporting a two‐staged precipitation‐dehydration formation hypothesis. In the interbedded unit 2 phyllosilicates have an abundance of ~40% and are interbedded with ~20% sulfates. The results, in combination with thermodynamic calculations performed previously, suggest that the interbedded phyllosilicates and sulfates likely formed through coupled basalt weathering and evaporation. The methodology developed in this study provides a powerful tool to derive the mineral abundances, aiming to better constrain the formation processes of minerals and past aqueous environment on Mars.

Temperature-dependent VNIR spectroscopy of hydrated Mg-sulfates

We investigate two poly-hydrated magnesium sulfates, hexahydrite (MgSO4 · 6H2O) and epsomite (MgSO4 · 7H2O), in the visible and infrared (VNIR) spectral range 0.5/4.0µm, as particulate for three different grain size ranges: 20–50µm, 75–100µm and 125–150µm. All samples were measured in the 93–298K temperature range. The spectra of these hydrated salts are characterized by strong OH absorption bands in the 1.0–1.5µm region, and by H2O absorption bands near 2 and 3µm. Other weak features show up at low temperatures near 1.75µm (in both hexahydrite and epsomite) and 2.2µm (only in hexahydrite). The spectral behavior of the absorption bands of these two minerals has been analyzed as a function of both grain size and temperature, deriving trends related to specific spectral parameters such as band center, band depth, band area, and band width. Hydrated minerals, in particular mono- and poly-hydrated sulfates, are present in planetary objects such as Mars and the icy Galilean satellites. Safe detection of these minerals shall rely on detailed laboratory investigation of these materials in different environmental conditions. Hence an accurate spectral analysis of such minerals as a function of temperature is key to better understand and constrain future observations.

Setting constraints on the nature and origin of the two major hydrous sulfates on Mars: Monohydrated and polyhydrated sulfates

AbstractMonohydrated Mg sulfate (MgSO4·H2O) and polyhydrated sulfate are the most common and abundant hydrous sulfates observed thus far on Mars. They are widely distributed and coexist in many locations. On the basis of results from two new sets of experiments, in combination with past experimental studies and the subsurface salt mineralogy observed at a saline playa (Dalangtan, DLT) in a terrestrial analogue hyperarid region on the Tibet Plateau, we can now set new constraints on the nature and origin of these two major Martian sulfates. Starkeyite (MgSO4·4H2O) is the best candidate for polyhydrated sulfate. MgSO4·H2O in the form of “LH‐1w,” generated from dehydration of Mg sulfates with high degrees of hydration, is the most likely mineral form for the majority of Martian monohydrated Mg sulfate. Two critical properties of Mg sulfates are responsible for the coexistence of these two phases that have very different degrees of hydration: (1) the metastability of a substructural unit in starkeyite at relatively low temperatures, and (2) catalytic effects attributed to coprecipitated species (sulfates, chlorides, oxides, and hydroxides) from chemically complex brines that help overcome the metastability of starkeyite. The combination of these two properties controls the coexistence of the LH‐1w layer and starkeyite layers at many locations on Mars, which sometimes occur in an interbedded stratigraphy. The structural H2O held by these two broadly distributed sulfates represents a large H2O reservoir at the surface and in the shallow subsurface on current Mars.

Alunite dissolution rates: Dissolution mechanisms and implications for Mars

Alunite (KAl3(SO4)2(OH)6) is a hydrated aluminous sulfate mineral associated with acidic, oxidizing aqueous environments on Earth. Additionally, orbiting spacecraft and rovers on Mars have reported spectral data that indicate a range of mono- and polyhydrated sulfate phases and hydroxysulfate phases, suggesting such conditions also existed on Mars in the past. This study examines alunite dissolution rates in aqueous systems with varying pH, temperature, and solution chemistry conditions. Alunite dissolution rates in dilute solutions are 2–3 orders of magnitude slower than jarosite dissolution rates measured under analogous conditions. Similar to jarosite, alunite dissolution rates vary as a function of activity of H+ and OH− following the rate law log r (molm−2s−1)=−0.133(±0.02)pH−10.65(±0.07) at pH<5 and log r=0.194(±0.04)pH−12.53(±0.26) at pH>5. However, minimum alunite dissolution rates are shifted to higher pH (5–5.5), likely due to differences in Fe and Al speciation. Alunite and jarosite rates converge in saturated NaCl and CaCl2 brines as the activity of water decreases, suggesting that differences in water exchange rates with Fe3+ and Al3+ control dissolution rates in dilute solutions, while metal–Cl− complexation occurs at similar rates within the brines. Particle lifetimes based on measured dissolution rates in dilute solutions show that alunite particles are expected to be preserved two orders of magnitude longer than jarosite particles over a range of pH and temperature conditions. In particular, alunite is more likely to be preserved in neutral to moderately alkaline systems compared to jarosite, which is expected to be preserved in more acidic conditions. Alunite dissolution produced amorphous Al-rich alteration products at moderate to high pH. Unlike jarosite, alunite dissolution does not show a clear trend as a function of temperature; alunite dissolution rates do not increase with increasing temperature, likely due to lower alunite solubility at higher temperature. Therefore, alunite is expected to be more prevalent in hydrothermal systems compared to jarosite.

Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability

We investigated the equatorial layered deposits (ELDs) of Arabia Terra, Mars, in Firsoff crater and on the adjacent plateau. We produced a detailed geological map that included a survey of the relative stratigraphic relations and crater count dating. We reconstructed the geometry of the layered deposits and inferred some compositional constraints. ELDs drape and onlap the plateau materials of late Noachian age, while they are unconformably covered by early and middle Amazonian units. ELDs show the presence of polyhydrated sulfates. The bulge morphology of the Firsoff crater ELDs appears to be largely depositional. The ELDs on the plateau display a sheet-drape geometry. ELDs show different characteristics between the crater and the plateau occurrences. In the crater they consist of mounds made of breccia sometimes displaying an apical pit laterally grading into a light-toned layered unit disrupted in a meter-scale polygonal pattern. These units are commonly associated with fissure ridges suggestive of subsurface sources. We interpret the ELDs inside the craters as spring deposits, originated by fluid upwelling through the pathways likely provided by the fractures related to the crater formations, and debouching at the surface through the fissure ridges and the mounds, leading to evaporite precipitation. On the plateau, ELDs consist of rare mounds, flat-lying deposits, and cross-bedded dune fields. We interpret these mounds as possible smaller spring deposits, the flat-lying deposits as playa deposits, and the cross-bedded dune fields as aeolian deposits. Groundwater fluctuations appear to be the major factor controlling ELD deposition.

Mars Reconnaissance Orbiter and Opportunity observations of the Burns formation: Crater hopping at Meridiani Planum

AbstractCompact Reconnaissance Imaging Spectrometer for Mars hyperspectral (1.0–2.65 µm) along‐track oversampled observations covering Victoria, Santa Maria, Endeavour, and Ada craters were processed to 6 m/pixel and used in combination with Opportunity observations to detect and map hydrated Mg and Ca sulfate minerals in the Burns formation. The strongest spectral absorption features were found to be associated with outcrops that are relatively young and fresh (Ada) or preferentially scoured of dust, soil, and coatings by prevailing winds. At Victoria and Santa Maria, the scoured areas are on the southeastern rims and walls, opposite to the sides where wind‐blown sands extend out of the craters. At Endeavour, the deepest absorptions are in Botany Bay, a subdued and buried rim segment that exhibits high thermal inertias, extensive outcrops, and is interpreted to be a region of enhanced wind scour extending up and out of the crater. Ada, Victoria, and Santa Maria outcrops expose the upper portion of the preserved Burns formation and show spectral evidence for the presence of kieserite. In contrast, gypsum is pervasive spectrally in the Botany Bay exposures. Gypsum, a relatively insoluble evaporative mineral, is interpreted to have formed close to the contact with the Noachian crust as rising groundwaters brought brines close to and onto the surface, either as a direct precipitate or during later diagenesis. The presence of kieserite at the top of the section is hypothesized to reflect precipitation from evaporatively concentrated brines or dehydration of polyhydrated sulfates, in both scenarios as the aqueous environment evolved to very arid conditions.

Mineralogy, morphology and stratigraphy of the light-toned interior layered deposits at Juventae Chasma

Juventae Chasma is a deep depression located north of Valles Marineris on Mars, with four bright mounds or light-toned interior layered deposits (ILDs) extending upwards from the Canyon floor. We present here the results of long-term imaging of Juventae Chasma including mounds A, B, C, and D using multiple datasets. Monohydrated sulfates (MHS) were deposited first on the canyon floor, followed by polyhydrated sulfates (PHS). The upper PHS-dominated units are largely eroded away at Juventae Chasma, but this material is still present in significant abundance at mound B. PHS are observed mixed with MHS in some areas of mounds A and C. Terraces are observed at the upper elevations of mound B that contain PHS at the steeper slopes and appear to be covered with dust on the horizontal surfaces. Current analyses of the MHS-rich unit indicate that kieserite (MgSO4⋅H2O) is the primary sulfate component, rather than szomolnokite (FeSO4⋅H2O) as previously thought. Formation of kieserite at Juventae Chasma likely required temperatures in the 150–200°C range. Geochemical modeling is most consistent with dissolution of mafic materials followed by precipitation of kieserite from solution. The dust exhibits ferric signatures and the sand is largely mafic material. Outcrops of olivine- and pyroxene-bearing rocks are best observed along the base of mound C and in the chaotic terrain surrounding mound D. This study summarizes the current understanding of Juventae Chasma and its ILDs using HRSC, HiRISE and CTX data, an expanded laboratory spectral library, and the latest calibrations available for CRISM.

Mixtures of clays and sulfates within deposits in western Melas Chasma, Mars

We have utilized several data sets from multiple spacecraft that have been acquired over a continuous observation campaign across the southwestern Melas Chasma region of Mars. The blocky deposit observed on the chasma floor and on portions of the southern wallrock consists of mixtures of light- and medium-toned materials that exhibit displacement structures consistent with transport down the wallrock and onto the chasma floor. CRISM visible and near-infrared reflectance spectra of the light-toned blocks suggest mixtures of nontronite, jarosite, Al-clays, hydrated silica, and/or an acid leached clay. The medium-toned blocks typically lack spectral features or exhibit signatures of polyhydrated sulfates. Bright layered mounds embayed by blocky deposit materials display spectra consistent with Ca-sulfates (gypsum and/or bassanite) that may have resulted from precipitation of less soluble Ca-rich minerals during ponding and evaporation of dilute fluids along the western chasma floor.Draping across some wallrock slopes is a light-toned deposit that sometimes overlies a blocky, layered, and spectrally bland medium-toned deposit. The light-toned deposit exhibits spectra consistent with mixtures of materials that may include jarosite, acid-leached clays, silica, poorly crystalline clays, and polyhydrated sulfates. Hesperian-age valleys dissect the medium-toned deposit along some wallrock slopes and may be partially filled in by light-toned deposits. The draping nature of these deposits can best be explained by airfall, either volcanic ash or atmospheric dust, with ice/snow accumulation perhaps controlling sedimentation along the wallrock. An examination of other regional hydrated deposits around southwestern Melas indicates similar draping deposits occur farther east and west along the wallrock, whereas layered sulfates to the southeast consist of monohydrated and polyhydrated sulfates that are similar to interior layered deposits found in many of the chasmata. Small opal and jarosite exposures within the Melas basin and along the nearby plateau are most likely the result of younger localized alteration and deposition. The diversity of hydrated minerals and fluvial features indicates multiple episodes of aqueous activity under distinct environmental conditions occurred throughout southwestern Melas Chasma during the Hesperian to Amazonian that may be contemporaneous with late aqueous activity elsewhere on Mars.

Stratigraphy and mineralogy of Candor Mensa, West Candor Chasma, Mars: Insights into the geologic history of Valles Marineris

Candor Mensa, an interior layered deposit (ILD) in Valles Marineris, Mars, consists of two stratigraphically distinct units, the lower of which comprises the bulk of the mensa. This lower unit is approximately 5 km thick and composed of parallel layers, 4 to 14 m in thickness and associated with monohydrated sulfates. The lower unit is disconformably overlain by an upper unit composed of thinner (< 3 m) layers with diagnostic polyhydrated sulfate signatures. The original extent of proto-Candor Mensa and its lower unit included neighboring Baetis Mensa. We suggest that the source material for both units is airborne dust or ash but that the depositional environment for the units differs. First, the lower unit was deposited during the subsidence of an enclosed water-filled basin. This basin/lake could have been frozen periodically, with freeze-thaw episodes possibly linked to Martian obliquity cycles. Erosion, including the potential action of glaciers, was able to remove large volumes of material out of the basin during the tectonism that produced the current geometry of Valles Marineris. Deposition of the upper unit postdates this event and took place in the absence of standing water at high elevation. Groundwater or snowmelt may have provided the water required for sulfate formation and deposit induration. We conclude that the major break in sedimentation recorded by this ILD deposit coincides with linking of ancestral basins into the current geometry of Valles Marineris chasmata and that it was possible to form hydrated minerals after this event.

Gypsum, opal, and fluvial channels within a trough of Noctis Labyrinthus, Mars: Implications for aqueous activity during the Late Hesperian to Amazonian

We investigate in detail the morphology, mineralogy, and stratigraphy of light-toned deposits within one trough of Noctis Labyrinthus, centered at −6.8°N, 261.1°E. CRISM spectra taken from light-toned layered deposits in the northern portion of the trough exhibit absorptions around 1.41, 1.92 and 2.21μm, consistent with mixtures of opal and Al-clays that are exposed beneath younger lava flows and between high-standing mesas of chaotic terrain. In the southern portion of the trough, opal occurs as a patchy surficial deposit along the southwestern lower wall. Gypsum appears to be present in the southern portion of the trough where spectra show triplet absorptions at 1.44, 1.48, and 1.54μm, and additional absorptions at 1.20, 1.74, 1.95, 2.22, 2.27, and 2.49μm. The gypsum-bearing materials consist of one to several beds that typically fill low-lying regions, including valleys. A bright mound on the trough floor exhibits spectral features at 1.43, 1.92 and 2.43µm, characteristic of polyhydrated sulfates. The bright mound appears distinct in morphology from chaotic terrain, and along its base are exposures of gypsum-bearing materials.Fluvial channels in the southwestern portion of the trough incise surface slopes at 4–6° and lack obvious sources. The channels display first and second order tributaries arranged in a parallel pattern and may have formed by localized surface discharge from melting snow and/or ice. Both opal and gypsum occur in close proximity in the southwestern region of the trough, but gypsum is found alone in the southeast and opal in association with Al-clays is found to the northwest. We do not believe gypsum and opal formed coevally because they are not always found together, they require different aqueous conditions (i.e., opaline deposits require high silica availability while the gypsum deposits require high Ca availability in solution), and they appear at stratigraphically distinct levels. Although we identified evidence for fluvial landforms within the trough, cross-cutting relations indicate their incision post-dates deposition of the opal and pre-dates deposition of the gypsum. Hence, several periods of aqueous activity and alteration likely occurred within the trough from the Late Hesperian into the Amazonian that reflect favorable localized conditions within the Noctis Labyrinthus region and may be contemporaneous with late aqueous activity occurring elsewhere on Mars.

Geologic relationships between gray hematite, sulfates, and clays in Capri Chasma

We used emissivity spectra from the Thermal Emission Spectrometer (TES) to identify the signature of crystalline gray hematite in Capri Chasma. Geologic units associated with major concentrations of hematite were then mapped using HiRISE, CRISM, and CTX images from the Mars Reconnaissance Orbiter (MRO). Along the northern portion of the Interior Layered Deposit (ILD), a lower polyhydrated sulfate (PHS) unit lies beneath a thicker kieserite unit, above which is a thinner upper PHS. An exposure at the thickest central portion of the ILD reveals additional sulfates, including a middle PHS, a mixture of lower hydration states PHS, and an intercalated unit comprised of kieserite and szomolnokite. We interpret these compositional transitions to reflect either changes in aqueous chemistry (e.g., iron levels and salinity) during groundwater upwelling events or successively buried layers of dust, ice and volcanic aerosols laid down over obliquity cycles. In addition to these sulfates, we identified a few small mounds along the chasma floor composed of either mixtures of ferric hydroxysulfate and Fe/Mg‐smectites, or possible opal, leached clays, and Fe/Mg‐smectites. Gray hematite is strongly spatially correlated to kieserite‐bearing slopes within the ILD and mantled PHS along the northern chasma floor. These results are consistent with sulfate and hematite formation found elsewhere on Mars, including Meridiani Planum, Aram Chaos, and several other chasmata and chaos regions within Valles Marineris.