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In the past two decades, near-infrared (NIR) hyperspectral imaging instruments have revolutionized our conception of planetary surfaces in terms of evolution, geology, mineralogy, and alteration processes. The cornerstone of this remote analysis technique is the synergy between imagery, giving the geomorphological context of the observations, and NIR spectroscopy whose spectral range is sensitive to the main absorption features of most of the minerals present on planetary surfaces. The development of a generation of space instrument based on Acousto-Optic Tunable Filters (AOTFs) increases the capacity of these spectrometers to be set up in a variety of space probes. The ExoCam concept, developed at Institut d'Astrophysique Spatiale and profiting from the lab's previous experience (MicrOmega onboard Phobos-Grunt, Hayabusa 2 and ExoMars), thus, proposes for the first time to do hyperspectral imagery through a wide aperture AOTF (15 × 15 mm2) in the 0.95–3.6 µm spectral range. The characterization of this instrumental concept, led on a representative breadboard built for this purpose, showed that the acousto-optic diffraction preserves the image quality up to the diffraction/resolution limit over the whole field of view. The spectral resolution (from 2 to 25 nm over the spectral range) and accuracy of the instrument are also consistent with the identification of planetary surface minerals. This paper describes the ExoCam concept and objectives, the setup of an optical breadboard representative of a space instrument based on this concept, and the results of performance characterizations realized on the breadboard. [ABSTRACT FROM AUTHOR]

MicrOmega is a near-IR hyperspectral microscope designed to characterize in situ the texture and composition of the surface materials of the Hayabusa2 target asteroid. MicrOmega is implemented within the MASCOT lander (Ho et al. in Space Sci. Rev., 2016, this issue, doi:10.1007/s11214-016-0251-6). The spectral range (0.99-3.65 μm) and the spectral sampling ( $20~\mbox{cm}^{-1}$ ) of MicrOmega have been chosen to allow the identification of most potential constituent minerals, ices and organics, within each 25 μm pixel of the $3.2\times 3.2~\mbox{mm}^{2}$ FOV. Such an unprecedented characterization will (1) enable the identification of most major and minor phases, including the potential organic phases, and ascribe their mineralogical context, as a critical set of clues to decipher the origin and evolution of this primitive body, and (2) provide the ground truth for the orbital measurements as well as a reference for the analyses later performed on returned samples. [ABSTRACT FROM AUTHOR]

Abstract: The Mars Express Observatoire pour la Minéralogie, l''Eau, les Glaces et l''Activité (OMEGA) collected an unprecedented visible and near-infrared hyperspectral dataset covering the low albedo regions of Mars. We investigate the ability to infer modal abundance of surfaces of these regions from a radiative transfer model developed by Shkuratov et al. [Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G., 1999. Icarus 137, 235–246] and adapted to basaltic surfaces by Poulet and Erard [Poulet F., Erard, S., 2004. J. Geophys. Res. 109 (E2), doi:10.1029/2003JE002179]. From OMEGA measurements of mafic surfaces, we develop several sensitivity tests to assess the extent to which the model can be applied to predict pyroxene composition (high-calcium phase and low-calcium phase), abundance of almost neutral components (plagioclase) in the near-infrared wavelength as well as grain sizes, by using a library of selected end-members. Results of the sensitivity tests indicate that the scattering model can estimate both abundances and grain sizes of major basaltic materials of low albedo regions within uncertainties (±5 to 15 vol%). The model is then applied to data from dissected cratered terrains located in Terra Meridiani. The derived grain size including uncertainties is in the 50–500 μm range. This is consistent with the thermal inertia and albedo of this region, which indicates a fine sand-sized surface with little dust. The abundances of plagioclase (43–57%) and pyroxenes (, including of low-calcium phase) are in good agreement with previous basalt-like compositions of low albedo regions from thermal infrared spectral measurements. The method presented in this paper will provide a valuable tool for evaluating the modal mineralogy of other mafic regions of Mars observed in the near-infrared wavelength range. [Copyright &y& Elsevier]

Abstract: We present an analysis comparing observations acquired by the Mars Express Observatoire pour la Minéralogie l’Eau, les Glaces et l’Activité (OMEGA) and Phoenix lander measurements. Analysis of OMEGA data provides evidence for hydrous and ferric phases at the Phoenix landing site and the surrounding regions. The 3μm hydration band deepens with increasing latitude, along with the appearance and deepening of a 1.9μm H2O band as latitude increases ∼60° polewards. A water content of 10–11% is derived from the OMEGA data for the optical surface at the Phoenix landing site compared to 1–2% derived for subsurface soil by Phoenix lander measurements. The hydration of these regions is best explained by surface adsorbed water onto soil grains. No evidence for carbonate or perchlorate-bearing phases is evident from OMEGA data, consistent with the relatively small abundances of these phases detected by Phoenix. The identification of spectral features consistent with hydrated phases (possibly zeolites) from OMEGA data covering regions outside the landing site and the ubiquitous ferric absorption edge suggest that chemical weathering may play a role in the arctic soils. [Copyright &y& Elsevier]

Highlights • We combined here for the first time all OMEGA NIR observations in the form of a 3D global-cube. • This new product is complete, well calibrated and adapted to fast analyses of the Martian composition. • It also can be used to illustrate the surface variability (in terms of albedo as well as composition for example). • Spectral indices can easily be applied to retrieve composition on the Martian surface. • Radiative transfer model can also be envisioned to retrieve modal mineralogy. Abstract This paper is the first paper of a series that will present the derivation of the modal mineralogy of Mars (M3 project) at a global scale from the near-infrared dataset acquired by the imaging spectrometer OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) on board ESA/Mars Express. The objective is to create and provide a global 3-D image-cube of Mars at 32 px/° covering most of Mars surface. This product has several advantages. First, it can be used to instantaneously extract atmospheric- and aerosol-corrected near-infrared (NIR) spectra from any location on Mars. Second, several new data maps can be built as discussed here. That includes new global mineral distributions, quantitative mineral abundance distributions and maps of Martian surface chemistry (wt% oxide) detailed in a companion paper (Riu et al, 2018). Here we present the method to derive the global hyperspectral cube from several hundred millions of spectra. Global maps of some mafic minerals are then shown, and compared to previous works. [ABSTRACT FROM AUTHOR]

The objective of this study is to identify and map spectral analogues of some key martian meteorites (basaltic shergottites Los Angeles, Shergotty, QUE 94201, lherzolitic shergottite ALH A77005, Nakhla, Chassigny and the orthopyroxenite ALH 84001) in order to localize terrain candidates for their source regions. We develop a best fit procedure to reproduce the near-infrared (NIR) spectral properties of the martian surface as seen by the hyperspectral imaging spectrometer OMEGA/MEx from the NIR spectra of the martian meteorites. The fitting process is tested and validated, and Root Mean Square (RMS) global maps for each meteorite are obtained. It is found that basaltic shergottites have NIR spectral properties the most representative of the martian surface with the best spectral analogues found in early Hesperian volcanic provinces. Sites with spectral properties similar to those of ALH A77005 are scarce. They are mainly localized in olivine-bearing regions such as Nili Fossae and small Noachian/early Hesperian terrains. The only plausible source region candidate for Chassigny is the Nili Patera caldera dated to 1.6 Ga. Widespread spectral analogues for the ALH 84001 meteorite are found northeast of Syrtis Major and northwest of the Hellas basin. While this distribution is in agreement with the low-calcium-pyroxene-rich composition and old age (4.1 Ga) of this meteorite, the modal mineralogy of these candidates is not consistent with that of this meteorite. No convincing spectral analogue is found for the Amazonian-aged Nakhla meteorite suggesting that its olivine/high-calcium-pyroxene-rich composition could be representative of the Amazonian terrains buried under dust. Finally, some young rayed craters are proposed as possible candidates for source craters of the studied martian meteorites. [ABSTRACT FROM AUTHOR]

The area of Meridiani Planum on Mars became of particular interest after the detection of coarse-grained, gray hematite, which led to the choice of this region as final landing site for the Mars Exploration Rover Opportunity. Multiple additional minerals have since been detected in the region, both from orbit and in situ. The present paper reassesses in detail the mineralogy and geomorphology of the area (between 3.5°S to 6.5°N latitude and 8.0°W to 8.0°E longitude) using visible and near-infrared hyperspectral data, merged with high spatial resolution images into a Geographic Information System. Fe/Mg-rich phyllosilicates, as well as several types of sulfates are identified within the kilometer of sedimentary deposits constituting the etched terrains and the overlying hematite-rich plains. The mineralogical stratigraphy of the etched terrains is characterized by a sulfate-rich unit, enriched in Fe/Mg-rich clays in its uppermost part. This clay-rich horizon is capped by another sulfate-rich unit, of different composition, associated with hematite detections and constituting the material of the hematite plains unit. The clay-bearing unit, which directly underlies the hematite plains, may be exposed in the rim of Endeavour Crater and could correspond to the material that is currently being analyzed by Opportunity. Diverse landforms including karsts and pan features are observed at the surface of the hydrated etched terrains and indicate past surface water and potential groundwater aquifers. Our analysis reveals that both surficial water and groundwater processes are required to explain the diversity of morphologies and mineralogies observed in the area. Although orbital detections are consistent with the hematite and sulfate detections made in situ by Opportunity in the hematite plains, they also show the presence of a clay-rich horizon at depth, overlying another, thicker, possibly chemically distinct, sulfate-rich unit within the etched terrains. Therefore we conclude that the very acidic conditions that prevailed during the formation of the topmost sulfate-rich unit, as observed in Meridiani Planum and in the hematite plains by the rover Opportunity, are not representative of the entire history of the etched terrains. In contrast, most hydrated minerals in Meridiani Planum likely formed at more neutral pH, at the beginning of the Hesperian. The overall sulfate/clay/sulfate sequence is similar to the one observed in Gale Crater, the landing site of the Mars Science Laboratory rover. [ABSTRACT FROM AUTHOR]

Abstract: The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1km to 42km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20km in diameter). Most detected minerals formed over relatively high temperatures (100–300°C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains. [Copyright &y& Elsevier]

Abstract: The 174km diameter Terby impact crater (28.0°S–74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100m to 1km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6–10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000km3 (a large amount considering classical martian fan deltas such as Eberswalde (6km3)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy. [Copyright &y& Elsevier]

Abstract: An extensive layered formation covers the high plateaus around Valles Marineris. Mapping based on HiRISE, CTX and HRSC images reveals these layered deposits (LDs) crop out north of Tithonium Chasma, south of Ius Chasma, around West Candor Chasma, and southwest of Juventae Chasma and Ganges Chasma. The estimated area covered by LDs is ∼42,300km2. They consist of a series of alternating light and dark beds, a 100m in total thickness that is covered by a dark unconsolidated mantle possibly resulting from their erosion. Their stratigraphic relationships with the plateaus and the Valles Marineris chasmata indicate that the LDs were deposited during the Early- to Late Hesperian, and possibly later depending on the region, before the end of the backwasting of the walls near Juventae Chasma, and probably before Louros Valles sapping near Ius Chasma. Their large spatial coverage and their location mainly on highly elevated plateaus lead us to conclude that LDs correspond to airfall dust and/or volcanic ash. The surface of LDs is characterized by various morphological features, including lobate ejecta and pedestal craters, polygonal fractures, valleys and sinuous ridges, and a pitted surface, which are all consistent with liquid water and/or water ice filling the pores of LDs. LDs were episodically eroded by fluvial processes and were possibly modified by sublimation processes. Considering that LDs correspond to dust and/or ash possibly mixed with ice particles in the past, LDs may be compared to Dissected Mantle Terrains currently observed in mid- to high latitudes on Mars, which correspond to a mantle of mixed dust and ice that is partially or totally dissected by sublimation. The analysis of CRISM and OMEGA hyperspectral data indicates that the basal layer of LDs near Ganges Chasma exhibits spectra with absorption bands at ∼1.4μm, and ∼1.9μm and a large deep band between ∼2.21 and ∼2.26μm that are consistent with previous spectral analysis in other regions of LDs. We interpret these spectral characteristics as an enrichment of LDs in opaline silica or by Al-phyllosilicate-rich layers being overlain by hydroxylated ferric sulfate-rich layers. These alteration minerals are consistent with the aqueous alteration of LDs at low temperatures. [Copyright &y& Elsevier]

Abstract: The Mawrth Vallis region contains an extensive (at least 300km×400km) and thick (⩾300m), finely layered (at meter scale), clay-rich unit detected by OMEGA. We use OMEGA, HRSC DTMs derived from stereoscopic imagery, HRSC color imagery and high resolution imagery such as MOC, CTX and HiRISE to characterize the geometry and the composition of the clay-rich unit at the regional scale. Our results show that the clay-bearing unit can be divided into sub-units on the basis of differences in color and composition. In false-color visible imagery, alternating white/bluish and orange/red colored units correspond to a compositional succession of, respectively, Al- and Fe- or Mg-phyllosilicate rich material. Geological cross-sections are presented along the principal outcrops of the region in order to define the stratigraphy of these sub-units. This method shows that the dips of the sub-units are frequently close to the slopes of the present topography, except for scarps visible at the dichotomy boundary, inside impact craters walls, and outcrops inside Mawrth Vallis. In addition to the Al- and Fe- or Mg-phyllosilicate rich sub-units, an altered surface is identified as the lower basement unit. We propose two possible end-member scenarios to explain the derived stratigraphy: (1) alteration of volcaniclastic, aeolian or aqueous layered deposits of various compositions by groundwater, resulting in distinct altered rocks; or (2) Alteration coeval with the deposition of sediments under varying chemical conditions, in wet pedodiagenetic environment. [Copyright &y& Elsevier]

Abstract: This paper reports on mapping of water frost and ice on Mars, in the range of latitudes between 30°S and 30°N. The study has been carried out by analysing 2485 orbits acquired during almost one martian year by the Mars Express/OMEGA imaging spectrometer. Water frost/ice is identified by the presence of ∼1.5μm, ∼2μm and ∼3.0μm absorptions. Although the orbits analysed in this study cover all seasons, water frost/ice is observed only near the aphelion seasons, at Ls=19° and at Ls=98–150°. Water frost/ice is detected mainly on the southern hemisphere between 15°S and 30°S latitude while it has not been identified within 15°S–15°N. In the northern hemisphere, the water frost/ice detection is complicated by the presence of clouds. Usually, water frost/ice is found in shadowed areas, while in few cases it is exposed to the sunlight. This indicates a clear relationship with the local illumination conditions on the slopes which favour the water frost/ice deposition on the surface when the temperatures are very low. OMEGA observations span from 10 to 17LT and the frost/ice is detected mainly between 15 and 16LT, with practically no detection before 13LT. We think this is due to the fact that the 10–12LT observations occur at large distances and it is not a local time effect. A thermal model is used to determine the deposition conditions on the sloped surfaces where water frost/ice has been found. There, daily atmospheric saturation does not occur on pole facing 10–25° slopes with current water vapour abundances but only by assuming values greater than 40prμm. Moreover, the water frost/ice is not detected during the northern winter, even if the thermal model foresees daily saturation on 25° slopes. [Copyright &y& Elsevier]

Abstract: The primary objectives of this paper are to determine the modal mineralogy of selected low albedo terrains of different ages ranging from Noachian to Amazonian exposed on the surface of Mars. This analysis is conducted using the spectral modeling of the Observatoire pour la Minéralogie, l''Eau, les Glaces, et l''Activité (OMEGA) reflectance data. Results from this work are consistent with the major results of previous spectroscopic studies: plagioclase (40–60% in volume) and high calcium pyroxene (20–40%, HCP) are the dominant minerals of the most regions. Low calcium pyroxene (10–15%, LCP) and minor amounts of olivine are also present. The oldest terrains are characterized by the largest amount of LCP and the lowest concentration of plagioclase. These overall compositions are consistent with two-pyroxene basalts. The particle sizes are in the range of a few hundreds of micrometers, which is in good agreement with the thermal inertia of the martian low albedo regions. In the region around the Nili Fossae, localized concentrations of olivine up to 40% with millimeter particle size similar to picritic basalts observed in situ by the Spirit rover in the Gusev crater are inferred. Chemical compositions are calculated for the first time from OMEGA spectra. They are quite consistent with Gusev rocks and shergottite compositions but they appear to be significantly SiO2-poorer than Thermal Emission Spectrometer data. A decreasing low calcium pyroxene abundance with the decreasing age of the low albedo regions is reported. This may be indicative of decreasing degree of partial melting as thermal flux decreases with time. We propose that the ancient Noachian-aged, LCP-rich terrains could have been formed from H2O-bearing melts. Then, dry, basaltic volcanism occurred leading to decreasing LCP abundance with time due to decreasing degree of partial melting. The olivine-bearing material modeled in Nili Fossae resembles the composition of ALH77005 and Chassigny meteorites consistent with prior studies. Implications on the formation of the basaltic Shergottites are discussed. [Copyright &y& Elsevier]

Abstract: The mineralogical composition of the Martian surface is investigated by a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully covered the surface of the red planet at medium to low resolution (2–4km per pixel). Several areas have been imaged at a resolution up to 300m per pixel. One difficulty in the data processing is to extract the mineralogical composition, since rocks are mixtures of several components. MELSUM is an algorithm that selects the best linear combination of spectra among the families of minerals available in a reference library. The best fit of the observed spectrum on each pixel is calculated by the same unmixing equation used in the classical Spectral Mixture Analysis (SMA). This study shows the importance of the choice of the input library, which contains in our case 24 laboratory spectra (endmembers) of minerals that cover the diversity of the mineral families that may be found on the Martian surface. The analysis is restricted to the 1.0–2.5μm wavelength range. Grain size variations and atmospheric scattering by aerosols induce changes in overall albedo level and continuum slopes. Synthetic flat and pure slope spectra have therefore been included in the input mineral spectral endmembers library in order to take these effects into account. The selection process for the endmembers is a systematic exploration of whole set of combinations of four components plus the straight line spectra. When negative coefficients occur, the results are discarded. This strategy is successfully tested on the terrestrial Cuprite site (Nevada, USA), for which extensive ground observations exist. It is then applied to different areas on Mars including Syrtis Major, Aram Chaos and Olympia Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region dominated by mafic minerals, with the oldest crustal regions composed of a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis volcanic edifice appears depleted in LCP (OPx) and enriched in HCP (CPx), which is consistent with materials produced with a lower partial fusion degree at an age younger to the surrounding crust. Strong olivine signatures are found between the two calderas Nili Patera and Meroe Patera and in Nili Fossae. A strong signature of iron oxides is found within Aram Chaos, with a spatial distribution also consistent with thermal emission spectrometer (TES). Gypsum is unambiguously detected in the northern polar region, in agreement with the study of Langevin et al. [2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars Express. Science 307(5715), 1584–1586]. Our results show that the linear spectral unmixing provides good first order results in a variety of mineralogical contexts, and can therefore confidently be used on a wider scale to analyze the complete archive of OMEGA data. [Copyright &y& Elsevier]

Abstract: Analyses of Mars Express OMEGA hyperspectral data (0.4–2.7 μm) for Terra Meridiani and western Arabia Terra show that the northern mantled cratered terrains are covered by dust that is spectrally dominated by nanophase ferric oxides. Dark aeolian dunes inside craters and dark streaks extending from the dunes into the intercrater areas in mantled cratered terrains in western Arabia Terra have similar pyroxene-rich signatures demonstrating that the dunes supply dark basaltic material to create dark streaks. The dissected cratered terrains to the south of the mantled terrains are dominated spectrally by both low-calcium and high-calcium pyroxenes with abundances of 20–30% each retrieved from nonlinear radiative transfer modeling. Spectra over the hematite-bearing plains in Meridiani Planum are characterized by very weak but unique spectral features attributed to a mixture of a dark and featureless component (possibly gray hematite) and minor olivine in some locations. Hydrated minerals (likely hydrous ferric sulfates and/or hydrous hydroxides) associated with poorly ferric crystalline phases are found in the etched terrains to the north and east of the hematite-bearing plains where erosion has exposed of section of layered outcrops with high thermal inertias. These materials are also found in numerous craters in the northern Terra Meridiani and may represent outliers of the etched terrain materials. A few localized spots within the etched terrain also exhibit the spectral signature of Fe-rich phyllosilicates. The ensemble of observations show that the evidence for aqueous processes detected by the Opportunity Rover in Meridiani Planum is widespread and confirms the extended presence of surface or near-surface water over this large region of Mars. The scenarios of formation of Terra Meridiani (“dirty” acidic evaporite, impact surge or weathering of volcanic ash) cannot satisfactorily explain the mineralogy derived from the OMEGA observations. The formation of the etched terrains is consistent with leaching of iron sulfides and formation of sulfates and hydrated iron oxides, either in-place or via transport and evaporation of aqueous fluids and under aqueous conditions less acidic than inferred from rocks examined by Opportunity. [Copyright &y& Elsevier]