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The region surrounding the Mawrth Vallis outflow channel on Mars hosts thick layered deposits containing diverse phyllosilicate minerals. Here we report detection of the Ca-sulfate bassanite on the outflow channel floor, requiring a more complex aqueous chemistry than previously inferred for this region. The sulfate-bearing materials underlie phyllosilicate-bearing strata, and provide an opportunity for testing proposed models of martian geochemical evolution with a future landed mission.

Previous orbital mapping of crystalline gray haematite, ferric oxides, and sulfates has shown an association of this mineralogy with light-toned, layered deposits on the floor of Valles Marineris, in chaos terrains in the canyon’s outflow channels, and in Meridiani Planum. The exact nature of the relationship between ferric oxides and sulfates within Valles Marineris is uncertain. The Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite (OMEGA) spectrometer initially identified sulfate and ferric oxides in the layered deposits of Valles Marineris. The Thermal Emission Spectrometer (TES) has also mapped coarse (gray) haematite in or at the base of these deposits. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra and Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) imagery from the Mars Reconnaissance Orbiter (MRO) to explore the mineralogy and morphology of the large layered deposit in central Capri Chasma, part of the Valles Marineris canyon system that has large, clear exposures of sulfate and haematite. We find kieserite (MgSO 4 ·H 2 O) and ferric oxide (often crystalline red haematite) in the lower bedrock exposures and a polyhydrated sulfate without ferric oxides in the upper bedrock. This stratigraphy is duplicated in many other basinal chasmata, suggesting a common genesis. We propose the haematite and monohydrated sulfate formed by diagenetic alteration of a sulfate-rich sedimentary deposit, where the upper polyhydrated sulfate-rich, haematite-poor layers either were not buried sufficiently to convert to a monohydrated sulfate or were part of a later depositional phase. Based on the similarities between the Valles Marineris assemblages and the sulfate and haematite-rich deposits of Meridiani Planum, we hypothesize a common evaporite and diagenetic formation process for the Meridiani Planum sediments and the sulfate-bearing basinal Interior Layered Deposits.

New high-resolution spectral and morphologic imaging of deposits on walls and floor of Ius Chasma extend previous geomorphic mapping, and permit a new interpretation of aqueous processes that occurred during the development of Valles Marineris. We identify hydrated mineralogy based on visible-near infrared (VNIR) absorptions. We map the extents of these units with CRISM spectral data as well as morphologies in CTX and HiRISE imagery. Three cross-sections across Ius Chasma illustrate the interpreted mineral stratigraphy. Multiple episodes formed and transported hydrated minerals within Ius Chasma. Polyhydrated sulfate and kieserite are found within a closed basin at the lowest elevations in the chasma. They may have been precipitates in a closed basin or diagenetically altered after deposition. Fluvial or aeolian processes then deposited layered Fe/Mg smectite and hydrated silicate on the chasma floor, postdating the sulfates. The smectite apparently was weathered out of Noachian-age wallrock and transported to the depositional sites. The overlying hydrated silicate is interpreted to be an acid-leached phyllosilicate transformed from the underlying smectite unit, or a smectite/jarosite mixture. The finely layered smectite and massive hydrated silicate units have an erosional unconformity between them, that marks a change in surface water chemistry. Landslides transported large blocks of wallrock, some altered to contain Fe/Mg smectite, to the chasma floor. After the last episode of normal faulting and subsequent landslides, opal was transported short distances into the chasma from a few m-thick light-toned layer near the top of the wallrock, by sapping channels in Louros Valles. Alternatively, the material was transported into the chasma and then altered to opal. The superposition of different types of hydrated minerals and the different fluvial morphologies of the units containing them indicate sequential, distinct aqueous environments, characterized by alkaline, then circum-neutral, and finally very acidic surface or groundwater chemistry.

Geochemical models for Mars predict carbonate formation during aqueous alteration. Carbonate-bearing rocks had not previously been detected on Mars' surface, but Mars Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock units and probably formed during the Noachian or early Hesperian era from the alteration of olivine by either hydrothermal fluids or near-surface water. The presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments.

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

Spectroscopic studies of analog terrestrial mineral assemblages are necessary to develop criteria to identify similar environments on Mars. We use visible/near infrared (VNIR) laboratory, field, and remotely acquired spectral data to identify the iron-bearing and hydrous minerals of Rio Tinto, Spain, an astrobiological analog. Mineralogy evolves from iron sulfate- and oxide-rich (jarosite, rozenite, gypsum, schwertmannite, copiapite, goethite, and hematite assemblages) in young sediments to hydrated iron oxides in preserved terraces. Using spectra from the Rio Tinto, we examine one of the key challenges of extraterrestrial exploration: how to identify promising targets from spatially coarse data for in situ investigation. We apply an index to quantify the expression of spectral diversity as a function of spatial scale from hand sample to landscape. To validate this method for use at the decimeter orbital scale, we apply the index to cm-scale point spectra and meter-scale gridded spectra collected in the field. This exercise in spatial scaling gives increased confidence in the ability of the Spectral Variance Index (SVI) method to locate regions with increased mineral diversity from remotely sensed data. We divide the remotely sensed data into 25 × 25 pixel (200 m × 200 m) cells and calculate the average mean (albedo) and spectral variance over all wavelengths for each cell. We next calculate the expected variance for each cell with a linear regression between mean and spectral variance. The number of standard deviations of each cell's spectral variance is from the expected variance is the SVI value. We locate ∼ 20 areas with high SVI values within the tailing piles and along wide riverbanks downstream of the active mine. This method uses spatially coarse VNIR spectra to recognize areas in Rio Tinto that would be ideal targets for future field exploration, and could also be applied to Mars orbital spectral datasets, such as OMEGA and CRISM.

[1] A closed depression in the Noctis Labyrinthus region of Mars (at 10.4°S, 98.6°W), believed to have formed in the Late Hesperian, holds an inner pit partially filled with several hundred meters of stratified material. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible-near infrared reflectance data reveal signatures of numerous hydrated minerals including halloysite/kaolinite, Fe-smectite, Si-OH bearing phases and Fe-sulfates (polyhydrated, monohydrated, and hydroxylated types, including jarosite). We use CRISM data, high resolution imagery (HiRISE) and HRSC (High Resolution Stereo Camera) derived elevation to analyze the morphology, composition and stratigraphy of these materials. We propose an alteration sequence including formation of acid sulfate solutions from groundwater and magmatic sulfur, which then locally altered the basaltic bedrock and layered sediments mainly deposited from volcanic tephra, forming Fe-smectite and Fe-sulfates. The mineral variability can mostly be explained by local variations in the pH of the altering fluids, with original acidity being buffered by dissolution of primary minerals; and by variable fluid input and evaporation and/or freezing rates (resulting in various water/rock ratios). This site shows local formation of almost all classes of minerals identified thus far on Mars without invoking global conditions. Processes related to local volcanic activity and associated hydrothermalism were able to produce, during an era in which the climate is believed to have been cold, a large variety of hydrated minerals. This study highlights the importance of the geological setting of hydrated minerals in the understanding of Mars geologic and climatic evolution.

Two troughs in Noctis Labyrinthus display a diversity of mineral assemblages rarely seen spatially collocated on Mars. Minerals identified from Mars Reconnaissance Orbiter data within the troughs include polyhydrated and monohydrated sulfates, an Al clay (e.g., kaolinite or beidellite), Fe/Mg smectites, hydrated silica and/or opal, and a leached clay or jarosite mixture with a doublet absorption between 2.2 and 2.3 μm. Units both pre-date and post-date smaller pits and depressions within the larger troughs, indicating that deposition was coeval with continued extension, collapse, and erosion in the Late Hesperian to Early Amazonian (2–3 Ga). The strata within each trough display a mineralogic diversity consistent with active aqueous processes and/or changing chemical conditions over time, perhaps due to hydrothermal alteration of volcanic ash, influxes of groundwater from nearby Tharsis volcanism, fumarole activity, and melting snow and/or ice. The superposition of younger Fe/Mg smectites over sulfates, Al clays, and hydrated silica and/or opal in both troughs indicates that this region is unique relative to most other locations on Mars, where the opposite progression is observed and the Fe/Mg smectites are Noachian (older than 3.6 Ga) in age. Consequently, these troughs may have been habitable regions on Mars at a time when drier conditions dominated the surface.

[1] Sedimentary deposits within the 280 km wide crater containing Aram Chaos (∼3°N, 339°E) have been differentially eroded by wind to expose a stratigraphic column 900–1000 m thick that unconformably overlies the chaos bedrock. A detailed stratigraphic and mineralogical description of the deposits is presented based on data from the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars, Context Imager, and High Resolution Imaging Science Experiment. Two sedimentary units overlie the basement chaos material representing the original plains fill in Aram Crater: the first and oldest is composed of (1) a 50–75 m thick dark-toned basal unit containing ferric hydroxysulfate intercalated with monohydrated-sulfate-bearing materials, (2) a 75–100 m thick light-toned unit with monohydrated sulfates, and (3) a 175–350 m thick light-toned resistant capping unit with nanophase ferric oxides and monohydrated sulfates. After a period of wind erosion, these deposits were partially and unconformably covered by the second sedimentary unit, a 75–100 m thick, discontinuous dark-toned unit containing crystalline hematite and polyhydrated sulfate material. These sedimentary deposits were formed by evaporite deposition during at least two distinct rising groundwater episodes fed by regional-scale recharge. Later groundwater event(s) formed the polyhydrated materials, indicating that environmental conditions changed to a higher water-to-rock ratio. Wind has continued to shape the landscape after the last wetting event to produce the features and exposures observed.

Definitive exposures of pristine, ancient crust on Mars are rare, and the finding that much of the ancient Noachian terrain on Mars exhibits evidence of phyllosilicate alteration adds further complexity. We have analyzed high-resolution data from the Mars Reconnaissance Orbiter in the well-exposed Noachian crust surrounding the Isidis basin. We focus on data from the Compact Reconnaissance Imaging Spectrometer for Mars as well as imaging data sets from High Resolution Imagine Science Experiment and Context Imager. These data show the lowermost unit of Noachian crust in this region is a complex, brecciated unit of diverse compositions. Breccia blocks consisting of unaltered mafic rocks together with rocks showing signatures of Fe/Mg-phyllosilicates are commonly observed. In regions of good exposure, layered or banded phyllosilicate-bearing breccia rocks are observed suggestive of pre-Isidis sedimentary deposits. In places, the phyllosilicate-bearing material appears as a matrix surrounding mafic blocks, and the mafic rocks show evidence of complex folded relationships possibly formed in the turbulent flow during emplacement of basin-scale ejecta. These materials likely include both pre-Isidis basement rocks as well as the brecciated products of the Isidis basin–forming event at 3.9 Ga. A banded olivine unit capped by a mafic unit covers a large topographic and geographic range from northwest of Nili Fossae to the southern edge of the Isidis basin. This olivine-mafic cap combination superimposes the phyllosilicate-bearing basement rocks and distinctly conforms to the underlying basement topography. This may be due to draping of the topography by a fluid or tectonic deformation of a previously flatter lying morphology. We interpret the draping, superposed olivine-mafic cap combination to be impact melt from the Isidis basin–forming event. While some distinct post-Isidis alteration is evident (carbonate, kaolinite, and serpentine), the persistence of olivine from the time of Isidis basin suggests that large-scale aqueous alteration processes had ceased by the time this unit was emplaced.

[1] This paper brings together initial results obtained of the high northern latitudes in Mars years 28 and 29, between October 2006 and October 2008. These measurements confirm many previous models and shed new light on the nature of polar surface materials, particularly in intermediate-albedo units of the polar layered deposits, many of which are found to be ice-rich. We identify hydrated non ice materials present in many low-albedo troughs, as well as in the circumpolar erg that was previously associated with gypsum. We identify icy outlier deposits that may be related to subsurface thermophysical properties and permafrost. New observations of the gypsum-rich dune material constrain models for its formation and distribution. Intrinsic properties of ice content and grain size are found to be independent of the albedo of fine layered units and may provide a novel method for stratigraphic identification and correlation.

[1] Juventae Chasma contains four light-toned sulfate-bearing mounds (denoted here as A–D from west to east) inside the trough, mafic outcrops at the base of the mounds and in the wall rock, and light-toned layered deposits of opal and ferric sulfates on the plateau. Hyperspectral visible/near-infrared Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra were used to identify monohydrated and polyhydrated sulfate (PHS) outcrops of layered material on the bright mounds. Most of the monohydrated sulfate signatures closely resemble those of szomolnokite (FeSO4·H2O), characterized by a water band near 2.08 μm, while some areas exhibit spectral features more similar to those of kieserite (MgSO4·H2O), with a band centered closer to 2.13 μm. The largest PHS outcrops occur on the top of mound B, and their spectral features are most consistent with ferricopiapite, melanterite, and starkeyite, but a specific mineral cannot be uniquely identified at this time. Coordinated analyses of CRISM maps, Mars Orbiter Laser Altimeter elevations, and High Resolution Imaging Science Experiment images suggest that mounds A and B may have formed together and then eroded into separate mounds, while mounds C and D likely formed separately. Mafic minerals (low-Ca pyroxene, high-Ca pyroxene, and olivine) are observed in large ∼2–10 km wide outcrops in the wall rock and in smaller outcrops ∼50–500 m across at the floor of the canyon. Most of the wall rock is covered by at least a thin layer of dust and does not exhibit strong features characteristic of these minerals. The plateau region northwest of Juventae Chasma is characterized by an abundance of light-toned layered deposits. One region contains two spectrally unique phases exhibiting a highly stratified, terraced pattern. CRISM spectra of one unit eroded into swirling patterns with arc-like ridges exhibit a narrow 2.23-μm band assigned to hydroxylated ferric sulfate. A thin layer of a fractured material bearing an opaline silica phase is observed at the contact between the older plateau unit and the younger hydroxylated ferric sulfate-bearing light-toned layered deposits. Hydrothermal processes may have produced an acidic environment that fostered formation of the hydrated silica and hydroxylated ferric sulfate units.

The Noachian terrain west of the Isidis basin hosts a diverse collection of alteration minerals in rocks comprising varied geomorphic units within a 100,000 km2 region in and near the Nili Fossae. Prior investigations in this region by the Observatoire pour l'Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) instrument on Mars Express revealed large exposures of both mafic minerals and iron magnesium phyllosilicates in stratigraphic context. Expanding on the discoveries of OMEGA, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) has found more spatially widespread and mineralogically diverse alteration minerals than previously realized, which represent multiple aqueous environments. Using CRISM near-infrared spectral data, we detail the basis for identification of iron and magnesium smectites (including both nontronite and more Mg-rich varieties), chlorite, prehnite, serpentine, kaolinite, potassium mica (illite or muscovite), hydrated (opaline) silica, the sodium zeolite analcime, and magnesium carbonate. The detection of serpentine and analcime on Mars is reported here for the first time. We detail the geomorphic context of these minerals using data from high-resolution imagers onboard MRO in conjunction with CRISM. We find that the distribution of alteration minerals is not homogeneous; rather, they occur in provinces with distinctive assemblages of alteration minerals. Key findings are (1) a distinctive stratigraphy, in and around the Nili Fossae, of kaolinite and magnesium carbonate in bedrock units always overlying Fe/Mg smectites and (2) evidence for mineral phases and assemblages indicative of low-grade metamorphic or hydrothermal aqueous alteration in cratered terrains. The alteration minerals around the Nili Fossae are more typical of those resulting from neutral to alkaline conditions rather than acidic conditions, which appear to have dominated much of Mars. Moreover, the mineralogic diversity and geologic context of alteration minerals found in the region around the Nili Fossae indicates several episodes of aqueous activity in multiple distinct environments.

[1] New results from the Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment and Context Imager cameras on Mars Reconnaissance Orbiter provide insights into the origin of interior layered deposits in Valles Marineris from analysis of a thick, well-exposed section in western Candor Chasma. Most of the deposit is dominated spectrally by nanophase ferric oxide like that found in the globally distributed eolian dust, with the addition of a prevalent component of monohydrated sulfates. A rippled mantle containing both pyroxene and monohydrated sulfate emanates from discrete layers, which are interpreted as interbedded basaltic sand. Ferric minerals are observed in most of the sulfate-rich layers, and locally a coarse-grained grayer component has been concentrated from the layers by sorting. Polyhydrated sulfates are concentrated in discrete layers high in the section, implying chasma-scale changes in brine chemistry during formation of the layered deposits. Hydrological models were constructed in order to assess whether evaporite deposition from groundwater discharge could have trapped eolian sediments to form the observed deposits. The predicted thickness and extent of the evaporite-trapped sediment is consistent with the distribution of interior layered deposits in Candor Chasma as well as in other chasmata of Valles Marineris. In this scenario, eolian dust and sand were trapped and lithified by evaporites formed by evaporation of groundwater discharge that was highly localized within the chasmata. Sulfates precipitated in the resulting saline conditions, and diagenetic alteration formed crystalline ferric minerals including hematite. This model links the layered deposits in Valles Marineris and those in Meridiani Planum to a common regional process.

International audience; The East Candor Interior Layered Deposit (ILD) has signatures of mono- and polyhydrated sulfate in alternating layers that give insight into the processes which formed these layered deposits and on the environmental conditions acting on them since then. We use orbital data to explore multiple hypotheses for how these deposits formed: (1) sulfate-bearing ILDs experience hydration changes on seasonal to a few years timescales under current Mars environmental conditions; (2) the deposits experience hydration under recent Mars conditions but require the wetter climate of high obliquity; and (3) the kieserite could be an original or diagenetic part of a complex evaporite mineral assemblage. Modeled climatology shows recent Mars environmental conditions might pass between multiple sulfate fields. However, comparison of Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) and Compact Reconnaissance Imaging Spectrometer (CRISM) observations of the same ILD do not show changes in hydration over 2 Mars years. Low temperatures might slow the kinetics of that transition; it is likely that more clement conditions during periods of high obliquity are needed to overcome mineral metastability and hydrate kieserite-bearing deposits. We find the alternate model, that the deposit is a cyclic evaporite sequence of mono- and polyhydrated sulfates, also plausible but with an unexplained dearth of Fe sulfates.

The previous discovery of extensive deposits of hydrated minerals in Olympia Planum in the north polar region of Mars by the Mars Express OMEGA instrument raises important questions about the origin and subsequent redistribution of these hydrated minerals. Here we present a new map of the distribution of hydrated minerals within the north polar region of Mars by applying both standard and new spectral analysis techniques to near-infrared spectral data from OMEGA. Our results are in agreement with the previous OMEGA observations but also show more extensive detections of hydrated minerals throughout the circumpolar plains, as well as new detections of hydrated minerals on the surface of Planum Boreum and within the polar troughs. We find that while the circumpolar plains hydration signatures appear to be correlated with the dark dunes of the north polar erg, hydration signatures in Planum Boreum instead appear to be correlated with the north polar veneers and their sources within the polar layered deposits. By applying laboratory-derived empirical models of the dependence of gypsum spectra on grain size and abundance, we provide approximate abundance estimates for the hydrated minerals we have identified in Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) and Compact Reconnaissance Imaging Spectrometer (CRISM) data. We find that the presence of hydrated minerals throughout the north polar region suggests (1) a complex cycle of sediment exchange between the Olympia Planum dunes and the other polar units; (2) an earlier origin for the hydrated minerals than originally postulated; and (3) the occurrence of significant water activity in this region during the Amazonian. This work was supported by grants from the Mars Data Analysis Program under contracts from NASA, the Mars Odyssey Participating Scientist program under contracts from the Jet Propulsion Laboratory, and the Canadian Space Agency. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2008JE003187

Hydrated minerals on Mars are most commonly found in ancient terrains dating to the first billion years of the planet’s evolution. Here we discuss the identification of a hydrated light-toned rock unit present in one Chasma of the Noctis Labyrinthus region. Stratigraphy and topography show that this alteration layer is part of a thin unit that drapes pre-existing bedrock. CRISM spectral data show that the unit contains hydrated minerals indicative of aqueous alteration. Potential minerals include sulfates such as bassanite (CaSO4� 1/2H2O) or possibly hydrated chloride salts. The proximity of a smooth volcanic plain and the similar crater model age (Late Amazonian