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The primary objective of NASA's Mars Science Laboratory (MSL) mission, which will launch in 2011, is to characterize the habitability of a site on Mars through detailed analyses of the composition and geological context of surface materials. Within the framework of established mission goals, we have evaluated the value of a possible landing site in the Mawrth Vallis region of Mars that is targeted directly on some of the most geologically and astrobiologically enticing materials in the Solar System. The area around Mawrth Vallis contains a vast (>1 × 10⁶ km²) deposit of phyllosilicate-rich, ancient, layered rocks. A thick (>150 m) stratigraphic section that exhibits spectral evidence for nontronite, montmorillonite, amorphous silica, kaolinite, saponite, other smectite clay minerals, ferrous mica, and sulfate minerals indicates a rich geological history that may have included multiple aqueous environments. Because phyllosilicates are strong indicators of ancient aqueous activity, and the preservation potential of biosignatures within sedimentary clay deposits is high, martian phyllosilicate deposits are desirable astrobiological targets. The proposed MSL landing site at Mawrth Vallis is located directly on the largest and most phyllosilicate-rich deposit on Mars and is therefore an excellent place to explore for evidence of life or habitability.

Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)-smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. A spectral feature attributed to an Fe2+ phase is present in many locations in the Mawrth Vallis region at the transition from Fe/Mg-smectite to aluminum/silicon (Al/Si)-rich units. Fe2+-bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica, and then Al-phyllosilicates implies a complex aqueous history.

A population of underrecognized volcanic constructs associated with explosive and effusive volcanism is present within Arabia Terra and perhaps throughout the ancient highlands on Mars. These features, termed "plains‐style caldera complexes," are characterized by 10s‐of‐km wide crustal collapse structures, lava flows, and potential pyroclastic deposits, but without a pronounced central edifice. Vast layered deposits around Eden Patera and throughout Arabia Terra are possibly linked to these explosive volcanisms. Remote sensing‐based morphological and geologic mapping was carried out along with geomorphological and structural analyses, and the application of terrestrial analogs to test the hypothesis that these large‐scale collapse features are controlled by magmatism and volcanic activity. Here, we map the surface geology, structure, and geomorphology of Eden Patera, providing a well‐exposed and documented benchmark for the identification of other plains‐style caldera complexes across the surface of the Martian crust. The unexplored population of explosive process‐dominated volcanic constructs could potentially be an important yet underrecognized aspect of the Martian geologic record. Plain Language Summary: Perhaps the most recognizable image of a volcano is a topographic rise, such as a cone or mountain, flanked by lava flows. While this image fits many volcanic settings on Earth and Mars, many known volcanic settings on Earth do not fit this iconic description at all and the question remains open as to whether that exception might also be true for Mars, where observations are limited to remote sensing interpretations. Several volcanoes in Arabia Terra on Mars lack recognizable topographic features, suggesting an important form of explosive volcanism associated with crustal collapse. These source vents could have produced huge amounts of ash airfall deposits, now widely observed throughout the region as layered deposits. These features are likely more common on Mars than is currently recognized and are likely a critical component of Mars' geologic puzzle. This work describes in detail the geological characteristics of Eden Patera (a large caldera with diameter over 60 km), located within Arabia Terra on Mars. This site is a "type‐locality" to which other collapsed calderas on Mars can be compared. Mapping results from Eden Patera illuminate a complex geologic history driven by both explosive (ash‐dominated) and effusive (lava‐dominated) eruptions. Key Points: Evidence for multiple episodes of caldera collapse at Eden Patera, a type‐example of plains style caldera complex on MarsRegional volcanic activity around Eden PateraComparison to Kīlauea Iki provides evidence for a lava lake [ABSTRACT FROM AUTHOR]

Arabia Terra is an ancient, geologically complex region of Mars. It is not typically described as volcanic because it lacks prominent, easily recognizable shield volcanoes. However, Arabia Terra contains flood lavas and widespread layered materials that might represent airfall pyroclastics from unknown sources. Here we explore the geology of some complex topographic depressions in Arabia Terra previously hypothesized to represent collapse features termed "plains style caldera complexes," an enigmatic category of volcanic feature on Mars characterized by surface collapse rather than construction of topographic shields. Geomorphological observations and geologic mapping carried out here support a volcanic origin of these features, providing a possible source for volcaniclastic sediments. The identification of caldera complexes in Arabia Terra raises the questions of how they are linked to the geotectonic setting of the region and whether similar features might be much more abundant in the ancient Martian crust than is currently recognized. Plain Language Summary: The most recognizable volcanoes on Mars correspond to large topographic shields, similar to Hawaiian volcanoes. These are constructional features built by layer upon layer of lava, with a summit caldera. It is possible that Mars contains many other volcanoes that are less recognizable, however, because they consist only of the caldera complex without the topographic rise built by lava. These calderas could signify much more explosive volcanism or collapse due to migration of magma in the subsurface, or both. In this work, we describe more identifications of plains style caldera complexes, which are collapsed calderas without the easily recognizable shield topography. We describe the geology of these calderas in the Arabia Terra region, their possible links to tectonic setting there, and the possibility that similar features are much more common on Mars than is currently understood. Key Points: Collapsed calderas are recognized in eastern Arabia TerraAncient Martian crust may have far more collapsed calderas than commonly recognizedPlains style caldera complexes could be a source of huge volumes of volcaniclastic materials on Mars [ABSTRACT FROM AUTHOR]

Potassium metasomatism is a common geologic process occurring in a range of environments on Earth. K‐metasomatism can dramatically change the chemistry of large volumes of crust resulting in formation of abundant K‐rich feldspar while preserving original rock textures. Remote sensing of Mars has revealed the presence of rare but striking felspar‐rich terrains as well as K‐rich phyllosilicates such as illite or muscovite. Furthermore, the Curiosity Rover has detected evidence for K‐enrichment in rocks at Gale Crater. In this study, we use spectroscopic and remote sensing analyses of K‐metasomatized rocks on Earth as a comparison to K‐rich and/or possible granitic rocks on Mars. The results suggest that evidence for K‐feldspar rich, quartz‐bearing, and illite‐bearing rocks on Mars is consistent with K‐metasomatism. K‐rich rocks in Gale Crater have themselves likely not been metasomatized, but the abundant feldspar within them might have been derived from metasomatized crust. Plain Language Summary: Mars is a volcanic planet with a surface dominated by basalt and sedimentary rocks derived from basalt, similar to oceanic crust on Earth (though Mars' crust is thicker). For decades, Mars has been considered a "one plate planet" where plate tectonics never occurred and where continental type rocks never formed. Recent observations have, however, revealed enticing evidence that felsic "continental type" rocks containing K‐feldspar and quartz are present. Such rock units were recognized in Gale Crater based on their high potassium abundances. Granite‐type rocks detected from orbit using infrared data would also presumably be rich in potassium. However, rocks of the same composition can be made through aqueous alteration rather than from magma in the crust. This study shows a striking example of potassium enrichment and mineralogical changes in basaltic rocks on Earth caused entirely by low‐temperature hydrothermal brine in the crust. We use analogous remote sensing measurements to study these potassic rocks on Earth and compare the results to Mars. It appears that at least some of the felsic rock compositions on Mars were caused by metasomatism. Key Points: Subsurface brines can deposit significant potassium in volcanic rocks while preserving original texturesSpectral signatures of K‐metasomatism include K‐feldspar and illiteFelsic compositions on Mars might occur because of low‐temperate alteration by subsurface brines [ABSTRACT FROM AUTHOR]

Allophane, a common component on Earth and a probable constituent of the amorphous component on Mars, is closely associated with Fe in the form of structural Fe and/or iron (oxyhydr)oxide coatings. However, until now, the formation and evolution of allophane as products of environmental Fe concentrations have rarely been studied. We investigated allophane precipitation from gels with different Fe/(Fe + Al) molar ratios (n, 0 ≤ n ≤ 1.0). X‐ray diffraction patterns and Fourier transform infrared spectra of the products showed that allophane was nearly the only product at n ≤ 0.2 and that the crystallinity of Fe‐rich allophane decreased with increasing n. Combined with the results of transmission electron microscopy and Mössbauer spectroscopy, Fe was found to be mainly incorporated into the gibbsite‐like sheet of allophane, forming clusters; the highest Fe‐for‐Al substitution content was roughly estimated to be 20 mol.%. As n increased further, the formation of allophane was increasingly suppressed, and the Fe phases began to separate from the Al‐Si phases, resulting in mixtures of incipient allophane and incipient akaganeite and finally akaganeite. The near‐infrared spectroscopic data (1.2–2.6 μm) of the products showed incapability in identifying poorly ordered minerals in Al‐Si‐Fe systems, while the features in the range of 0.4–1.2 μm were powerful for studying iron occurrence in the products. These findings not only offer insights into the formation, evolution and geological role of allophane in Al‐Si‐Fe systems on Earth but also help constrain the paleoenvironment of locations where allophane and akaganeite co‐occur on Mars. Key Points: Environmental Fe concentrations control the formation and evolution of allophane in Al‐Si‐Fe systemsNear‐infrared spectra (1.2–2.6 μm) show incapability in identifying short‐range ordered minerals in Al‐Si‐Fe systemsCo‐occurrence of allophane and akaganeite would constrain the local palaeoenvironment on Mars [ABSTRACT FROM AUTHOR]

McLaughlin crater is a 92‐km diameter complex crater that formed on Mars ~4 billion years ago. The resulting basin was the site of a large (~3,000 km2), deep (~500 m), voluminous (~1,500 km3) Martian lake circa 3.8 Ga. While there is strong evidence that hundreds of lakes have existed on Mars at some point during the same time period, the geology of McLaughlin crater is extraordinary for a number of reasons. Detailed spectral analyses show that the deep‐water sediments include detrital inputs of olivine and pyroxene, but the lake‐floor sediments include lithologies with abundant Fe‐rich, Mg‐bearing smectite, serpentine‐rich deposits, and ferrihydrite. For astrobiologists, this site provides a treasure trove of high‐priority targets. Serpentinization reactions are thought to have played a key role in abiogenesis on Earth, and within McLaughlin crater, deposits of subterranean and probably sublacustrine serpentinites are well preserved. In addition, delta sequences are well exposed throughout the east side of the basin; such deposits are endorsed by some as the highest priority targets for preservation of organics on Mars. Yet deep‐water turbidites, which might have flowed through hydrothermal environments, may be the most intriguing aspect of this geology. Such rapid sedimentation could have sequestered and preserved any potential organic materials for future exploration by a rover. Plain Language Summary: Though Mars is cold, dry, and inhospitable today, it contains evidence for many dry lake beds—relics of an ancient climate that was at least episodically warmer and wetter. Considering that lakes can be an excellent environments for life on Earth, a common thread of Mars research has been to characterize the environmental conditions under which Martian lakes formed in order to better understand their implications for past habitability of the red planet. The goal of this work is to characterize the geology of McLaughlin crater, where a lake existed on Mars over 3.8 billion years ago. McLaughlin lake was vast, deep (~ 500 m), and probably long lived. Many characteristics of McLaughlin lake are unlike those seen in any other ancient lake basins on Mars. This lake contained a range of sedimentary environments including delta deposits, shallow‐water fan deposits, and deep‐water, fine‐grained materials. It is the only known place on Mars where fine‐grained, subaqueous "landslides" called turbidites are identified, and these are important because we know that such deposits can be sites of excellent preservation of organic matter on Earth (due to rapid burial). Key Points: McLaughlin crater contained a deep, voluminous lake >3.8 Ga with features unlike any other known Martian paleolakesSedimentary units include delta deposits, turbidites, and deep‐water sediments; and hydrothermal deposits include serpentinitesGeochemical gradients occurred in a long‐lived lake that was in sustained communication with groundwater and subsurface environments [ABSTRACT FROM AUTHOR]

Earth analogs are indispensable to investigate mineral assemblages on Mars because they enable detailed analysis of spectroscopic data from Mars and aid environmental interpretation. Samples from four sites in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using mineralogical, chemical, and spectroscopic techniques, with a focus on clay minerals and alteration environments. They represent Earth analogs of areas on Mars that underwent acidic alteration. X‑ray diffraction and transmittance mid-infrared data indicate that the rocks were subjected to several degrees of acid alteration corresponding to assemblages characterized by the following mixtures: (1) illite, chlorite, interstratified chlorite-vermiculite, kaolinite-smectite, and kaolinite; (2) illite, kaolinite, and alunite; and (3) jarosite and goethite. According to mineral stability data, these three assemblages correspond to pH values 7–5, 5–3, and <3, respectively. The lack of goethite in the illite-kaolinitealunite assemblage suggests an alteration in reducing conditions. Illite was progressively dissolved by acidic alteration but is sufficiently resilient not to be diagnostic of the intensity of the alteration. Illite and kaolinite were the two most abundant phyllosilicate minerals observed, and the main reaction involving phyllosilicates was the alteration of illite to kaolinite. Mixed-layer phases appeared mainly in the mildest degree of acid alteration, with few exceptions. This suggests a transition from a mechanism dominated by transformation to a mechanism dominated by dissolution-precipitation as the intensity of the acid alteration increases. Our results highlight the sparse kaolinite–alunite occurrences on Mars as worthy of specific investigation. Acid alteration on Mars is expected to be patchy and/or consisting of fine alteration rims. Alunite occurrences on Mars in the absence of goethite may indicate an acid alteration in reducing conditions. Kaolinite produced through acid alteration on Mars is expected to exist mainly as an end-member phase of low crystallinity, which would enhance IR absorption and increase its visibility. [ABSTRACT FROM AUTHOR]

Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal–OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27–2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal–chemical substitution and mixed layering affect the position of the diagnostic metal–OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified di–trioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates. [ABSTRACT FROM AUTHOR]

Highlights: [•] The origin of compositional stratigraphy is a global scale problem on Mars. [•] The Mawrth Vallis area provides insights into key geological relationships. [•] Compositional stratigraphy likely formed from chemical weathering of dust or ash. [•] The source of water was likely acidic ice and snow. [Copyright &y& Elsevier]

Sulfate-dominated sedimentary deposits are widespread on the surface of Mars, which contrasts with the rarity of carbonate deposits, and indicates surface waters with chemical features drastically different from those on Earth. While the Earth's surface chemistry and climate are intimately tied to the carbon cycle, it is the sulfur cycle that most strongly influences the Martian geosystems. The presence of sulfate minerals observed from orbit and in-situ via surface exploration within sedimentary rocks and unconsolidated regolith traces a history of post-Noachian aqueous processes mediated by sulfur. These materials likely formed in water-limited aqueous conditions compared to environments indicated by clay minerals and localized carbonates that formed in surface and subsurface settings on early Mars. Constraining the timing of sulfur delivery to the Martian exosphere, as well as volcanogenic HO is therefore central, as it combines with volcanogenic sulfur to produce acidic fluids and ice. Here, we reassess and review the Martian geochemical reservoirs of sulfur from the innermost core, to the mantle, crust, and surficial sediments. The recognized occurrences and the mineralogical features of sedimentary sulfate deposits are synthesized and summarized. Existing models of formation of sedimentary sulfate are discussed and related to weathering processes and chemical conditions of surface waters. We also review existing models of sulfur content in the Martian mantle and analyze how volcanic activities may have transferred igneous sulfur into the exosphere and evaluate the mass transfers and speciation relationships between volcanic sulfur and sedimentary sulfates. The sedimentary clay-sulfate succession can be reconciled with a continuous volcanic eruption rate throughout the Noachian-Hesperian, but a process occurring around the mid-Noachian must have profoundly changed the composition of volcanic degassing. A hypothetical increase in the oxidation state or in water content of Martian lavas or a decrease in atmospheric pressure is necessary to account for such a change in composition of volcanic gases. This would allow the pre mid-Noachian volcanic gases to be dominated by water and carbon-species but late Noachian and Hesperian volcanic gases to be sulfur-rich and characterized by high SO content. Interruption of early dynamo and impact ejection of the atmosphere may have decreased the atmospheric pressure during the early Noachian whereas it remains unclear how the redox state or water content of lavas could have changed. Nevertheless, volcanic emission of SO rich gases since the late Noachian can explain many features of Martian sulfate-rich regolith, including the mass of sulfate and the particular chemical features (i.e. acidity) of surface waters accompanying these deposits. How SO impacted on Mars's climate, with possible short time scale global warming and long time scale cooling effects, remains controversial. However, the ancient wet and warm era on Mars seems incompatible with elevated atmospheric sulfur dioxide because conditions favorable to volcanic SO degassing were most likely not in place at this time. [ABSTRACT FROM AUTHOR]

Ongoing research on martian meteorites and a new set of observations of carbonate minerals provided by an unprecedented series of robotic missions to Mars in the past 15 years help define new constraints on the history of martian climate with important crosscutting themes including: the CO budget of Mars, the role of Mg-, Fe-rich fluids on Mars, and the interplay between carbonate formation and acidity. Carbonate minerals have now been identified in a wide range of localities on Mars as well as in several martian meteorites. The martian meteorites contain carbonates in low abundances (<1 vol.%) and with a wide range of chemistries. Carbonates have also been identified by remote sensing instruments on orbiting spacecraft in several surface locations as well as in low concentrations (2-5 wt.%) in the martian dust. The Spirit rover also identified an outcrop with 16 to 34 wt.% carbonate material in the Columbia Hills of Gusev Crater that strongly resembled the composition of carbonate found in martian meteorite ALH 84001. Finally, the Phoenix lander identified concentrations of 3-6 wt.% carbonate in the soils of the northern plains. The carbonates discovered to date do not clearly indicate the past presence of a dense Noachian atmosphere, but instead suggest localized hydrothermal aqueous environments with limited water availability that existed primarily in the early to mid-Noachian followed by low levels of carbonate formation from thin films of transient water from the late Noachian to the present. The prevalence of carbonate along with evidence for active carbonate precipitation suggests that a global acidic chemistry is unlikely and a more complex relationship between acidity and carbonate formation is present. [ABSTRACT FROM AUTHOR]

Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars' crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars' atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars' atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet. [ABSTRACT FROM AUTHOR]

The cameras on the Pathfinder probe (Imager for Pathfinder) and the rovers Spirit and Opportunity (Panoramic Camera), Curiosity (Mast Camera) and Perseverance (Mast Camera-Z) produce visible-range spectra of limited wavelength resolution but of great target resolution which allows mineralogical analysis of selected areas within martian rocks. Laboratory spectra of relevant specimens were transformed into the spectra corresponding to each of the above cameras to increase our capability to interpret martian spectral data collected in-situ. The focus was on finding spectral features that can be detected by the cameras on Mars and are diagnostic of specific minerals. The samples are a collection of (1) Fe/Mg-phyllosilicates from submarine hydrothermal sites and (2) of rocks from acid alteration environments containing goethite, hematite, jarosite and Fe-bearing chlorite as these minerals are detectable in the extended visible range. Among all the samples, interstratified glauconite-nontronite has the most unique spectral features and should be easily detectable with the rover cameras. The spectral features of talc from Fe-bearing interstratified specimens are described. These data are especially relevant as glauconite and talc have been proposed to be fairly abundant on Mars and their detections are suggested from remote-sensing near-IR data. Several indices are proposed to assess Fe content on the investigated samples as well as mineral concentration of goethite and hematite. Among these indices, the normalized spectral slope in the range 420–600 nm increases with total Fe content for all samples, whether phyllosilicates, oxides or sulphates (R2 = 0.7–0.8). For pure phyllosilicates, the slope from 600 to 1010 nm decreases with increasing octahedral Fe (R2 = 0.75). An index for goethite produced excellent results assessing goethite concentration (R2 = 0.84). Of all cameras, Mast Camera reproduces the spectra with lowest fidelity and generates the poorest correlations between indices and tested variables. The other three cameras perform similarly. [Display omitted] • Lab spectra of Earth analogues shown with the resolution of martian rover cameras. • Spectral fidelity of Fe-clays and Fe oxides is: IMP, PanCam, MastCam-Z > MastCam. • Interstratified glauconite-nontronite easily recognizable on Mars by rover cameras. • Spectral features of Fe-rich talc and interstratified talc layers are shown. • Spectral indexes correlate well with Fe content and goethite content in samples. [ABSTRACT FROM AUTHOR]

Abstract: Clay mineral-bearing deposits previously discovered on Mars with near infrared () remote sensing data are of major significance for understanding the aqueous history, geological evolution, and past habitability of Mars. In this study, we analyzed the thermal infrared () surface properties of the most extensive phyllosilicate deposit on Mars: the Mawrth Vallis area. Clay mineral-bearing units, which in visible images appear to be relatively light-toned, layered bedrock, have thermal inertia values ranging from 150 to 460 J m−2 K−1 s−1/2. This suggests the deposits are composed of a mixture of rock with sand and dust at 100-meter scales. Dark-toned materials that mantle the clay-bearing surfaces have thermal inertia values ranging from 150 to 800, indicating variable degrees of rockiness or induration of this younger sedimentary or pyroclastic unit. Thermal Emission Spectrometer (TES) spectra of the light-toned rocks were analyzed with a number of techniques, but none of the results shows a large phyllosilicate component as has been detected in the same surfaces with near-infrared data. Instead, TES spectra of light-toned surfaces are best modeled by a combination of plagioclase feldspar, high-silica materials (similar to impure opaline silica or felsic glass), and zeolites. We propose three hypotheses for why the clay minerals are not apparent in thermal infrared data, including effects due to surface roughness, sub-pixel mixing of multiple surface temperatures, and low absolute mineral abundances combined with differences in spatial sampling between instruments. Zeolites modeled in TES spectra could be a previously unrecognized component of the alteration assemblage in the phyllosilicate-bearing rocks of the Mawrth Vallis area. TES spectral index mapping suggests that (Fe/Mg)-clays detected with near infrared data correspond to trioctahedral (Fe2+) clay minerals rather than nontronite-like clays. The average mineralogy and geologic context of these complex, interbedded deposits suggests they are either aqueous sedimentary rocks, altered pyroclastic deposits, or a combination of both. [Copyright &y& Elsevier]

Clay minerals detected spectroscopically in the Mawrth Vallis region of Mars correspond to a complex, layered, thick (>600 m) stratigraphic section of ancient bedrock. Because the lighttoned, clay-bearing rocks are lithologically diverse over a broad area (>80,000 km2), have significant internal layering and complexity, and contain buried impact craters within the section (demonstrating that they were deposited over a geologically significant duration of time), we interpret the host rocks as sedimentary or possibly pyroclastic. Crater counts date the claybearing rocks to early-middle Noachian time (estimated as 3.8-4.1 Ga). Geomorphic observations suggest that the rocks were lithified early and deeply eroded during the late Noachian-early Hesperian. The combination of a probable ancient sedimentary context of the clays and a moderate pH formation environment implied by the occurrence of smectites clearly places these rocks among the most important targets for future astrobiological exploration. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the mineralogy of basalt weathering rinds and fresh basaltic rocks using visible/near-infrared (VNIR) (λ =0.4–2.5 μm) and thermal emission (λ =6–30 μm) spectroscopy to 1) constrain the effects of chemical weathering on rock spectra, and 2) further understand the context of infrared spectra of Mars, which may contain evidence for weathered rocks and particulates derived from them. VNIR spectra of weathered rock surfaces are generally redder and brighter than fresh surfaces. Thermal infrared spectra of weathered basalts show evidence for aluminous opal and clay minerals (or clay precursor mineraloids) in natural surfaces. Supporting VNIR observations generally do not show the same evidence for neoformed clays or silica because of their textural occurrence as thin coatings and microfracture-fill, and possibly due to poor crystallinity of the aluminosilicate weathering products in this context. Spectral trends observed at Mars, such as the detection of low to moderate (10–25%) abundances of silica and clay that are observed in the thermal infrared but not in the VNIR, are therefore consistent with trends observed for natural rock surfaces in the laboratory. The combined use of thermal infrared and VNIR suggest that vast areas of martian dark regions contain sandy–rocky basaltic materials with weathering rinds and thin coatings that could have formed in conditions of relatively low water/rock ratios. [Copyright &y& Elsevier]

Abstract A range of phyllosilicate compositions have been detected spectroscopically on Mars, but the largest fraction by far corresponds to clay minerals rich in Fe and Mg. Given that most of our understanding of Martian clays comes from remote sensing data, it is critically important to explore the details of how compositional variation affects spectral features of phyllosilicates. The greatest efforts have focused so far on near-infrared (NIR) spectroscopy. Recently, ambiguities have been detected in the NIR spectra of 2:1 phyllosilicates with intermediate Fe Mg content that preclude mineral and chemical discrimination. Such ambiguities highlight the relevance of exploring the visible spectral range as a complementary tool to characterize Martian phyllosilicates precisely. This article reports the investigation of laboratory reflectance spectra (330–800 nm) from 34 Earth analogues of Martian phyllosilicates with a wide range of Mg Fe composition, including nontronite, celadonite and saponite end-members, as well as interstratified glauconite-nontronite, talc-nontronite, and talc-saponite. The spectra indicated the presence of Fe(III) by absorption modulations and a decrease in total reflectance, especially in samples with tetrahedral Fe(III). Absorption bands at 370 and 420 nm were diagnostic of octahedrically and tetrahedrally coordinated Fe(III), respectively. Band amplitudes in the second derivative of the Kubelka-Munk function correlated positively with Fe(III) content (R 2 > 0.8). Standard color analyses of the visible reflectance spectra under the CIE illuminant D65 indicated that the CIELAB color parameter a * 10 was positively correlated with tetrahedral Fe(III), b * 10 was positively correlated with octahedral Fe(III), and L * 10 was negatively correlated with Fe(III) in both structural sites. Because Fe(II) was in relatively low amount, it did not provide clear spectral evidence. Multiple regression models using the amplitude of the diagnostic absorption bands predicted well absolute Fe content in the phyllosilicates (R 2 = 0.89) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.84). CIELAB color parameters improved the prediction of total Fe (R 2 = 0.92) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.93). Application of these analyses to Martian data has challenges set by Fe oxide dust coating and spatial and spectral resolution. However, these results mark an avenue to develop testable tools using visible-wavelength spectral data from both satellite and lander probes to help establishing Fe content and mineral identification of Martian phyllosilicates. Highlights • Visible-range spectra of phyllosilicates shaped by Fe content and site occupancy. • Phyllosilicate Fe content is predicted well by 2nd-derivative and color analyses. • Vis spectra from Martian rocks with phyllosilicates comparable to lab spectra. • 2nd-derivative and color analyses hopeful tool to estimate Fe in Martian clays. [ABSTRACT FROM AUTHOR]

Transverse Aeolian Ridges (TARs) are the most pervasive aeolian feature on Mars. Their small size requires high-resolution data for thorough analyses. We have utilized Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and High Resolution Image Stereo Experiment (HiRISE) images, along with MRO Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectroscopic data to study TARs in detail. TAR deposits, along with related dark dune material and layered terrains, have been mapped in six study areas in order to identify sediment pathways and determine whether TARs are sourced locally or from global wind-borne materials. TAR morphology and orientation were mapped in grids within each study area; the results show that TARs are probably locally sourced. We constructed four HiRISE Digital Terrain Models (DTMs) to measure TAR heights, widths, spacing, areas, symmetry, and to calculate sediment volumes. Results show that TARs have average heights of ∼1.5 m, are very symmetrical, and are similar in form to terrestrial megaripples. Orthorectified HiRISE images taken 3 years apart were analyzed for TAR movement and none was found. Superposed craters on equatorial TARs give ages of ∼2 Ma, suggesting that these are relatively ancient and generally inactive aeolian deposits. CRISM data were analyzed over TAR deposits, dark dune material, and light-toned terrains. Although the surfaces were somewhat obscured by dust cover, the results did not show any remarkable difference between TARs and other deposits. We conclude that TARs may be sourced from local materials and form in a similar way to terrestrial megaripples. [ABSTRACT FROM AUTHOR]

On Earth, trioctahedral (Mg-rich) vermiculite occurs as a transient alteration product of Mg/Fe2+-mica and chlorite, pyroxene, olivine, amphibole and even plagioclase where there is a nearby Mg source, indicating mild/short-lived weathering or hydrothermal conditions. On Mars, the abundant mafic rocks and low water availability suggest the likelihood of widespread vermiculite formation from the Noachian to the early Hesperian. Although it can be challenging to identify vermiculite by remote sensing or in situ infrared spectroscopy alone, we show that numerous previous remote sensing results suggest that vermiculite is widespread on the martian surface and perhaps a more common mineral on that planet than on Earth. Investigation of rocks from Riotinto (SW Spain) provides an example of how chlorite alters to vermiculite in an acidic environment. Similar incomplete vermiculite replacements of Mg/Fe2+-mica, chlorite and mafic minerals could have resulted on Mars from acidic to neutral weathering at low water regimes. While possible bias exists towards remote sensing detection of vermiculite on Mars in sites (1) with advanced alteration and (2) where vermiculite is not mixed (at pixel level) with other phyllosilicates, we find that vermiculite occurs in Noachian terrains, in sites (wider than pixel level) where other phyllosilicates are detected, and that vermiculite has more Fe3+ than is typical on Earth. [Display omitted] • Vermiculite is a transient alteration product of mafic minerals, mica and chlorite. • Vermiculite is widespread on Mars and found in Noachian altered terrains. • The mineral might be more commonplace on Mars than on Earth. • Most identified vermiculite on Mars so far is Fe3+-rich. • Existing biases suggest more occurrences and wider composition range. [ABSTRACT FROM AUTHOR]

The compositional stratigraphy of Al-rich clays overlying Fe/Mg-rich clays on Mars has been viewed as a window to understanding the atmospheric conditions of early Mars. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) indicates that nontronite is a main component of Fe/Mg-rich clays. However, the role of the ancient climate in the alteration process, which produced and dissolved these phyllosilicates, remains under debate. The present study experimentally modeled the dissolution of nontronite and precipitation of kaolinite in an acidic solution, which enables us to enhance the interpretation of near-infrared remote sensing data from compositional stratigraphy on Mars. The obtained results show that the transformation started from the losses of both tetrahedral Si and interlayer alkali(earth) cations in nontronite, and Fe3+ in the dioctahedral sheet was released and formed hematite with progressive damage of Si O tetrahedral sheets. TG analyses also show that the dissolution of nontronite and precipitation of kaolinite led to an increase in metal-OH (metal = Al, Fe3+ and Mg) content and a simultaneous decrease in H 2 O content. Accordingly, the gradual increasing trend of visible/near-infrared reflectance (VNIR) spectra ratios of BD1400/BD1900 (BD, band depth) in the kaolinized nontronite can be linked to a gradual upward transition from nontronite to a mixture of nontronite and kaolinite, and finally into kaolinite and hematite, by comparing their spectral features at different evolution stages. Such a trend suggests a weathering process known as ferrallitization, i.e., partial leaching of Si and enrichment of Fe3+ and Al3+. The observed weathering process is consistent with a warm and wet climate capable of sustaining acidic liquid water on its surface over extended geological periods. • A semi-quantitative investigation was conducted to examine VNIR proxies for characterizing nontronite ferrallitization. • Formation of hematite and kaolinite in the compositional stratigraphy on Mars was resulted from nontronite ferrallitization. • A warm and wet climate capable of sustaining acidic liquid water possibly existed on martian surface. [ABSTRACT FROM AUTHOR]

• Zhurong landed at 109.92°E, 25.066°N in the southern Utopia Planitia in May 2021. • The landing site exhibits volatile-driven periglacial landforms and pitted cones. • The landing site is located near a key geomorphological boundary between landforms. • Penetrating radar will provide critical ice detection and resource characterization. On May 15, 2021, Tianwen-1's rover Zhurong landed successfully at 109.926°E, 25.066°N, in southern Utopia Planitia on Mars. The Zhurong landing site contains a wide range of geomorphic exploration targets including troughs, raised ridges, pitted cones, mesas, sand dunes and crater ejecta. Aspects of all of these features suggest formation through interactions between volatiles, sediments, and magma. Pitted cones are invaluable windows into the subsurface and intriguing astrobiology targets for Martian life considering that they potentially formed from diapiric upwelling of fine-grained sediments (i.e. mud volcanism), a process that on Earth is often associated with methane release. The ground-penetrating radar onboard Zhurong will provide fundamentally new perspectives on the presence, distribution, and abundance of subsurface water-ice, a strategic natural resource for future crewed Mars exploration. [ABSTRACT FROM AUTHOR]