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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]

Abstract: Aluminous clay deposits on Mars are recognized from remotely sensed infrared spectral features similar to those of montmorillonite, beidellite, and/or kaolinite. The nature of aluminous clay deposits on Mars is of interest because they likely indicate a different formation mechanism than that of Fe–Mg clays, which are widespread on Mars and likely alteration products of the Fe–Mg-rich basaltic crust. The near-infrared reflectance spectra of aluminous martian clay deposits frequently display characteristics typical of both montmorillonite and kaolinite. The question arises whether such mixed character is due to the existence of end-member phases or to kaolinite–smectite mixed-layer (K–S). The issue is relevant because K–S implies the existence of a smectite precursor that alters into kaolinite, and thus constrains the timing and intensity of the alteration processes that generates it. A mixture of kaolinite and smectite end-members may indicate locally heterogeneous alteration processes, or alternatively, could result from the physical mixing of altered materials of different provenance. A group of natural K–S samples and synthetic kaolinite/smectite mixtures of known proportion, all of which had been thoroughly characterized in previous work using several analytical techniques, were investigated here using near-infrared (NIR) spectroscopy. The NIR spectral features correlate well with their kaolinite–smectite relative proportions. The shape of spectral features attributed to Al–OH in K–S is subtly different from those in physical mixtures of kaolinite and smectite. Based on qualitative comparison, some regions on Mars appear to have spectral signatures similar to K–S. We also applied a quantitative technique using the second derivative of spectra. In this technique, plots of the height of the features at (λ=) 2.21μm (band present in kaolinite and montmorillonite) and 2.17μm (kaolinite only) were able to discriminate between K–S and kaolinite–smectite physical mixtures, as they generated correlations with different slopes. The method of discrimination was applied to Mars spectra, which resulted in reasonable evidence for the existence of K–S in Nili Fossae and Mawrth Vallis, and mixtures of end-members in Mawrth Vallis and Leighton Crater. This is one of the first times that evidence for mixed-layer clay minerals, and particularly K–S, on Mars has been gathered. The ability to detect mixed-layer clays is an important step forward for further development of our understanding of the processes that generated clay on Mars. [Copyright &y& Elsevier]

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]

Abstract: Diverse phyllosilicate deposits discovered previously in the Nili Fossae region with near infrared reflectance data are a window into the complex history of aqueous alteration on Mars. In this work, we used thermal infrared data from the Thermal Emission Spectrometer (TES) in combination with near infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) to better constrain the mineralogy and geologic origin of these deposits. We developed a TES spectral index for identification of clay minerals, which correctly identifies the phyllosilicates in the Nili Fossae area and points to several other interesting deposits in the Syrtis Major region. However, detailed inspection of the TES spectral features of Nili Fossae phyllosilicates shows a feature at low wavenumbers (350–550cm−1) that is not an exact match to any specific Fe3+-, Al-, or Mg-rich phyllosilicate phase. Instead, the feature is more similar to basaltic glass and may indicate that the phyllosilicates in this region are: (1) rich in Fe2+ (based on similarity to trends seen in laboratory data of clay minerals), (2) poorly crystalline/extremely disordered, and/or (3) present within a matrix of actual basalt glass. This feature is similar to spectral features seen in altered rocks in the Columbia Hills region of Gusev Crater by previous authors. By calibrating measured spectral index values against mathematical spectral mixtures of typical martian dark surfaces and known abundances of alteration minerals, we are able to estimate an enrichment in abundance of alteration minerals in the altered surfaces. Many dark, Noachian deposits in the Nili Fossae area are enriched phyllosilicates by 20–30% (±10–15%) relative to dark, volcanic surfaces in the same region. The distribution and abundance of these phases indicates that alteration in the region was pervasive, but did not completely erase the original mineralogy of what was likely an Fe-rich basalt protolith. As a group, the Nili Fossae phyllosilicate deposits are fundamentally different from those found in the Mawrth Vallis region. Nili Fossae deposits have strong thermal infrared features related to admixed pyroxene, plagioclase, and occasionally olivine, whereas the Mawrth Vallis deposits contain no mafic minerals. Comparison of TES and OMEGA data also illustrates some more general differences between the datasets, including the impact of physical character of the martian surface on detectability of minerals in each spectral range. [Copyright &y& Elsevier]

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]

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: The Thermal Emission Spectrometer (TES) has observed a high-silica material in the dark regions of Mars that is spectrally similar to obsidian glass and may have a volcanic origin. An alternate interpretation is that the spectrally amorphous material consists of clay minerals or some other secondary material, formed by chemical alteration of surface rocks. The regions where this material is observed (e.g., Acidalia Planitia) have relatively high spectral contrast, suggesting that the high-silica material exists as coarse particulates, indurated soils or cements, within rocks, or as indurated coatings on rock surfaces. The geologic interpretation of this spectral result has major implications for understanding magmatic evolution and weathering processes on Mars. One of the complications in interpreting spectral observations of glasses and clay minerals is that both are structurally and compositionally complex. In this study, we perform a detailed spectroscopic analysis of indurated smectite clay minerals and relate their thermal emission spectral features to structural and crystal chemical properties. We examine the spectral similarities and differences between smectite clay minerals and obsidian glass from a structural-chemical perspective, and make further mineralogical interpretations from previous TES results. The results suggest that neither smectite clays nor any clay mineral with similar structural and chemical properties can adequately explain TES observations of high-silica materials in some martian dark regions. If the spectrally amorphous materials observed by TES do represent an alteration product, then these materials are likely to be poorly crystalline aluminosilicates. While all clay minerals have Si/O ratios 0.4, the position of the emissivity minimum at Mars suggests a Si/O ratio of 0.4–0.5. The spectral observation could be explained by the existence of a silica-rich alteration product, such as Al- or Fe-bearing opal, an intimate physical mixture of relatively pure silica and other aluminosilicates (such as clay minerals or clay precursors), or certain zeolites. The chemical alteration of basaltic rocks on Mars to phyllosilicate-poor, silica-rich alteration products provides a geologically reasonable and consistent explanation for the global TES surface mineralogical results. [Copyright &y& Elsevier]

Visible/Near Infrared Reflectance spectroscopy (350–2500 nm) is an efficient and effective tool to identify and semi-quantify clay minerals, Fe3+ (oxyhydr)oxides, and their crystal-chemistry/structure. Even so, there have been few studies systematically describing how spectroscopy can be applied to mineralogical evolution during weathering. This study investigated reflectance spectra of a long core of a thick basaltic weathering sequence in Hainan Island and a series of transformation products from montmorillonite to kaolinite to present spectral features associated with weathering trends. In combination with X-ray diffraction and geochemical results, mineralogical transitions and weathering intensity of the core samples were described from a spectroscopic perspective. Several different protoliths were spectroscopically recognized in the Hainan weathering sequence, consistent with geological observation and geochemical analysis. Overprinted on these multiple protoliths was a discernable weathering sequence observable by infrared spectroscopic methods. Clay minerals changed upwards in section from TOT type (nontronite) to TO type (kaolinite), and finally to O type (gibbsite) hydroxide clay-size particles as the result of gradual hydrolysis of silicates and leaching of mobile elements with weathering. The relative abundance of hydroxyl to interlayer water, observable by the band depth ratio of spectral absorptions around 1400 nm and 1900 nm (BD1400/BD1900), increased progressively and correlated positively with the chemical index of alteration and Al 2 O 3 concentration. The increasing BD1400/BD1900 index value with weathering was the result of gradual leaching of interlayer cations (e. g. Na+, K+), and their water complexes within interlayers, and gradual formation of M-OH layers by hydrolysis. Owing to leaching of Mg and release of Fe, the octahedral M-OH formed by hydrolysis was gradually dominated by Al-OH. Reflectance spectroscopy is a powerful tool to identify protolith changes in weathering sequences and for studying clay mineralogical transitions and weathering intensity. This spectroscopic approach could have great value for understanding weathering related to paleoclimate on Earth or other planets using field spectroscopy and remote sensing. • • Infrared reflectance spectroscopy was applied to a drill core through weathered rocks. • • Spectral indices of clay minerals were developed to characterize weathering trends. • • Ratios of spectral band depths measure interlayer water and hydroxyl content. • • These indices can be applied to remote sensing data of Mars. [ABSTRACT FROM AUTHOR]