Polarized absorption spectra of single crystals of lunar pyroxenes and olivines

Measurements have been made of the polarized absorption spectra (360-2200 nm.) of compositionally zoned pyroxene minerals in rocks 10045, 10047 and 10058 and olivines in rocks 10020 and 10022. Specimens in the form of petrographic thin sections were mounted on polarizing microscopes equipped with three-axis universal stage attachments and inserted into a Cary 17 spectrophotometer. The Apollo 11 pyroxenes with relatively high Ti/Fe ratios were chosen initially to investigate the presence of crystal field spectra of Fe²⁺ and Ti³⁺ ions in the minerals.Broad intense bands at about 1000 and 2100 nm. arise from spin-allowed, polarization-dependent transitions in Fe²⁺ ions in pyroxenes. Several weak sharp peaks occur in the visible region. Peaks at 402, 425, 505, 550 and 585 nm. represent spin-forbidden transitions in Fe²⁺ ions, while broader bands at 460–470 nm. and 650–660 nm. are attributed to Ti³⁺ ions. Charge transfer bands, which in terrestrial pyroxenes often extend into the visible region, are displaced to shorter wavelengths in lunar pyroxenes. This feature correlates with the absence of Fe³⁺ ions in these minerals. The magnitudes of the intensity ratios: band 465 nm. (Ti³⁺) to band 1000 nm. (Fe²⁺) are similar to Ti/Fe ratios from lunar pyroxene bulk chemical analyses, suggesting that an appreciable amount of titanium occurs as Ti³⁺ ions in the lunar pyroxenes. The 505 nm. spin-forbidden peak in Fe²⁺, together with absorption at 465 nm. by Ti³⁺, contribute to the pink or pale reddish-brown colors of lunar pyroxenes in transmitted lights.The absorption spectral measurements not only provide information on the redox behavior and crystal chemistry of lunar pyroxenes, but also form a basis for interpreting spectral reflectivity properties of lunar rocks and the Moon's surface.     

Exploration of faint absorption bands in the reflectance spectra of the asteroids by method of optimal smoothing: Vestoids

An optimization method of smoothing noisy spectra was developed to investigate faint absorption bands in the visual spectral region of reflectance spectra of asteroids and the compositional information derived from their analysis. The smoothing algorithm is called “optimal” because the algorithm determines the best running box size to separate weak absorption bands from the noise. The method is tested for its sensitivity to identifying false features in the smoothed spectrum, and its correctness of forecasting real absorption bands was tested with artificial spectra simulating asteroid reflectance spectra. After validating the method we optimally smoothed 22 vestoid spectra from SMASS1 [Xu, Sh., Binzel, R.P., Burbine, T.H., Bus, S.J., 1995. Icarus 115, 1-35]. We show that the resulting bands are not telluric features. Interpretation of the absorption bands in the asteroid spectra was based on the spectral properties of both terrestrial and meteorite pyroxenes. The bands located near 480, 505, 530, and 550 nm we assigned to spin-forbidden crystal field bands of ferrous iron, whereas the bands near 570, 600, and 650 nm are attributed to the crystal field bands of trivalent chromium and/or ferric iron in low-calcium pyroxenes on the asteroids' surface. While not measured by microprobe analysis, Fe³⁺ site occupancy can be measured with Mössbauer spectroscopy, and is seen in trace amounts in pyroxenes. We believe that trace amounts of Fe³⁺ on vestoid surfaces may be due to oxidation from impacts by icy bodies. If that is the case, they should be ubiquitous in the asteroid belt wherever pyroxene absorptions are found. Pyroxene composition of four asteroids of our set is determined from the band position of absorptions at 505 and 1000 nm, implying that there can be orthopyroxenes in all range of ferruginosity on the vestoid surfaces. For the present we cannot unambiguously interpret of the faint absorption bands that are seen in the spectra of 4005 Dyagilev, 4038 Kristina, 4147 Lennon, and 5143 Heracles. Probably there are other spectrally active materials along with pyroxenes on the surfaces of these asteroids.     

Near-infrared reflectance spectroscopy of Ca-rich clinopyroxenes and prospects for remote spectral characterization of planetary surfaces

We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo_45-50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809-28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe²⁺ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe³⁺/Fe²⁺ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe²⁺ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe²⁺ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites) that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.     

Reflectance spectra of analog anorthosites: Implications for lunar highland mapping

Lunar highland region and associated craters are mostly composed of anorthosite. In the present study, we studied the reflectance spectra of terrestrial anorthosites collected from Sittampundi Anorthosites Complex, which is considered as equivalent (simulant) of lunar highland anorthosites. The objective of the study is to interpret diagnostic spectral features of analog anorthosite for remotely exploring lunar highland region. Reflectance spectra of anorthosites were measured under two different environments, such as controlled field and laboratory conditions. In these two procedures, the laboratory spectra give clear, diagnostic spectral information in the present study. Reflectance spectra captured under 350-2500 nm covering UV, Visible, NIR, and SWIR part of the electromagnetic spectrum. The spectral characteristics of anorthosites measured under various parts of electromagnetic spectrum have diagnostic absorption features at 380-387, 700-740, 930-1100, 1160-1200, 1415, 1920, 2200 and 2330 nm correspondingly due to plagioclase UV absorption, Fe³⁺ electron transition absorption, Fe²⁺ pyroxene and olivine absorption, OH/Mn³⁺ crystal transition absorption, pyroxene absorption, Al-OH absorption and Mg-OH absorption. Mineralogical and chemical analyses were carried out for four anorthosites and compared with the results of chemical component of lunar anorthosite. The percentage of plagioclase content, relative abundance of low and high calcium pyroxene and olivine in different anorthosite samples are correlated with the albedo range, absorption shape, absorption centers and band depth. The similarity in the diagnostic spectral features of the anolog anorthosite with lunar anorthosites could be effectively utilized for remotely mapping the lunar highland region.     

Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites

We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56 μm and 2.4 μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra - both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3-2.5 μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56 μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1 μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4-0.5, 0.65, 2.2 μm). The depths of the 0.7, 0.9, and 1.1 μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7 μm region (due to Fe³⁺-Fe²⁺ charge transfers), and the 0.9-1.2 μm region (due to Fe²⁺ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7 μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65 μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral-compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9 μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.     

Near‐infrared spectra of clinopyroxenes: Effects of calcium content and crystal structure

Abstract– Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe²⁺ is responsible for the dominant pyroxene absorptions in the visible and near‐infrared, but substitutions of other cations such as Ca²⁺ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca²⁺, Mg²⁺, and Fe²⁺), we focus on a suite of pyroxenes synthesized with only Ca²⁺, Mg²⁺, and Fe²⁺ in the two octahedral sites, specifically examining the effect of Ca²⁺ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthopyroxenes and low‐Ca clinopyroxenes, Ca²⁺‐content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo₃₀, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca²⁺ is added. At Wo₅₀, when most of the M2 sites are filled by Ca²⁺, band positions do not change drastically, although the presence and strengths of the 1 and 2 μm bands are affected by even trace amounts of Fe²⁺ in the M2 site. It is thus apparent that next‐nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe²⁺ cations and control absorption band properties.     

Ultraviolet spectral reflectance properties of common planetary minerals

Ultraviolet spectral reflectance properties (200-400 nm) of a large number of minerals known or presumed to exist on the surfaces of Mars, the Moon, and asteroids, and in many meteorites, were investigated. Ultraviolet reflectance spectra (200-400 nm) of these minerals range from slightly blue-sloped (reflectance decreasing toward longer wavelengths) to strongly red-sloped (reflectance increasing toward longer wavelengths). Most exhibit one or two absorption features that are attributable to FeO charge transfers involving Fe³⁺ or Fe²⁺. The UV region is a very sensitive indicator of the presence of even trace amounts (<0.01 wt%) of Fe³⁺ and Fe²⁺. The major Fe³⁺O absorption band occurs at shorter wavelengths (∼210-230 nm), and is more intense than the major Fe²⁺O absorption band (∼250-270 nm). Ti-bearing minerals, such as ilmenite, rutile and anatase exhibit UV absorption bands attributable to Ti⁴⁺O charge transfers. While the positions of metal-O charge transfer bands sometimes differ for different minerals, the variation is often not diagnostic enough to permit unique mineral identification. However, iron oxides and oxyhydroxides can generally be distinguished from Fe-bearing silicates in the 200-400 nm region on the basis of absorption band positions. Within a given mineral group (e.g., low-calcium pyroxene, olivine, plagioclase feldspar), changes in Fe²⁺ or Fe³⁺ abundance do not appear to result in a measurable change in absorption band minima positions. Absorption band positions can vary as a function of grain size, however, and this variation is likely due to band saturation effects. The intensity of metal-O charge transfers means that some minerals will exhibit saturated UV absorption bands even for fine-grained (<45 μm) powders. In cases where absorption bands are not saturated (e.g., Fe²⁺O bands in some plagioclase feldspars and pyroxenes), changes in Fe²⁺ content do not appear to cause variations in band position. In other minerals (e.g., olivine), changes in band positions are correlated with compositional and/or grain size variations, but this is likely due to increasing band saturation rather than compositional variations. Overall, we find that the UV spectral region is sensitive to different mineral properties than longer wavelength regions, and thus offers the potential to provide complementary capabilities and unique opportunities for planetary remote sensing.     

Vestoid surface composition from analysis of faint absorption bands in visible reflectance spectra

Faint absorption bands in the visible range of the smoothed vestoid spectra have been found. The bands centered near 505, 530, and 550 nm are attributed to ferrous iron in low-calcium pyroxene and are typical for pyroxene-bearing vestoid surfaces. In accordance with characteristics of the faint absorption bands around 600 and 650 nm the studied vestoid spectra can be sorted into five types. Since the same absorptions are also seen in the laboratory spectra of the minerals and meteorites, which appear to be similar to vestoid material, spectral types of the vestoids can be related to their surface compositions. Regolith of the Type-I vestoids consists of pure low-calcium pyroxenes. Minor amount of olivine along with pyroxene appear to be on the Type-II vestoids whereas the mixtures of low-calcium pyroxene with minor chromite define the Type-III and -IV. The causes for the fifth spectral type in terms of minor mineral phases are unclear now. Simulation of the spectra of vestoids was employed to estimate content of olivine (∼6-12 vol%) and chromite (∼12-30 vol%) on their surfaces.     

Study of the continuum removal method for the Moon Mineralogy Mapper(M~3) and its application to Mare Humorum and Mare Nubium

The absorption band center of visible and near infrared reflectance spectra is a key spectral parameter for lunar mineralogical studies, especially for the mafic minerals(olivine and pyroxene) of mare basalts, which have two obvious absorption bands at 1000 nm(Band I) and 2000 nm(Band II). Removal of the continuum from spectra, which was developed by Clark and Roush and used to isolate the particular absorption feature, is necessary to estimate this parameter. The Moon Mineralogy Mapper(M3) data are widely used for lunar mineral identification. However, M3 data show a residual thermal effect, which interferes with the continuum removal, and systematic differences exist among optical data taken during different optical periods. This study investigated a suitable continuum removal method and compared the difference between two sets of M3 data taken during different optical periods, Optical Period 1B(OP1B)and Optical Period 2A(OP2A). Two programs for continuum removal are reported in this paper. Generally,a program respectively constructs two straight lines across Band I and Band II to remove the continuum,which is recommended for locating band centers, because it can find the same Band I center with different right endpoints. The optimal right endpoint for continuum removal is mainly dominated by two optical period data at approximately 2480 and 2560 nm for OP1 B and OP2 A data, respectively. The band center values derived from OP1 B data are smaller than those derived from OP2 A data in Band I but larger in Band II, especially for the spectra using longer right endpoints(2600 nm). This may be due to the spectral slopes of OP1 B data being steeper than those of OP2 A data in Band I but gentler in Band II. These results were applied to Mare Humorum and Mare Nubium, and the measurements were found to mainly vary from intermediate- to high-Ca pyroxene.     

NIR spectral trends of HED meteorites: Can we discriminate between the magmatic evolution, mechanical mixing and observation geometry effects?

The Howardite–Eucrite–Diogenite (HED) suite is a family of differentiated meteorites that provide a unique opportunity to study the differentiation of small bodies. The likely parent-body of this meteorite group, (4) Vesta is presently under study by the Dawn mission, scrutinizing its surface in the visible and NIR infrared range. Here, we discuss how well the magmatic trends observed in HED might be retrieved from NIR spectroscopy, by studying laboratory spectra of 10 HED meteorites together with spectra from the RELAB database. We show that although an exsolution process did occur for most eucrites (i.e. decomposition of a primary calcic pyroxene into a high-Ca and low-Ca pyroxene), it does not affect the “bulk pyroxene” trend retrieved from the location of the pyroxene crystal field bands (Band I with a maximum of absorption around at about 1 μm and Band II around 2 μm). Absolute values of the chemical composition appears however to deviate from the expected chemical composition. We show that mechanical mixture (i.e. impact gardening) will produce a linear mixing in the pyroxenes band position diagram (Band I position vs Band II position). This diagram also reveals that howardite are not pure mixtures of an average eucrite and average diogenite. Because asteroid surfaces are expected to show topography, we also study the effect of observation geometry on the NIR spectra of an eucrite and a diogenite by measuring the bi-directional reflectance spectra from 0.4 to 4.6 μm. Results show that these meteorites tend to act as forward scatterers, leading to a decrease of integrated band area (relative to the continuum) at high phase angles. The position of the two strong crystal field bands shows only small variability with observation geometry. Retrieval of the magmatic trends from the Band I vs Band II diagram should not be affected by observation geometry effects. Finally we performed NIR reflectance measurement on olivine diogenites. The presence of olivine can be suggested by using the Band Area Ratio vs Band I diagram, but this phase might affect the retrieval of pyroxene composition from the position of Band I and Band II.     

Mineralogy of Asteroid 1459 Magnya and implications for its origin

Detailed near-infrared spectral observations of Asteroid 1459 Magnya reveal an asteroid that is primarily composed of pyroxene and plagioclase feldspar, confirming earlier suggestions that Magnya has a basaltic composition. The average Magnya spectrum for March 23, 2002 has a Band I center of 0.926 μm and a Band II center of 1.938 μm. Observations over  hours show little variation in band center positions. The feldspar-to-pyroxene ratio is ∼0.6 on Magnya's surface. Comparing Magnya with the spectral parameters from 4 Vesta shows discordant pyroxene chemistries; Magnya's pyroxenes contain ∼10 mol% less Fs than Vesta's pyroxenes. This suggests that Magnya originated from a parent body other than 4 Vesta and that its progenitor formed in a more chemically reduced region of the solar nebula within the asteroid belt.     

Detection of ferric iron in an exsolved lunar pyroxene using electron energy loss spectroscopy (EELS): Implications for space weathering and redox conditions on the Moon

To shed light on the mechanism of formation of nanophase iron particles (npFe) in space-weathered materials from airless bodies, we analyzed exsolved and unexsolved space-weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space-weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe⁰, Fe²⁺, and Fe³⁺ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe²⁺ and Fe³⁺ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe³⁺. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe²⁺ and Fe³⁺ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe³⁺ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe³⁺-rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.     

Spectroscopy of synthetic Mg‐Fe pyroxenes I: Spin‐allowed and spin‐forbidden crystal field bands in the visible and near‐infrared

Abstract— Understanding the fundamental crystal chemical controls on visible and near‐infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis of spectroscopic measurements of a comprehensive set of synthetic Mg‐Fe pyroxenes from the visible through the near‐infrared (0.3–2.6 μm) to address the constraints of crystal structure and Fe2+ content on spin‐forbidden and spin‐allowed crystal field absorptions in Ca‐free orthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mössbauer spectroscopy is used to determine site occupancy of Fe2+ in the M1 and M2 crystallographic sites. Properties of visible and near‐infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 μm spin‐allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin‐allowed crystal field absorption band at 1.2 μm is explicitly verified, even at low total iron contents, indicating that some portion of Fe2+ resides in the M1 site. The 1.2 μm band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin‐forbidden crystal field absorptions are observed in all iron‐bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe2+ in the M1 and M2 sites.     

Characterization of the 1.2 μm M1 pyroxene band: Extracting cooling history from near‐IR spectra of pyroxenes and pyroxene‐dominated rocks

Abstract— The 1.2 μm band in near‐infrared spectra of pyroxenes results from Fe²⁺in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near‐infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 μm band as a function of Fe²⁺in the M1 site. Near‐infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe²⁺in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene‐dominated rocks.     

Infrared micro-spectroscopy of the martian meteorite Zagami: Extraction of individual mineral phase spectra

Zagami, a well characterized SNC meteorite, represents a reference sample to verify the feasibility of the non-destructive infrared micro-spectroscopy technique to extract spectral signatures from individual mineral phases in a meteorite sample. For the first time individual infrared spectra of the major mineral phases, in the 6000-600 cm⁻¹ (1.67-16.7 μm) spectral interval, whose identification is confirmed by energy dispersive X-ray analysis and backscattered imaging, are measured. The signatures of the main mineral phases we identified in the Zagami chip are: (1) maskelynite characterized by broad and smooth SiO vibrational bands in the 1000 cm⁻¹ spectral region; (2) crystalline pyroxenes showing well defined fine structures; and (3) an oxide mineral phase with an almost featureless and flat spectrum. In the part of the spectrum centered around 2 μm, by analyzing the different positions of the Fe²⁺ bands, we were able to discern the high-Ca from the low-Ca pyroxene phases. This result demonstrates that by means of the infrared micro-spectroscopy technique it is possible to retrieve directly the composition of pyroxenes in the En-Fs-Wo system, without relying on the use of deconvolution techniques. In addition IR signatures due to water and aliphatic hydrocarbons were observed to be more abundant in the pyroxenes than in maskelynite. This could be an indication that the organic and water signatures are due to indigenous compounds in Zagami rather than laboratory contamination, however, further investigations are necessary before this conclusion can be confirmed.     

Spectral characterization of weathering products of elemental iron in a Martian atmosphere: Implications for Mars hyperspectral studies

Visible and near-infrared spectroscopic properties have been measured on elemental iron experimentally weathered in simulated Martian atmosphere and correlated to mineralogical compositions determined by X-ray diffraction. Two main features are observed in the reflectance spectra, corresponding to two deep bands located at 0.9 and 3.1 μm, respectively the iron band and the hydration band. In early weathering stages both Fe²⁺ and Fe³⁺ bands are identified. In addition, whereas the water band position does not change with time, the Fe²⁺ band disappears, and the Fe³⁺ band shifts towards longer wavelength (from 0.88 to 0.92 μm) because of transition from Fe²⁺ phases (siderite) to Fe³⁺ phases (ferrihydrite and goethite). Apart from these spectral signatures, other bands more specific of each phase are not clearly evidenced, especially for siderite. This is due to relatively low abundance of siderite (<20 wt%), but also to the very small grain size of secondary phase as well as surface coatings of iron (oxy)hydroxides. Therefore, our results suggest that carbonates, even if not detected, could be present in the form of very small grains in the surface of Mars.     

Complexities in pyroxene compositions derived from absorption band centers: Examples from Apollo samples,HED meteorites,synthetic pure pyroxenes,and remote sensing data

We reexamine the relationship between pyroxene composition and near‐infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two‐part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simple approach which can easily be automated and applied to remote sensing data. Employing a suite of synthetic pure pyroxenes, the PLC technique is shown to derive similar band centers to the modified Gaussian model. PLC analyses are extended to natural pyroxene‐bearing materials, including (1) bulk lunar basalts and pyroxene separates, (2) diverse lunar soils, and (3) HED meteorites. For natural pyroxenes, the relationship between composition and absorption band center differs from that of synthetic pyroxenes. These differences arise from complexities inherent in natural materials such as exsolution, zoning, mixing, and space weathering. For these reasons, band center measurements of natural pyroxene‐bearing materials are compositionally nonunique and could represent three distinct scenarios (1) pyroxene with a narrow compositional range, (2) complexly zoned pyroxene grains, or (3) a mixture of multiple pyroxene (or nonpyroxene) components. Therefore, a universal quantitative relationship between band centers and pyroxene composition cannot be uniquely derived for natural pyroxene‐bearing materials without additional geologic context. Nevertheless, useful relative relationships between composition and band center persist in most cases. These relationships are used to interpret M³ data from the Humboldtianum Basin. Four distinct compositional units are identified (1) Mare Humboldtianum basalts, (2) distinct outer basalts, (3) low‐Ca pyroxene‐bearing materials, and (4) feldspathic materials.     

Lithological mapping of central part of Mare Moscoviense using Chandrayaan-1 Hyperspectral Imager (HySI) data

We conducted spectral analysis of central region of the Mare Moscoviense area on the far side of the Moon using the Hyperspectral Imager (HySI) data from the Chandrayaan-1 mission in an effort to identify and map the major lithological units present in the area. Various spectral band parameters, namely, band curvature, band tilt and band strength have been used for lithological discrimination based on the nature of the spectral profile. These band parameters essentially measure the shape, position and strength of the absorption feature near 1000 nm arising due to electronic transition of Fe²⁺ in crystallographic sites of major rock forming silicates. Spectral band parameters have been used for generating rock type composite image. Based on spectral studies and rock type composite image as obtained using band parameters, five major compositional units have been identified: highland basin soils, ancient mature mare, highland contaminated mare, buried unit with abundant low-Ca pyroxene (LCP), and youngest mare unit. In the present study, a multispectral approach in the form of spectral band parameters has been adopted for analysing the HySI hyperspectral data from Chandrayaan-1 mission. Present study clearly shows that the spectral band parameters obtained using selected HySI channels could efficiently be used to discriminate and delineate the major litho-units present across the central part of Mare Moscoviense and the same approach can thus be used for lithological mapping of other parts of lunar surface using HySI data.     

Near-infrared reflectance spectra from bulk samples of the two Martian meteorites Zagami and Nakhla

Abstract— Zagami and Nakhla are achondrites and belong to the Shergotty-Nakhla-Chassigny (SNC) meteorite group. It is generally accepted that Mars is their parent body. Mineralogical and chemical analyses have revealed that the major mineral phases of these two meteorites are pyroxene, olivine, maskelynite, and plagioclase. In this work, near-infrared biconical reflectance measurements were performed on sawed surfaces of chips from Zagami and Nakhla. Spectra obtained with an analytical spot diameter on the order of the mineral grain size reflect the heterogeneous distribution of different mineral phases. The characteristic absorption bands of the pyroxenes are numerically evaluated in terms of the modified Gaussian model. Spectra with overlapping absorption features are resolved into the basic absorption bands. From these results, it can be estimated what kind of clinopyroxenes belong to the investigated mineral assemblages. As a result, the major clinopyroxene phase in Nakhla is Ca-rich augite, whereas in Zagami both Ca-rich and Ca-poor pyroxenes are present. By means of such a procedure, laboratory spectra of minerals become more informative and may help in discussing Martian remote sensing data in the near-infrared region.     

Spectral reflectance properties of carbonaceous chondrites: 6. CV chondrites

Multiple reflectance spectra of 11 CV chondrites have been measured to determine spectral–compositional relationships for this meteorite class and to aid the search for CV parent bodies. The reflectance of CV chondrite spectra is variable, ranging from ～5% to 13% at 0.56 μm, and ～5% to 15% at the 0.7 μm region local reflectance maximum. Overall slopes range from slightly blue to red for powders, while slab spectra are strongly blue-sloped. With increasing average grain size and/or removal of the finest fraction, CV spectra generally become more blue-sloped. CV spectra are characterized by ubiquitous absorption features in the 1 and 2 μm regions. The 1 μm region is usually characterized by a band centered near 1.05–1.08 μm and a band or shoulder near 1.3 μm that are characteristic of Fe-rich olivine. Band depths in the 1 μm region for powdered CVs and slabs range from ～1% to 10%. The 2 μm region is characterized by a region of broad absorption that extends beyond 2 μm and usually includes band minima near 1.95 and 2.1 μm; these features are characteristic of Fe²⁺-bearing spinel. The sample suite is not comprehensive enough to firmly establish whether spectral differences exist between CV_R, CV_OxA, and CV_OxB subclasses, or as a function of metamorphic grade. However, we believe that the mineralogic and petrologic differences that exist between these classes, and with varying petrologic subtype (CV3.0–>3.7), may not be significant enough to result in measurable spectral differences that exceed spectral variations within a subgroup, within an individual meteorite, or as a function of grain size. Terrestrial weathering seems to affect CV spectra most noticeably in the visible region, resulting in more red-sloped spectra for finds as compared to falls. The search for CV parent bodies should focus on the detection of olivine and spinel absorption bands, specifically absorption features near 1.05, 1.3, 1.95, and 2.1 μm, as these are the most commonly seen spectral features of CV chondrites.