Band Depth Research Articles

Near-infrared spectroscopy of boulders with dust or patina coatings on the Moon: A two-layer radiative transfer model

Previous remote sensing studies focus on lunar surface regolith, which contains abundant mixtures of rock fragments and dust, making it hard to track the petrologic origin. Igneous boulders exposed on lunar surface, however, carry pristine mineralogy and chemistry since its formation, therefore are direct evidence of lunar thermal evolution events. High spatial-resolution remote sensing images and rover explorations of the Moon allow us to study the spectroscopy of igneous boulders. We modeled the optical scattering properties of rocks using the Legendre and Double Henyey–Greenstein phase functions, porosity parameter and grain size, and provided a modified radiative transfer model for rocks rather than powdered minerals. Considering that space weathering could generate a layer of dust or patina on the surface of boulders, we introduced a two-layer radiative transfer modeling algorithm to solve the spectroscopy of the substrate rock for dust- or patina-coated boulder. The modeled substrate rock spectra show less reddening, larger reflectance, and stronger absorption band depth compared to dust- or patina-coated rock, consistent with the measurements of Apollo rock samples. We applied this two-layer model on the dust-coated boulder detected by Yutu-2 rover and derived the spectrum of the substrate rock. Using Kaguya Multiband Imager data, we calculated the substrate rock spectra for an anorthosite boulder, and our result shows good consistency with laboratory measured anorthosite rock spectrum. This work is a beginning of understanding lunar boulder spectroscopy for a more precise interpretation of lunar thermal history.

Active multiband varifocal metalenses based on orbital angular momentum division multiplexing

Metalenses as miniature flat lenses exhibit a substantial potential in replacing traditional optical component. Although the metalenses have been intensively explored, their functions are limited by poor active ability, narrow operating band and small depth of field (DOF). Here, we show a dielectric metalens consisting of TiO2 nanofins array with ultrahigh aspect ratio to realize active multiband varifocal function. Regulating the orbital angular momentum (OAM) by the phase assignment covering the 2π range, its focal lengths can be switched from 5 mm to 35 mm. This active optical multiplexing uses the physical properties of OAM channels to selectively address and decode the vortex beams. The multiband capability and large DOFs with conversion efficiency of 49% for this metalens are validated for both 532 nm and 633 nm, and the incidence wavelength can further change the focal lengths. This non-mechanical tunable metalens demonstrates the possibility of active varifocal metalenses.

The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It

We present evidence, via a large survey of 191 new spectra along with previously published spectra, of a divide in the 3 μm spectral properties of the low-albedo asteroid population. One group (“sharp types,” or STs, with band centers <3 μm) has a spectral shape consistent with carbonaceous chondrite meteorites, while the other group (“not sharp types,” or NSTs, with bands centered >3 μm) is not represented in the meteorite literature but is as abundant as the STs among large objects. Both groups are present in most low-albedo asteroid taxonomic classes, and, except in limited cases, taxonomic classifications based on 0.5–2.5 μm data alone cannot predict whether an asteroid is an ST or NST. Statistical tests show that the STs and NSTs differ in average band depth, semimajor axis, and perihelion at confidence levels ≥98% while not showing significant differences in albedo. We also show that many NSTs have a 3 μm absorption band shape like comet 67P and likely represent an important small-body composition throughout the solar system. A simple explanation for the origin of these groups is formation on opposite sides of the ammonia snow line, with the NST group accreting H2O and NH3 and the ST group only accreting H2O, with subsequent thermal and chemical evolution resulting in the minerals seen today. Such an explanation is consistent with recent dynamical modeling of planetesimal formation and delivery and suggests that much more outer solar system material was delivered to the main asteroid belt than would be thought based on the number of D-class asteroids found today.

Field-scale spatial correlation between soil and Vis-NIR spectra in the Cerrado biome of Central Brazil

Visible and near-infrared (Vis-NIR) spectroscopy is a tool to determine soil spatial variability and has been used to map soils and their properties. Considering that physical, chemical, mineralogical, and morphological soil properties can affect the intensity and the depth of the spectral reflectance band in the Vis-NIR region, the objectives of this work were to: (i) evaluate the potential of the reflectance inflection difference (RID) to discriminate soils; and (ii) verify potential spatial correlations of the RID with soil properties, compared with the full spectra, in order to build thematic maps at a field scale. In a farm of 375 ha, 78 soil samples from the 0.87–0.92-m depth were collected in a regular grid of 200 m, with a focus on the soil diagnostic horizon (Bw horizon). The sampled soils were a Latossolo Vermelho-Amarelo ácrico (Haplic Ferralsol) and a Latossolo Vermelho distrófico (Rhodic Ferralsol). Twenty-two physical, chemical, mineralogical, and morphological soil properties were determined, and the Vis-NIR spectra between 400 and 2500 nm were measured. Considering the presence of an inflection band and its relationship with soil properties, the spectral bands used to calculate the RID were between (base 1/base 2): 400–510, 730–930, 1290–1450, 1800–1950, 2000–2218, and 2218–2290 nm. The RID failed to map the spatial variability of soil properties, with a Kappa index of 39%; therefore, it is not a good parameter for building thematic maps of soil parameters. In addition, the complete spectrum (mainly in 400–510, 730–930, 1290–1450, 1800–1950, 2000–2218, and 2218–2290 nm) was better spatially correlated with soil properties than the decomposition of the spectrum by the RID. Soil classification and level of discretization as affected by spectral variability were also discussed here. Three soil groups were discriminated mainly by the Ki and Kr indexes and clay content. Moreover, the variability of the spectra was conditioned by the spatial variability of the mentioned variables. The clay content for soils with a discrepant particle size (group 1 compared with groups 2 and 3) and the Ki and Kr indexes for soils with a homogeneous particle size (between groups 2 and 3), associated with the full Vis-NIR spectral analysis, allowed building thematic maps with a good precision, without the need of mathematical models; this was possible by the modification of the reflectance intensity and the size of the concavity of the spectral band.

Automatic element and mineral detection in thin sections using hyperspectral transmittance imaging microscopy (HyperTIM)

Abstract. In this study we present a novel method for the automatic detection of minerals and elements using hyperspectral transmittance imaging microscopy measurements of complete thin sections (HyperTIM). This is accomplished by using a hyperspectral camera system that operates in the visible and near-infrared (VNIR) range with a specifically designed sample holder, scanning setup, and a microscope lens. We utilize this method on a monazite ore thin section from Steenkampskraal (South Africa), which we analyzed for the rare earth element (REE)-bearing mineral monazite ((Ce,Nd,La)PO4), with high concentrations of Nd. The transmittance analyses with the hyperspectral VNIR camera can be used to identify REE minerals and Nd in thin sections. We propose a three-point band depth index, the Nd feature depth index (NdFD), and its related product the Nd band depth index (NdBDI), which enables automatic mineral detection and classification for the Nd-bearing monazites in thin sections. In combination with the average concentration of the relative Nd content, it permits a destruction-free, total concentration calculation for Nd across the entire thin section.

Ultraviolet Photooxidation of Smectite‐Bound Fe(II) and Implications for the Origin of Martian Nontronites

AbstractClay minerals detected with orbital and in situ instruments in ancient Martian terrains constrain Mars' climate and aqueous alteration history. Early in its history, Mars experienced an atmospheric redox change and iron‐bearing clay minerals may preserve the effects of that transition. Ferrous smectites, the thermodynamically predicted product of chemical weathering of basalts under anoxic conditions, may have undergone oxidation by exposure to chemical oxidants in the atmosphere or regolith, or by direct photooxidation at the surface. To assess these potential oxidation pathways, ferrous trioctahedral smectites of varying initial iron content were synthesized and subjected to oxidation by ultraviolet (UV) irradiation. Experimental UV irradiation under an anoxic atmosphere equivalent to approximately 7 years of flux on the Martian surface caused partial oxidation of smectite‐bound Fe (Fe3+/ΣFe = 16–18%) and octahedral sheet contraction. Metal‐OH vibrational bands in visible/near infrared (VNIR) reflectance spectra of oxidized smectites changed in band depth and asymmetry with higher iron content. X‐ray diffraction patterns of UV irradiated samples indicate the formation of a mixed di‐ and trioctahedral smectite or a secondary nontronite phase, possibly on the surfaces of higher iron content smectites. These experiments suggest that UV irradiation is able to oxidize structurally bound iron in smectites without the presence of other chemical oxidants. Photooxidation may have influenced the mineralogy, both syndepositionally and postdepositionally, of Martian alteration assemblages formed near the surface and this process needs not be limited to one part of their formation history.

Reflectance spectra (1–5 μm) at low temperatures and different grain sizes of ammonium-bearing minerals relevant for icy bodies

It has been proposed that ammonium-bearing minerals are present in a varying amount in icy planetary bodies. Their observation at the surface of large objects was related to the upwelling and cryovolcanism of ammoniated water from possible subsurface oceans forming ammonium-bearing minerals (NH4+) mixed with ice at the surface. We analyzed the temperature evolution of the near-infrared spectra of a selected number of anhydrous and hydrated ammonium-bearing minerals containing different anions and water content. Reflectance spectra were collected in the 1–4.8 μm spectral range at cryogenic temperatures ranging from 293 K to ~65 K and the effect of sample's grain size between 32 and 150 μm was also investigated at room temperature. Reflectance spectra of anhydrous samples show well-defined absorption bands in the 1–2.5 μm range. The bands located at ~1.06, 1.3, 1.56, 2.02, and 2.2 μm could be useful to discriminate these salts and their characteristics are examined in detail in this work. On the other hand, the reflectance spectra of water-rich samples show H2O fundamental absorption bands strongly overlapping the NH4+ bands, thus dominating the spectra from 1 to 2.8 μm and fully saturating above 2.8 μm. The position of the absorption bands changes with temperature and grain size, shifting to higher frequencies as temperature decreases. The low-temperature spectra also reveal a fine structure compared to the room temperature ones and display narrower and more defined absorption bands. Granulometry mainly affects the band depth and band area parameters. Moreover, mascagnite, salammoniac, ammonium phosphate, tschermigite, and ammonium nitrate are subjected to a reversible low-temperature phase transition, which is manifested in the spectra by a progressive growth and shift of the bands towards shorter wavelengths with an abrupt change in their depth. This new set of spectra at cryogenic temperatures can be directly compared with remote sensing data to detect the presence of ammonium-bearing minerals on the surface of icy bodies. Their identification can impact our knowledge of the internal composition and dynamics of these bodies as well as their potential habitability.

Investigating S-type asteroid surfaces through reflectance spectra of ordinary chondrites

The search for asteroidal parent bodies of chondrites through various techniques is an ongoing endeavor. A link between ordinary chondrites (OCs) and S-type asteroids has previously been established by the sample return of the Hayabusa space mission. OCs are the class with the most abundant samples in our meteorite collection. We present an in-depth study of the reflectance spectra of 39 equilibrated and 41 unequilibrated ordinary chondrites (EOCs and UOCs). We demonstrate that consistent measuring conditions are vital for the direct comparison of spectral features between chondrites, otherwise hampering any conclusions. We include a comparison with a total of 466 S-type asteroid reflectance spectra from various databases. We analyze (i) if a difference between EOCs and UOCs as well as between H, L and LL can be seen, (ii) if it is possible to identify unequilibrated and equilibrated S-type asteroid surfaces and (iii) if we can further constrain the match between OCs and S-type asteroids all based on reflectance spectra.As a first step, we checked the classification of the 31 Antarctic UOCs analyzed in the present work, using petrography and magnetic measurements, and evidenced that 74% of them were misclassified. Reflectance spectra were compared between EOCs and UOCs as well as between H, L and LL chondrites using a set of spectral features including band depths and positions, peak reflectance values, spectral slopes and the Ol/(Ol + Px) ratio. UOCs and EOCs reflectance spectra show no clear-cut dichotomy, but a continuum with some EOCs showing stronger absorption bands and peak reflectance values, while others are comparable to UOCs. Moreover, we show by the example of 6 EOCs that their band depths decrease with decreasing grain size. Based on reflectance spectra alone, it is thus highly challenging to objectively identify an unequilibrated from an equilibrated S-type surface. There is no clear distinction of the chemical groups: only LL EOCs of petrographic type >4 can be distinguished from H and L through less deep 2000 nm band depths and 1000 nm band positions at longer wavelengths. No dichotomy of S-type asteroids can be seen based on the Ol/(Ol + Px) ratio. Their average Ol/(Ol + Px) ratio matches EOCs better than UOCs. A principal component analysis (PCA) was performed illustrating that both the unknown degree of space weathering and the unknown regolith grain size on asteroid surfaces hinder the distinction between equilibrated and unequilibrated surfaces. Lastly, an anti-correlation between the diameter of the asteroids and their 1000 nm band depth is found indicating that larger sized S-type asteroids show finer grained surfaces.

Hydrothermal alteration mapping using ASTER data, Takab-Baneh area, NW Iran: a key for further exploration of polymetal deposits

A large number of polymetallic deposits exist in the Takab-Baneh area, NW Iran. Different methods such as minimum noise fraction (MNF), pixel purity index (PPI), n-D visualization, mixture tuned matched filtering (MTMF), relative absorption band depth (RBD), band ratio (BR), and principal component analysis (PCA) were used for extraction of alteration zone characteristics from ASTER data in the Takab-Baneh area. The overall accuracy of MTMF is around 97% for phyllic, and 98% for argillic propylitic, which validates the potential and suitability of ASTER imagery for classification of alteration type. Subsequently, the alteration maps were confirmed by ground-truthing, i.e., field observation and petrographic studies. The results show a wide distribution of three main phyllic, argillic and propylitic alteration zones, that can be considered as targets for future exploration in NW Iran.

Optical band gap, photocatalysis, surface structures and depth profile properties of Cu2Se nanostructured thin films

The optical behaviour, stoichiometry, surface configuration of Cu2Se nanostructured thin layers prepared at different reaction times by the chemical process has been considered. The optical band gap of the deposited thin films decreases from 2.92 eV to 2.08 eV for the first transition and from 2.18 to 1.2 eV for the second transition. In addition, it was shown that Urbach energy is independent from the film thickness, and tail values of the localized states has decreased from 1.15 to 0.33 eV. It was found that the concentration of Cu: Se remains approximately constant across the depth of the deposited film at higher deposition time. The degradation efficiency of 98% was achieved by Congo red for the prepared Cu2Se thin film at optimal condition which make it a good candidate for organic dye. From these measurements and by controlling the band gap energy, the Cu2Se can find application in solar cell application.

A Novel Method for Hyperspectral Mineral Mapping Based on Clustering-Matching and Nonnegative Matrix Factorization

The emergence of hyperspectral imagery paved a new way for rapid mineral mapping. As a classical hyperspectral classification method, spectral matching (SM) can automatically map the spatial distribution of minerals without the need for selecting training samples. However, due to the influence of noise, the mapping accuracy of SM is usually poor, and its per-pixel matching method is inefficient to some extent. To solve these problems, we propose an unsupervised clustering-matching mapping method, using a combination of k-means and SM (KSM). First, nonnegative matrix factorization (NMF) is used and combined with a simple and effective NMF initialization method (SMNMF) for feature extraction. Then, k-means is implemented to get the cluster centers of the extracted features and band depth, which are used for clustering and matching, respectively. Finally, dimensionless matching methods, including spectral angle mapper (SAM), spectral correlation angle (SCA), spectral gradient angle (SGA), and a combined matching method (SCGA) are used to match the cluster centers of band depth with a spectral library to obtain the mineral mapping results. A case study on the airborne hyperspectral image of Cuprite, Nevada, USA, demonstrated that the average overall accuracies of KSM based on SAM, SCA, SGA, and SCGA are approximately 22%, 22%, 35%, and 33% higher than those of SM, respectively, and KSM can save more than 95% of the mapping time. Moreover, the mapping accuracy and efficiency of SMNMF are about 15% and 38% higher than those of the widely used NMF initialization method. In addition, the proposed SCGA could achieve promising mapping results at both high and low signal-to-noise ratios compared with other matching methods. The mapping method proposed in this study provides a new solution for the rapid and autonomous identification of minerals and other fine objects.

An Investigation on the Morphological and Mineralogical Characteristics of Posidonius Floor Fractured Lunar Impact Crater Using Lunar Remote Sensing Data

Lunar floor-fractured craters (FFCs) are a distinguished type of crater found on the surface of the Moon with radial, concentric, and/or polygonal fractures. In the present study, we selected the Posidonius FCC to explore the mineralogy, morphology and tectonic characteristics using remote sensing datasets. The Posidonius crater is vested with a wide moat of lava separating the crater rim inner wall terraces from the fractured central floor. Lunar Reconnaissance Orbiter’s (LRO) images and Digital Elevation Model (DEM) data were used to map the tectonics and morphology of the present study. The Moon Mineralogy Mapper (M3) data of Chandrayaan-1 were used to investigate the mineralogy of the region through specified techniques such as integrated band depth, band composite and spectral characterization. The detailed mineralogical analysis indicates the noritic-rich materials in one massif among four central peak rings and confirm intrusion (mafic pluton). Spectral analysis from the fresh crater of the Posidonius moat mare unit indicates clinopyroxene pigeonite in nature. Integrated studies of the mineralogy, morphology and tectonics revealed that the study region belongs to the Class-III category of FFCs. The lithospheric loading by adjacent volcanic load (Serenitatis basin) generates a stress state and distribution of the fracture system.

Exploring beach placer minerals in the east coast of Tamil Nadu, India, using EO-1 Hyperion data

Recent advancements in remote sensing facilitate the mapping of mineral occurrences but lack quantitative estimation of mineral abundances. In the present study, EO-1 Hyperion data are used for mapping and estimating beach placer minerals in the Cuddalore coastal stretch of Tamil Nadu, India, using spectral angle mapper (SAM) algorithm, continuum removed band depth analysis, and random forest (RF) regression. Strategic beach placer minerals are exposed in the image due to their specific spectral reflection/absorption characteristics; these unique settings are used to explore their spatial distribution from associated features. EO-1 Hyperion images are analyzed for (i) atmospheric correction; (ii) selection and transformation of optimum bands; (iii) fixing the input pixels from selected bands; (iv) generating n-D angle—for the preparation of end-members with unique spectra of mineral classes; (v) comparative analysis between true end-member and reference spectra of minerals using SAM, spectral feature fitting, and binary encoding algorithms; and (vi) image classification using SAM. The result of SAM shows the occurrence of placer mineral deposits such as zircon with an overall accuracy of 81.06% and Kappa coefficient of 0.71. The zircon shows strong absorption in spectral geometric parameters of EO-1 Hyperion data, such as band depth and band area at the spectral range of 1075 to 1150 nm. The positive correlation between geometric parameters of mineral spectra in its absorption characteristics and the mineral concentration of in situ samplings is used for developing an RF prediction model for estimating the placer minerals from beach sand deposits. Significantly, the lower RMSE values estimated at 0.082 and 0.156 for reference spectra (in situ) and image spectra respectively proved the ability of EO-1 Hyperion data for quantifying mineral resources.

A non-motorized spectro-goniometric system to measure the bi-directional reflectance spectra of particulate surfaces in the visible and near-infrared.

Reflectance spectroscopy is a powerful tool for remotely identifying the compositional and physical properties of surface materials. Due to the anisotropic scattering nature of most surfaces, the spectral features, including the absolute reflectance value, spectral slope, and band depth, are influenced by illumination and viewing configurations. Therefore, it is important to understand how spectral features vary with illumination and observation geometries for various particulate surfaces through laboratory measurements. Here, we describe a non-motorized spectro-goniometric system capable of measuring the bi-directional reflectance of particulate surfaces in the upper hemisphere in the wavelength range from 350 to 2150nm. The incident and the viewing zenith angles can be varied from 0° to 55° and from 0° to 70°, respectively. The relative viewing azimuth angle can be varied from 0° to 360°. Measurements on Labsphere Spectralon agree well with measurements done with other instruments. We also present measurement results on two typical planetary analog materials, the JSC-1A Martian soil simulant and the JSC-1A lunar regolith simulant.

Bathymetric Inversion and Mapping of Two Shallow Lakes Using Sentinel-2 Imagery and Bathymetry Data in the Central Tibetan Plateau

High-accuracy lake bathymetry and mapping are crucial for estimating lake water storage on the Tibetan Plateau (TP). In this study, we constructed traditional empirical (TE) models and machine learning (ML) models to compare the prediction accuracy and remote sensing bathymetric mapping performance by using Sentinel-2 satellite imagery and in situ measured water depth from Caiduochaka (CK) and QiXiang Co in the central TP. We analyzed the relationship between the band reflectance and depth and explored the universality of the model in different lakes. The results indicated that when using the TE model, the mean absolute percentage error (MAPE) varied between 14.5% and 26.5% for the test dataset at different study sites. When using the ML models, the MAPE varied between 7.6% and 18.9%, and it was the better choice overall. For the test dataset of the random forest (RF) model with the highest accuracy, in the CK with the maximum depth of approximately 16 m, the mean absolute error (MAE) and root mean square error (RMSE) were 0.54 m and 0.89 m, and the precision was the highest with an MAE of 1.13 m and RMSE of 1.67 m in QiXiang Co with a maximum depth of approximately 28 m, whereas the portability of the model was not satisfactory. Overall, the results indicated that the ML model can obtain bathymetric maps with high accuracy, good visual performance, and reliability, outperforming the TE model. It can be used effectively for deriving accurate and updated high-resolution bathymetric maps for shallow lakes.

Application of ASTER Data for Differentiating Carbonate Minerals and Evaluating MgO Content of Magnesite in the Jiao-Liao-Ji Belt, North China Craton

Numerous reports have successfully detected or differentiated carbonate minerals such as calcite and dolomite by using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). However, there is a need to determine whether existing methods can differentiate magnesite from other carbonate minerals. This study proposes optimal band ratio combinations and new thresholds to distinguish magnesite, dolomite, and calcite using ASTER shortwave-infrared (SWIR) data. These were determined based on the spectral and chemical analysis of rock samples collected from Liaoning, China and Danchon, North Korea and the reflectance values from ASTER images. The results demonstrated that the simultaneous use of thresholds 2.13 and 2.015 for relative absorption band depths (RBDs) of (6 + 8)/7 and (7 + 9)/8, respectively, was the most effective for magnesite differentiation. The use of RBDs and band ratios to discriminate between dolomite and calcite was sufficiently effective. However, talc, tremolite, clay, and their mixtures with dolomite and calcite, which are commonly found in the study area, hampered the classification. The assessment of the ASTER band ratios for magnesite grade according to magnesium oxide content indicated that a band ratio of 5/6 was the most effective for this purpose. Therefore, this study proved that ASTER SWIR data can be effectively utilized for the identification and grade assessment of magnesite on a regional scale.

A Fast and Effective Irregular Stripe Removal Method for Moon Mineralogy Mapper (M3)

Hyperspectral imagery (HSI) is one of the emerging tools to explore the physical properties and chemical composition of the lunar surface. Moon mineralogy mapper (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) is the most widely used lunar HSI data set with the widest coverage and the excellent resolution; however, dense and nonperiodic stripes distributed across all bands in M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> images hinder visual interpretation as well as their use in subsequent applications. In this article, a fast destriping approach for M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is proposed using the Hodrick–Prescott decomposition embedded in the low-rank framework (LRHP) to overcome this limitation. The integration of a statistical filter and variational model tackles the problem stemming from a lack of the correct residual information when certain pixels are corrupted in every band, thereby restoring severely degraded hyperspectral images (HSIs). Simulated and real experiments conducted on typical regions on the Moon with various levels of corruption demonstrate that the proposed LRHP rapidly achieves favorable performance against state-of-the-art approaches. Also, expanding tests on interference imaging spectrometer (IIM) data of Chang’E-1 and commonly used terrestrial remote sensing images show that LRHP has good generalization capability. Moreover, the integrated band depth (IBD) maps further verify the necessity of destriping and the high spectral fidelity of LRHP that benefits further applications.

Dependence of the Hydration of the Lunar Surface on the Concentrations of TiO2, Plagioclase, and Spinel

We investigate the interrelation between the hydration of the lunar regolith and the mineral composition of the surface of the Moon with respect to the concentrations of plagioclase, TiO2 (highly correlated with the oxide mineral ilmenite), and Mg-spinel. The spectral properties of lunar regions with a low concentration of plagioclase or a high concentration of TiO2 or Mg-spinel show a significant reduction in hydration at lunar midday compared to other compositions. This suggests that these oxide minerals contain less of the strongly bound OH component, which is not removed at lunar midday. The time-of-day-dependent variation of the 3 μm band depth is greater in TiO2-rich areas compared to other mare regions. The TiO2-rich regions therefore appear to have a strong tendency to adsorb solar wind-induced hydrogen into binding states of low energy that can more readily desorb and readsorb OH/H2O on a daily basis.

Saturn’s icy satellites investigated by Cassini - VIMS. V. Spectrophotometry

Albedo, spectral slopes, and water ice band depths maps for the five midsized saturnian satellites Mimas, Enceladus, Tethys, Dione, and Rhea have been derived from Cassini-Visual and Infrared Mapping Spectrometer (VIMS) data. The maps are systematically built from photometric corrected data by applying the Kaasalainen-Shkuratov model (Kaasalainen et al., 2001; Shkuratov et al., 2011). In this work a quadratic function is used to fit phase curves built by filtering observations taken with incidence angle i≤70∘, emission angle e≤70∘, phase angle 10∘≤g≤120∘, and Cassini-satellite distance D≤100.000 km. This procedure is systematically repeated for a subset of 65 VIMS visible and near-infrared wavelengths for each satellite. The average photometric parameters are used to compare satellites’ properties and to study their variability with illumination conditions changes. We derive equigonal albedo, extrapolated at g = 0∘, not including the opposition effect, equal to 0.63 ± 0.02 for Mimas, 0.89 ± 0.03 for Enceladus, 0.74 ± 0.03 for Tethys, 0.65 ± 0.03 for Dione, 0.60 ± 0.05 for Rhea at 0.55μm. The knowledge of photometric spectral response allows to correct individual VIMS spectra used to build maps through geolocation. Maps are rendered at a fixed resolution corresponding to a 0.5∘×0.5∘ bin on a longitude by latitude grid resulting in spatial resolutions of 1.7 km/bin for Mimas, 2.2 km/bin for Enceladus; 4.7 km/bin for Tethys; 4.5 km/bin for Dione; 6.7 km/bin for Rhea. These spectral maps allow establishing relationships with morphological features and with endogenic and exogenic processes capable to alter satellites’ surface properties through several mechanisms. The hemispheric dichotomies in albedo and spectral indicators between leading and trailing hemispheres are common properties of all midsized satellites: the accumulation of fine E ring grains is responsible for the higher albedo measured across the leading hemispheres of Tethys, Dione, Rhea, and the trailing side of Mimas. Conversely, the dark and red-colored material visible across the trailing hemispheres of Tethys, Dione, and Rhea is associated with the implantation of cold plasma particles. The thermal anomaly lenses located on the leading equatorial regions of Mimas and Tethys have been resolved and mapped. VIMS data evidence that the distribution of water ice band depths on Mimas lens is biased by the presence of the large Herschel impact crater pointing to a different regolith size distribution with respect to Tethys. Maps show local changes of albedo and spectral indicators in correspondence of recent impact craters (Inktomi on Rhea, Creusa on Dione) and on Dione’s wispy terrains are caused by the exposure of pristine water ice. Enceladus’ tiger stripes, the active sources of plumes in the southern polar region, despite being partially resolved on VIMS maps allow measuring an exceedingly high band depths in comparison with the rest of the satellite’s surface. Moreover, Enceladus’ smooth terrains located on the leading hemisphere around (lon,lat)=(90∘,30∘) show peculiar properties (low infrared albedo, positive 0.35-0.55μm slope and maximum band depth) possibly associated with the presence of a buried diapir in this area. The variability of average spectral albedos and indicators induced by phase angle is investigated and exploited to establish a comparison among satellites’ properties as a function of orbital distance from Saturn.

Chronological and compositional mapping of the Mare Orientale basin using Chandrayaan-1 – M3 and LRO datasets

The lunar multiring mare basins have different ages, compositions and are associated with the diverse unit of mare deposits, providing evidence for the origin and evolution of the lunar crust. The multiring basins offer the opportunity to assess many unknown questions that may reveal the lunar geologic standpoint. The Mare Orientale basin is one of the youngest impact multiring basins on the Moon, which covers about 930 km in diameter and coordinates centered at 200S 950W. In the present study, the compositional, topographical, and chronological studies have been done by using Moon Mineralogy Mapper (M3) from Chandrayaan-1 orbiter and Lunar Reconnaissance Orbiter Camera-Wide Angle Camera (LROC-WAC) image from Lunar Reconnaissance Orbiter (LRO). The M3 data used to map the Optical maturity (OMAT), FeO & TiO2 concentrations of the basin. The Standard Band Ratio (SBR) of the Orientale basin has been prepared to discriminate the different mare and highland lithologies. The 1 μm and 2 μm band depths have been mapped to demarcate the mafic minerals such as olivine and pyroxenes. The age of the Orientale basin mare units was mapped as Orientale event (3.72 Ga - Upper Imbrian), Orientale South West (3.7 Ga), Orientale West unit (3.37 Ga), Lacus Veris North unit- 1, 2, and 3 (3.1 Ga, 2.63 Ga, and 3.3 Ga), Lacus Veris East (2.9 Ga), Lacus Autumni - North (3.1 Ga), Middle (2.01 Ga), South (2.27 Ga) and Kopff crater (mare) (2.92 Ga) by using LROC-WAC data through Crater Size Frequency Distribution (CSFD) technique. Characterization of M3 reflectance spectra profiles was done to map the Orientale basin lithologies like olivine, pyroxene (low calcic and high calcic), plagioclase, ilmenite, Mg-spinel, and olivine – pyroxene mixtures that are present in the anorthositic, basaltic, noritic, and gabbroic rocks. It is revealed that the Orientale basin could have been formed either by global thermal evolution and basin mare volcanic evolution or pressure release melting and associated with secondary convection.