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Spectral reflectance properties of magnetites: Implications for remote sensing
Abstract
Magnetite (Fe³⁺(Fe²⁺Fe³⁺)2O4) is ubiquitous in Earth and planetary materials, forming in igneous, metamorphic, and sedimentary settings, sometimes influenced by microbiology. Magnetite can be used to study many and varied planetary processes, such as the oxidation state of magmas, paleomagnetism, water–rock interactions such as serpentinization, alteration and metamorphism occurring on meteorite parent bodies, and for astrobiology. The spectral reflectance signature of magnetite in the ultraviolet, visible, and near-infrared is somewhat unusual compared to common planetary materials, suggesting that remote detection and characterization of magnetite should be possible. Here we present a systematic investigation of the reflectance spectral properties of magnetite using natural and synthetic samples. We investigate the effects of chemical substitutions, grain size variations, and mixtures with other phases in order to better constrain remote spectral searches for, and interpretation of, magnetite-bearing lithologies. Magnetite is characterized by high extinction over the entire wavelength range considered here, and therefore surface scattering dominates over volume scattering. Magnetite reflectance spectra are strongly influenced by the presence of delocalized electrons above the Verwey transition temperature (∼120 K), leading to metal-like scattering behavior, that is, high extinction, surface scattering dominant, and a general increase in reflectance with increasing wavelength, i.e. “red-sloped and featureless”. Superimposed upon the metal-like reflectance are local reflectance maxima which we ascribe to Fresnel reflectance peaks corresponding to Fe–O oxygen–metal charge transfer processes (∼0.27 and ∼0.39 µm) and Fe-related field–d orbital transitions (∼0.65 µm). We also find a systematic shift in the wavelength position of the 0.65 µm Fresnel peak with increasing chemical impurity in magnetite. Magnetite reflectance spectra are most similar to those of titanomagnetite and wüstite, and unlike those of other Fe–(Ti) oxides, such as ilmenite, hæmatite, ulvospinel, maghemite, pseudobrookite, and armalcolite.
... However, the subordinate amounts of high-grade iron ore in China is dominated by magnetite, whether they are of skarn or BIF-related type (Wang E et al. 2014;Zhang et al. 2014;. As we all know, the magnetite is characterized by surface scattering dominant and high extinction over the entire visible-near infrared-shortwave infrared (0.35-2.5 lm) wavelength range (Izawa et al. 2019), resulting in 'flat and featureless' reflectance spectroscopic characteristics (Hunt 1971;Izawa et al. 2019). This means magnetite is difficult to be discriminated by remote sensing method. ...
... However, the subordinate amounts of high-grade iron ore in China is dominated by magnetite, whether they are of skarn or BIF-related type (Wang E et al. 2014;Zhang et al. 2014;. As we all know, the magnetite is characterized by surface scattering dominant and high extinction over the entire visible-near infrared-shortwave infrared (0.35-2.5 lm) wavelength range (Izawa et al. 2019), resulting in 'flat and featureless' reflectance spectroscopic characteristics (Hunt 1971;Izawa et al. 2019). This means magnetite is difficult to be discriminated by remote sensing method. ...
There are two kinds of chlorites occurred in Anshan area, among which the hydrothermal Fe-chlorite with longer Fe-OH wavelength is spatially related to the high-grade magnetite ore, meaning that wavelength mapping of chlorites can be used to target the BIF-hosted high grade iron ore. In this article, quadratic polynomial, cubic spline, and quartic polynomial method were used to interpolate the absorption wavelength near 2250 nm of China ZY1-02D satellite hyperspectral image. The result of quadratic polynomial is continuous without data overlapping or intervals, thus most suitable for discrimination of the chlorites. The field truth shows that the spatial distribution of Fe-chlorite, validated by XRD analysis, is not only in accordance with that of high grade magnetite ore bodies, but also consistent with the recently discovered concealed iron bonanza. The study shows that hyperspectral remote sensing techniques can play significant role in the exploration of magnetite iron ore in east Liaoning Province.
... These Fresnel peaks are superimposed on a featureless red sloped spectrum (Izawa et al., 2019). The unirradiated magnetite spectrum presents all these features except for the 0.35 band, as in our samples it is centered at ~0.32 μm. ...
... Pressed pellets might not perfectly mimic the physical and spectral characteristics of asteroidal regoliths. However, increasing the compaction and reducing the porosity of the simulated regolith will likely create higher reflectance, bluer slopes in the short wavelengths, and strengthening of bands compared to loose powders as shown in Izawa et al. (2019). To identify the spectral modifications caused by pulsed laser irradiation, we compared the reflectance spectra of a magnetite pressed pellet before and after 1, 2, 3, and 5 pulses ( Fig. 2), calculated band depths relative to Fresnel peaks (Fig. 3a,b), and the NIR spectral slope (μm − 1 ) between 1 and 2.3 μm (Fig. 3c) using the derivative of the spectrum as described in Loeffler and Prince (2022). ...
... Magnetite is known to form in igneous, metamorphic, and sedimentary settings and through different reactions such as gas and liquid interactions on Earth, in the solar nebula, and on small Solar System parent bodies (Kerridge et al., 1979;Mittlefehldt et al., 1998;McSween and Treiman, 1998;Krot et al., 1997;Keller, 1998;Klein, 2005;Chaumard et al., 2009;Hicks et al., 2017;Barth et al., 2018;Izawa et al., 2019). As a consequence, it is a useful mineral for studying a variety of processes, such as variation in oxygen fugacity, paleomagnetism, water-rock interactions, and biogenic activity (McKay et al., 2003;Izawa et al., 2019;Sato et al., 2022). ...
... Magnetite is known to form in igneous, metamorphic, and sedimentary settings and through different reactions such as gas and liquid interactions on Earth, in the solar nebula, and on small Solar System parent bodies (Kerridge et al., 1979;Mittlefehldt et al., 1998;McSween and Treiman, 1998;Krot et al., 1997;Keller, 1998;Klein, 2005;Chaumard et al., 2009;Hicks et al., 2017;Barth et al., 2018;Izawa et al., 2019). As a consequence, it is a useful mineral for studying a variety of processes, such as variation in oxygen fugacity, paleomagnetism, water-rock interactions, and biogenic activity (McKay et al., 2003;Izawa et al., 2019;Sato et al., 2022). In chondrites, petrographic evidence suggests that magnetite is often formed on the meteorite parent body during postaccretional aqueous alteration processes (Kerridge et al. 1979;Krot et al., 1997;Hua and Buseck, 1998;Brearley, 2006;Alfing et al., 2019). ...
... For models which use only a single darkening agent, the use of magnetite was found to provide the best single spectral match to the telescopic data. While some magnetites have a broad local minimum in reflectance near 1.0 µm (see discussion in Izawa et al. 2019), we note that the reason magnetite is preferred is not based on any curvature detected from 0.7-1.3 µm. ...
... Lab Mix,Izawa et al. 2019) ...
Nereid, Neptune's third largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on spectral characterization of Nereid using its reflectance spectrum from 0.8-2.4 $\mu m$, providing the first measurements over the range of 0.8-1.4 $\mu m$. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 $\mu m$ when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for sub-micron sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of Tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. (2019). Comparison to spectra of large TNOs and satellites of Uranus show that Nereid's low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by incomplete understanding of spectral variability among $\sim$100km-sized icy bodies.
... It also causes bluing in the 0.7-1.0 μm range of hydrated meteorite (CM) spectra (Izawa et al. 2019). However, the effect of magnetite is more subtle in the short wavelength region, indeed they showed that for chondritic materials with carbonaceous macromolecules at their surface, the 0.4-0.65 μm region is little affected by the presence of magnetite even in large quantities, the absorption being dominated by organic matter inducing thus an important reddening in this wavelength range. ...
... Magnetite could also have an effect on the reflectance level of particles. In fact, magnetite is a relatively dark material with reflectance level around 5%-8% (Izawa et al. 2019). Thus, a layer of magnetite formed on the surface of an initially shiny material (reflectance level >8%) will tend to darken this material. ...
Visible near-infrared (Vis–NIR) reflectance spectroscopy is a powerful nondestructive technique allowing the parent bodies identification of cosmomaterials such as meteorites, micrometeorites, and interplanetary dust particles (IDPs) studied in the laboratory. Previous studies showed that meteorites do not represent the full diversity of the solar system small bodies. We present here an experimental setup we developed for measuring Vis–NIR microspectroscopy of individual IDPs. We acquired diffuse Vis–NIR reflectance spectra of 15 particles ranging 7–31 μ m in size. We discuss the requirements, the abilities, as well as the limitations of the technique. For sizes smaller than 17 μ m, the slopes increase with decreasing particles sizes, while for sizes larger than 17 μ m, the slopes are randomly distributed. The visible reflectance levels do not appear to be affected by the size of the IDPs, and show a bimodal distribution. Among the studied particles, we identified an IDP (L2079C18) exhibiting a feature at 0.66 μ m, which is similar to the one observed by remote sensing on the surface of hydrated asteroids. This is the first detection of a hydration band in the reflectance spectrum of an IDP that could indicate a possible link between hydrated IDPs with hydrated asteroid surfaces.
... Some instances show spectral absorption at 0.55 µm, which has not previously been reported for any dark asteroid 21 . Magnetite (Fe 3 O 4 ) is the most likely source of this spectral feature 22 . This interpretation is consistent with the emissivity spectra obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES), which show features at 18 µm and 29 µm that may be due to magnetite 15 . ...
... µm. Figure 2 provides a comparison of the MapCam spectrum of Bennu's dark outcrop with the MAG105 reflectance spectrum presented in figure 7b of ref. 22 , which was found to be a good match with our data. This comparison is conducted by sampling the MAG105 spectrum at the effective wavelengths of the MapCam colour bands and then shifting the spectrum linearly into the reflectance range of Bennu's surface (linearly reduced by −0.031292095, which is the difference between the reflectance of MAG105 and Bennu's dark outcrop in the b′ band). ...
NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth¹. Bennu is a low-albedo B-type asteroid² that has been linked to organic-rich hydrated carbonaceous chondrites³. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes⁴. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5–11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid’s properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu’s thermal inertia¹² and radar polarization ratios¹³—which indicated a generally smooth surface covered by centimetre-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres⁴. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.
... However, TWI only changes at large scales, whereas in small regions (especially hilly areas), the difference in topography is more evident. While the present study only used a single-phase remote sensing image to obtain NDVI, multi-phase remote sensing images are often required to link this variable to SPM types [55][56][57]. Land use types vary in many areas as a result of anthropogenic activities, with SPM having a limited impact. This study was conducted in a county where TWI, NDVI, and land use type did not characterise the differences among various SPM types well. ...
Conventional maps of soil parent material (SPM) types obtained by field survey and manual mapping or predictions from other map data have limited accuracy. Digital soil mapping of SPM types necessitates accurate acquisition of SPM distribution information, which is still a challenge in hilly areas. This study developed a high-accuracy method for SPM identification in hilly areas at the county scale. Based on geographic information system technology, seven feature variables were extracted from the geological map, geomorphic map, digital elevation model, and remote sensing image data of Shanggao County, Jiangxi Province, China. Different feature combination schemes were designed to develop SPM identification models based on random forest (RF), support vector machine (SVM), and maximum likelihood classification (MLC) algorithms. The best SPM identification results were obtained from the RF algorithm using the combination of geological type, geomorphic type, elevation, and slope. Confusion matrices were constructed based on a field survey of 586 validation samples, and the results were evaluated in terms of overall accuracy, precision, recall, F1 score, and Kappa coefficient. The overall accuracy and Kappa coefficient of the results from the optimal RF model were 83.11% and 0.79, respectively, which were 26.11% and 0.31 higher than those of the conventional map, respectively. Its precision and recall for various SPM types were greater than 75%. A comprehensive comparison of the accuracy, uncertainty, and plotting performance of the SPM recognition results reveals that the RF algorithm outperforms the SVM algorithm and the MLC algorithm. Geological type was the largest contributor to SPM identification, followed by geomorphic type, elevation, and slope. The importance of different feature variables varied for distinct SPM types. The accuracy of SPM identification was not improved by selecting more feature variables, such as land use type, normalised difference vegetation index, and topographic wetness index. This study demonstrates the feasibility of high-accuracy county-level SPM mapping in hilly areas based on the RF algorithm using geological type, geomorphic type, elevation, and slope as feature variables. As hilly areas have typical topographic features and SPM types, the proposed method of SPM mapping can be useful for application in other similar areas. There are a few limitations in this study with regard to data quality and resolution, feature variable selection, classification algorithm generalisation, and study area representativeness, which may affect the outcomes and need to be solved.
... This suggests that the parent body of rubble pile Ryugu formed far from the Sun, incorporated ices of water and carbon dioxide, and that subsequent increases in temperature caused intensive aqueous alteration to convert primary silicate phases to Mg-rich phyllosilicates and carbonates. Other phases formed by the alteration include pyrrhotite and magnetite, but they were not detected in the spectra due to the lack of diagnostic absorption bands in the wavelength range probed (68,69). Small Vis-NIR spectral variations among Ryugu samples can be explained by variations in sample surface texture and orientation ( fig. ...
The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.
... A slope break near 0.5 μm is seen in some spectra of the CI1 CC Orgueil (Cloutis et al., 2011), but its cause is uncertain. Some magnetite reflectance spectra do show a flat spectral slope or shallow absorption band in the 0.5-0.6 μm interval (Izawa et al., 2019), but also show other features not consistent with the martian moon spectra, such as a broad 1 μm region absorption feature. A 0.5-0.6 μm feature is seen in some plagioclase feldspar spectra, and a slope break is seen in many lunar regolith samples. ...
Spectroscopic measurements are a powerful tool to investigate the surface composition of airless bodies and provide clues of their origin. The composition and origin of Phobos and Deimos are still unknown and are currently widely debated. We present spectroscopic measurements of Phobos and Deimos at ultraviolet and visible wavelengths (250–650 nm) made by the NOMAD‐Ultraviolet and Visible Spectrometer (UVIS) on the ExoMars TGO mission. These new spectra cover multiple areas on Phobos and Deimos, and are of generally higher spectral resolution and signal‐to‐noise than previous spectra, and extend to lower wavelengths than most previous measurements. The UVIS spectra confirm a red‐sloped spectrum lacking any strong absorption features; however, we confirm the presence of a previously identified absorption feature near 0.65 μm and tentative absorption near 0.45 μm. The observed Phobos and Deimos spectra are similar to D‐ and T‐type asteroids, adding weight to the captured asteroid hypothesis for the moons' origins. We also find, however, that the UVIS Phobos reflectance spectra of Phobos' red unit is a relatively close match to the olivine‐rich, highly shocked Mars meteorite NWA 2737, with a low overall reflectance, a red‐sloped spectrum, and lack of olivine‐associated absorption bands in the UVIS spectral range. This meteorite, however, exhibits spectral features at longer wavelengths that not observed in the Martian moon spectra, indicating a need for further investigation at longer wavelengths to interpret whether this material could inform our understanding of Phobos' origin.
... Similarly, Valaee et al. (2016) highlighted the major role of soil moisture regimes in the concentration of the prevailing soil Fe oxide. Demattê et al. (2004) and Izawa et al. (2019) also concluded that high total iron content and the presence of magnetite influenced the lower reflectance intensity. Formaggio et al. (1996) pointed out that the existence of opaque minerals such as magnetite and ilmenite can coat the absorption bands at wavelengths of 1400 and 1900 nm. ...
Soil spectroscopy has become an applicable method for monitoring and management of the spatial variability of soil characteristics. However, no widely knowledge about the potential application of this method in arid regions is still available. This research aimed to ascertain the performance of visible and near-infrared spectroscopy (Vis-NIR) to detect soil variations related to pedogenetic processes and pedoenvironmental conditions and to show the perspective of the most important variables changing spectral behavior in Lut Watershed. Twenty-three pedons in three clusters, including site 1 (xeric-mesic soil moisture-temperature regimes), site 2 (aridic-mesic, and aridic-thermic), and site 3 (extreme aridic-hyperthermic), with different geomorphology and lithology, were selected. Physicochemical properties and different iron forms were analyzed; X-ray diffraction (XRD), magnetic susceptibility (χlf), and Vis-NIR spectroscopy were also performed. The most important attributes influencing spectral morphology were related to soil texture, organic carbon content, clay mineralogy, evaporitic minerals (carbonate, halite, gypsum, and anhydrite), and iron oxides (goethite, hematite, and magnetite). Geometric parameters (area and depth) of absorption features located in 400–1000 nm (iron oxide), 2200 nm (clay), and 2340 nm (CaCO3) were significantly decreased by organic carbon in site 1. Increasing χlf values masked the wavelengths in 1400, 1540, 1750, and 1900 nm (CaSO4.2H2O) in site 2. Moreover, increasing salinity and anhydrite content covered the features of iron oxides in 400–570 nm in site 3. The finger-shaped absorption in 1450–1475 nm (CaSO4.2H2O) disappeared with an increasing amount of anhydrite and its degree of dehydration. Principal component analysis (PCA) showed a suitable performance for soil discrimination in the three climatic sites with different pedogenetic processes. Validation results by the partial least squares regression (PLSR) models indicated excellent accuracy in predicting CEC, EC, χlf, and gypsum content (RPIQ = 4.17, 3.00, 3.00, and 2.58, respectively) and good accuracy in predicting Fed and CCE (RPIQ = 2.30 and 2.06, respectively). Overall, spectral sensing was recognized as a suitable pedoenvironmental monitoring indicator for pedological assessments in Lut Watershed.
... Poorly crystalline, nanophase Fe oxides lack these distinct spectral absorptions and are characterized by a steep spectral edge between 400 and 750 nm (Bell et al., 1990(Bell et al., , 2000. Magnetite, a mixed-valence Fe oxide, has a relatively flat, low-reflectance spectrum with a local minimum near 550 nm and local maximum near 650 nm, caused by closely colocated Fe 2+ and Fe 3+ in both tetrahedral and octahedral coordination (Izawa et al., 2019). Although a variety of factors (e.g., particle size, opaque minerals) can influence the strength of an absorption band in reflectance spectra, calculating relevant band depth, band positions, and spectral slopes can provide a measurement of relative absorption strength between samples. ...
During the first 2934 sols of the Curiosity rover's mission 33,468 passive visible/near‐infrared (NIR) reflectance spectra were taken of the surface by the mast‐mounted Chemistry and Camera (ChemCam) instrument on a range of target types. ChemCam spectra of bedrock targets from the Murray and Carolyn Shoemaker formations on Mt. Sharp were investigated using principal component analysis and various spectral parameters including the band depth at 535 nm and the slope between 840 and 750 nm. Four end‐member spectra were identified. Passive spectra were compared to Laser Induced Breakdown Spectroscopy (LIBS) data to search for correlations between spectral properties and elemental abundances. The correlation coefficient between FeOT reported by LIBS and BD535 from passive spectra was used to search for regions where iron may have been added to the bedrock through oxidation of ferrous‐bearing fluids but no correlations were found. Rocks in the Blunts Point‐Sutton Island transition that have unique spectral properties compared to surrounding rocks, that is flat NIR slopes and weak 535 nm absorptions, are associated with higher Mn and Mg in the LIBS spectra of bedrock. Additionally, calcium‐sulfate cements, previously identified by Ca and S enrichments in the LIBS spectra of bedrock, were also shown to be associated with spectral trends seen in Blunts Point. A shift toward a steeper NIR slope is seen in the Hutton interval, indicative of changing depositional conditions or increased diagenesis.
... Within the first group, except Ba, in which case the highest positive significant correlation was registered around 2250 nm (r ≈ 0.26), other PTE in Fig. 4a were almost entirely negatively correlated with VIS-NIR spectra. Highest significant negative correlations (r = − 0.5 ± 0.03) were observed for V, Ti and Fe around the goethite (440 nm) (Stenberg et al., 2010), ilmenite (540, 630 nm) (Izawa et al., 2021) and magnetite (400 nm) (Izawa et al., 2019) absorption regions, which are responsible regions of the iron oxides and compound. For Co significant negative correlation was detected at 400-508 nm and 2491-2498 nm narrow region. ...
PTE contamination of soils remains one of the global environmental concerns. The ways of detecting and monitoring PTE concentrations in soils varies including traditional field sampling accompanied by sample preparation and chemical analysis and state of the art visible and near-infrared (Vis-NIR) spectroscopic approaches. Among the different Machine Learning (ML) to extract soil information from spectra and to explore the relationship between spectral reflectance data and soil PTE content PLSR method is a well-established one to construct a soil PTE estimation model. This study aimed to explore the relationship of soil PTE geochemical and VIS-NIR spectroscopy characteristics in agricultural soils near Cu-Mo mine area in Armenia. PLSR method is applied to identify the links between the spectra and agricultural soil Ti, V, Cr, Mn, Fe, Co, Ba, Pb, Zn, Cu, Sr, Zr and Mo contents to reveal the potential of VIS-NIR spectroscopy in ex-situ monitoring of Kajaran soils. The results show that different portions of VIS-NIR spectra are responsible for Ti (1100-1200 nm, 2350-2500 nm), V (350-500 nm, 700-750 nm, 1000-1100 nm, 1400-2500 nm), Cr (1300-1400 nm, 1900-2100 nm) and Ba (450-500 nm, 600-800 nm, 1050-1700 nm, 2000-2100 nm, 2350-2400 nm) estimations through PLSR correspondingly. However, among the studied PTEs Ti and V, which shows significant negative correlations in VIS-NIR spectra registered at around 400-600 nm and 850-1150 nm regions, are remarkable and promising with the PLSR estimation results using VIS-NIR spectra Ti (R2Test = 0.74), V (R2Test = 0.71). This study shows that VIS-NIR spectroscopy has a high potential for the estimation of at least several PTE in soils and PLSR modelis reliable for deriving information from there.
... local minimum in reflectance near 1.0 μm (see discussion in Izawa et al. 2019), we note that the reason magnetite is preferred is not based on any curvature detected from 0.7 to 1.3 μm. As can be seen in the fits for magnetite (Figure 3) versus the fits for amorphous carbon (Figure 4), both components can provide good quality fits across the Figure 3. Best-fit models for mixtures of water ice with magnetite as the darkening agent. ...
Nereid, Neptune’s third-largest satellite, lies in an irregular orbit and is the only outer satellite in the system (apart from Triton) that can be spectroscopically characterized with the current generation of Earth-based telescopes. We report our results on the spectral characterization of Nereid using its reflectance spectrum from 0.8 to 2.4 μ m, providing the first measurements over the range of 0.8–1.4 μ m. We detect spectral absorption features of crystalline water ice in close agreement with previous measurements. We show that model fits of simple intimate mixtures including water ice do not provide simultaneous matches to absorption band depths at 1.5 and 2.0 μ m when accounting for the spectral continuum. Possible solutions include invoking a more complex continuum, including both crystalline and amorphous water ice, and allowing for submicron-sized grains. We show that mixtures including magnetite and the CM2 chondrite Murchison provide a flexible framework for interpreting spectral variation of bodies with neutral-sloped spectra like that of Nereid. Magnetite in particular provides a good match to the spectral continuum without requiring the presence of tholin-like organics. We note that carbonaceous chondrites and their components may be useful analogs for the non-ice components of outer solar system bodies, consistent with recent findings by Fraser et al. Comparison to spectra of large trans-Neptunian objects and satellites of Uranus show that Nereid’s low albedo, deep water bands, and neutral color is distinct from many other icy objects, but such comparisons are limited by an incomplete understanding of spectral variability among ∼100 km-sized icy bodies.
... [30]), various geological studies (e.g. [25]), interpretation of astronomical observations on cometary (e.g. [32]), asteroidal (e.g. ...
Surfaces of solid solar system objects are covered by layers of particulate materials called regolith originated from their surface bedrock. They preserve important information about surface geological processes. Often regolith is composed of more than one type of particle in terms of composition, maturity, size, etc. Experiments and theoretical works are carried out to constrain the result of mixing and extract the abundance of compositional end-members from regolith spectra. In this work, we have studied photometric light scattering from simulated surfaces made of two different materials—one is highly bright quartz particles (average diameter 78.336μm) and the other is moderately bright sandstone particles (average diameter 253.757μm). The samples were mixed with varying proportions and investigated at normal illumination conditions to avoid the shadowing effect. Said combinations may resemble ice mixed regolith on various solar system objects, and therefore, it is important for in situ observations. We find that the combinations show a linear trend in the corresponding reflectance data in terms of their mixing proportion and some interesting facts come out when compared to previous studies.
... [30]), various geological studies (e.g. [25]), interpretation of astronomical observations on cometary (e.g. [32]), asteroidal (e.g. ...
Surfaces of solid solar system objects are covered by layers of particulate materials called regolith originated from their surface bedrock. They preserve important information about surface geological processes. Often regolith is composed of more than one type of particle in terms of composition, maturity, size, etc. Experiments and theoretical works are being carried out to constrain the result of mixing and extract the abundance of compositional end-members from regolith spectra. In this work we have studied, photometric light scattering from simulated surfaces made of two different materials – one is highly bright quartz particles (average diameter 78.336 µm) and the other moderately bright sandstone particles (average diameter 253.757 µm). The samples were mixed with varying proportions and investigated at normal illumination conditions to avoid the shadowing effect. Said combinations may resemble ice mixed regolith on various solar system objects and therefore it is important for in situ observations. We find that the combinations show a linear trend in the corresponding reflectance data in terms of their mixing proportion and some interesting facts come out when compared to previous studies.
... 71−74 Moreover, in the outer space, a large number of bodies is (or has been) characterized by the presence of volatile-rich elements as well as hydrous minerals or water ices (e.g., Mars, Ceres, asteroids, and comets) that affect the chemistry and physics of space weathering processes. 15,29,75 In particular, magnetite is quite ubiquitous on Earth and in planetary materials, and it is often a marker for a variety of processes as for water−rock interactions (e.g., serpentinization, see ref 76 and references therein). ...
... Magnetite, a mixed valence iron oxide, has spectral properties distinct from many of the other iron oxides, due in part to the surface scattering dominating at visible wavelengths caused by the delocalized nature of electrons associated with octahedral Fe. Its spectrum is generally dark and flat, with subtle Fresnel peaks near 0.65 µm and a local reflectance minimum near 0.55 µm [56]. ...
Phyllosilicates, sulfates, and Fe oxides are the most prevalent secondary minerals detected on Mars from orbit and the surface, including in the Mars Science Laboratory Curiosity rover’s field site at Gale crater. These records of aqueous activity have been investigated in detail in Gale crater, where Curiosity’s X-ray diffractometer allows for direct observation and detailed characterization of mineral structure and abundance. This capability provides critical ground truthing to better understand how to interpret Martian mineralogy inferred from orbital datasets. Curiosity is about to leave behind phyllosilicate-rich strata for more sulfate-rich terrains, while the Mars 2020 Perseverance rover is in its early exploration of ancient sedimentary strata in Jezero crater. It is thus an appropriate time to review Gale crater’s mineral distribution from multiple perspectives, utilizing the range of chemical, mineralogical, and spectral measurements provided by orbital and in situ observations. This review compares orbital predictions of composition in Gale crater with higher fidelity (but more spatially restricted) in situ measurements by Curiosity, and we synthesize how this information contributes to our understanding of water-rock interaction in Gale crater. In the context of combining these disparate spatial scales, we also discuss implications for the larger understanding of martian surface evolution and the need for a wide range of data types and scales to properly reconstruct ancient geologic processes using remote methods.
... This is explained by the differential pedogenic development of the soil layers. The more superficial layers were submitted to a relatively longer time of exposition to bioclimatic agents (Buol et al., 2011), which causes a greater residual concentration of magnetite and/or ilmenite, opaque minerals that cause a sharp drop in reflectance (Izawa et al., 2019). The layers below are closest to the parent material, however they have a smaller residual content of such opaque minerals, which is diluted by the occurrence of easilyweatherable primary minerals, and generally higher silt and sand contents, which cause greater reflectance intensities (Demattê and Garcia, 1999). ...
The technological and computational development has raised new possibilities for soil scientists to describe and infer about soil-forming processes and functions. Thus, why not to insert the contrasts between soil layers in a profile revealed by proximal sensing? We proposed to study the soil profile heterogeneity by reflectance (Vis-NIR-SWIR), X-ray fluorescence and magnetic susceptibility aiming to quantify it with the proposed “Profile Heterogeneity Index” (PHI) through seven soil sequences in twenty-seven locations over different hillslope positions and parent materials. The study was conducted in a 165-ha farm, at Rio das Pedras municipality, São Paulo State, Brazil. The differences between two consecutive soil layers were calculated by subtracting the data to the bottom and the immediately above for each soil profile. The results are depicted as a depth function with proportional colors to highlight the heterogeneity in the soil profiles. This approach provided a new method to identify where the transitions of soil attributes occur exactly and improve the understanding about soil-formation factors and processes. In this study, greater PHI values were mainly related to erosion and redeposition process through the landscape, differences in soil parent materials and also soil-forming processes. The sensors can furnish important and complementary information to the characterization of soil profile heterogeneity.
... Currently, the Legendre polynomial coefficient of the scattering phase function in spectral simulations mainly refers to the approximate average value of the forward scattering coefficients of minerals proposed by Mustard and Pieters [22]. Generally, b is set to −0.4, c is set to 0.25 [9,36], and then the SSA w is retrieved from the single-angle spectra. ...
In this paper, a self-made, mineral, multi-angle, spectrum measurement device is employed to measure the multi-angle spectra of olivine and plagioclase; the multi-angle spectra of ilmenite in the Reflectance Experiment Laboratory (RELAB) Spectral Library are collected; and the optimized retrieval of the photometric parameters of the Hapke model is realized. Importantly, the derived result of the single-scattering albedo (SSA) is stable and has both mathematical meaning and physical meaning. The derived Legendre polynomial coefficients of the phase function can better simulate the variation in the mineral spectra with angle. This paper compares the effects of multi-angle and single-angle spectral data on the photometric parameter derived results. The setting of the Legendre polynomial coefficient of the scattering phase function mainly affects the simulation accuracy of the mineral spectra as a function of angle. Using this coefficient to optimize the retrieval, the simulation accuracy is moderately improved compared with the single-angle simulation. The estimation of photometric parameters based on multi-angle spectral data can provide a basis for setting the empirical values of the phase function parameters from single-angle spectral calculations, which can more truly reflect the law of reflectance spectra changing with angle than Lucey’s traditional empirical value of the phase function (b = −0.4 and c = 0.25). The results of multi-angle spectra retrieval in this paper show that the Legendre polynomial coefficients of the phase function vary with wavelength rather than being constant and that different minerals differ greatly.
... Moreover, the reflected energy responds to the upper few centimeters of the soil surface, presenting no direct effects of PM reflectance. However, soil attributes such as clay, sand, Fe 2 O 3 , and TiO 2 are directly affected by the PM and are also manifested on the spectral reflectance (Dematte et al., 2009;Izawa et al., 2019;Silva et al., 2020). ...
Parent material is the main source for soil textural, mineralogical, and other physical attributes. The knowledge over this factor is explored generally in low scale geology maps, insufficient for most users. Remote sensing can offer assistance in this regard, since it allows the evaluation of soil properties, as largely indicated in literature, being a potential tool to delineate parent material. Thus, we explored a multi temporal Landsat image composition with bare soil reflectance to extract soil properties and distinguish discrepant lithological classes at the western plateau, São Paulo State, Brazil. The area is 247,737 ha large, where 981 soil samples were collected at 0–20 cm depth. We acquired the synthetic soil image and linked the pixel's spectra with soil attributes. We performed a digital soil mapping procedure to generate maps of attributes related to parent material. The soil maps offered a great input on identifying the transitions between sandstone and basalt as soils from these formations have significant differences in clay, sand, Fe2O3 and TiO2 contents. Therefore, the use of remote sensing coupled with digital soil mapping is a strong alternative to conventional methods to improve low scale PM maps to enhance detail on regional and local scales.
... Metal-oxygen charge transfers can cause 'Fresnel peaks' due to strongly anomalous dispersion, rather than typical absorption bands in opaque minerals such as magnetite (Izawa et al., 2019). Ilmenite also exhibits what we interpret as a Fresnel peak near 245 nm, likely due to a Ti 4+ -O transition. ...
Ilmenite is an important mineral for understanding lunar evolution. Ilmenite is also a primary ore of titanium on the Earth. Here we present a comprehensive examination of the spectral-compositional-structural relationships of ilmenites and related FeTi oxides. Ilmenite spectral features of interest include maxima near ~250 nm (due Ti⁴⁺-O and Fe²⁺-O charge transfers), ~335 nm (due to Fe²⁺-Ti⁴⁺ charge transfer), and 950 nm (interband maximum), and absorption features near ~540 nm (due to Fe²⁺-Ti⁴⁺ charge transfers), ~630 nm (due to Ti³⁺-Ti⁴⁺ charge transfers), and a ~ 1300/1600 nm absorption doublet (due to Fe²⁺ crystal field transitions). Absorption features transition from Fresnel peaks to valley around 400 nm, as absorption coefficients decrease toward longer wavelengths. Ilmenite powders are generally darkest and show the greatest spectral contrast and reflectance rise beyond ~1300 nm for the smallest grain sizes. Other Ti and FeTi oxides share some spectral properties with ilmenites but also exhibit many differences. The most common feature they share is a rise in reflectance toward shorter wavelengths below ~500 nm (i.e., blue spectral slope). Ti ± Fe oxide spectra can also exhibit absorption features attributable to Ti, Fe, and Ti ± Fe, and these features vary in intensity, shape and wavelength position due to factors such as Ti and Fe oxidation states, coordination environment, and nearest neighbor cation types. Ilmenite differs most from silicates in the region below ~500 nm: it shows a reflectance increase versus decrease toward shorter wavelengths for silicates, as well as diagnostic Ti/Ti-Fe maxima or minima. Thus, detection of ilmenite in mixtures is best accomplished by including the UV region in spectral analysis. With increasing ilmenite in mixtures, its diagnostic spectral features become increasingly apparent.
... For example, the delocalization of magnetite electrons produces a broad absorption minimum at 1000 nm that dominantly suppresses reflectance at 982 nm and, to a lesser extent, also 651 nm and 852 nm. Conversely, ilmenite and ulvöspinel have a local absorption maximum near 1000 nm, albeit reflectance is diminished unless coarse grained (Izawa et al. 2019). Magnesite and siderite have strong broad absorption features at 1100 nm due to transition metal cations, e.g. ...
The accurate characterization of mafic and ultramafic rocks is a challenging but necessary task given the spatial and genetic relationship of mineralization with specific lithologies (e.g. komatiite hosted nickel-sulfides preferentially associated with cumulate-rich ultramafic rocks). Rock classification is further complicated as most mafic and ultramafic rocks have undergone varying degrees of alteration. The accuracy and reproducibility of characterization can be significantly improved by using portable energy dispersive X-ray Fluorescence (pXRF) chemical data with portable Visible and Near-Infrared (pVis-NIR) mineralogical data.
A new workflow using pXRF and pVis-NIR is presented and used to reliably characterize mafic and ultramafic rocks from the Yilgarn Craton, Western Australia. The workflow involves 6 steps: Mitigate and identify compound processing and closure issues. For example, we used a pXRF with helium flush to reliably and rapidly measure light elements and mitigate closure, i.e. problems related to data failing to sum to 100%.
Identify and exclude geochemically heterogeneous samples. Heterogeneity may be unrelated to alteration and caused by veining or small-scale structure interleaving of different rock types. Geochemical heterogeneity was evaluated using skewness and kurtosis of SiO 2 data.
Relate rocks from similar magmatic, weathering and alteration events. This was achieved by interpreting data grouping of Vis-NIR ferric and ferrous iron data via a 852 nm/982 nm reflectance v. 651 nm/982 nm reflectance plot and the Ferrous Abundance Index. Unrepresentative data were omitted.
Correct XRF iron data, and characterize lithology and alteration. Values ascribed to regions in the TAS (Total Alkali Silica) diagram were used to approximate FeO and Fe 2 O 3 . Subsequently, geochemical indices (e.g. Mg#) were used to characterize the alteration box plot.
Characterize fractionation in detail. Fractionation variation diagrams were used to interpret fractionation, e.g. MgO v. Al 2 O 3 , Ca/Al v. Al 2 O 3 , Ni/Cr v. Ni/Ti, and MgO v. Cr.
Identify and quantify talc alteration and serpentinization. This included the use of a new alteration plot (Mg# v. 1410 nm RAD /Albedo) to estimate serpentinization and identify relationships between serpentine, carbonate, chlorite and talc abundances.
The results and observations contained in this contribution have important implications for progressive technologies such as core logging platforms that are equipped with pXRF and pVis-NIR instruments.
... μm region versus MM-38 than it did versus HG-42. This is likely due to the differences in the low wavelength boundary of the magnetite absorption feature described for these two samples in Izawa et al. (2015) and suggests a higher magnetite abundance and/or smaller grain size in HG-42 than in MM-38 (Izawa et al., 2019;Izawa et al., 2010a;Izawa et al., 2010b). 17274 (2000 is very close in shape and reflectance in the ~1.5-1.8 μm region and displays less reflectance difference in the longest wavelength segment than the corresponding HG-42 segment. ...
Asteroids are the origin point for most meteorites impacting Earth. Terrestrial meteorite samples provide evidence of what actually occurred in the early solar system at the formation location of the meteorite, and when it occurred. The ability to connect a meteorite sample to an asteroid parent body provides its starting location as a meteoroid. To date, only a handful of chondritic meteorite types have been credibly connected to an asteroid parent. For the past two decades, D-type asteroids, a dark, spectrally reddish, and featureless taxonomic type have been speculated to be the parent body of the tiny family of ungrouped chondrites. These include the Tagish Lake Meteorite (TLM), a ~ 4 m meteorite “fall” in Canada's Yukon territory recovered in 2000. The quest to identify the TLM parent has been a baffling one as D-type asteroids are dominant among the Jovian Trojans, rare in main asteroid belt, and extremely rare in the inner asteroid belt as well as Near-Earth space.
This study employed Near Infrared (NIR) spectra (0.7–2.45 μm) of 86 D-types and a variety of analysis techniques including visual analysis, slope analysis, curve fitting, Fréchet analysis, dynamical analysis and Shkuratov radiative transfer theory to search for the TLM parent body. Sixteen TLM samples from the NASA Reflectance Experiment Laboratory (RELAB) plus five additional mineralogically well-constrained samples measured using X-ray diffraction (XRD) and Rietveld refinement were compared to D-type asteroid spectra. Our results indicate, out of several promising candidates, Near-Earth asteroid 17274 (2006 LC16), a ~ 3 km diameter Amor asteroid is a plausible parent body for TLM.
... Consequently, the development of an innovative synthetic methodology is crucial to enrich the convenient properties of these compounds [14]. As a spinel-group mineral, magnetite crystallized in a space group of Fd-3 m, and due to its nature as an inversed spinel group, Fe 2+ ions occupy the octahedral sites, whilst Fe 3+ ions are distributed on both tetrahedral and octahedral sites [15]. However, the structure resilience of magnetite allows being substituted with a numerous number of cations which lead to the formation of many compositional variations [16,17]. ...
Magnetite nanoparticles constitute a class of nanoparticles which is easily manipulated using a magnetic field. Magnetite nanoparticles doped with ruthenium (Ru) ions [RuxFe(3−x)O4] were synthesized via co-precipitation method where 0.0 ≤ x ≤ 0.5 with step 0.1. The obtained nanopowder was investigated via x-ray diffraction, FTIR, FESEM. It was shown that Ru ions were incorporated successfully into a magnetite structure with a slight influence on the value of the lattice parameter which increased from 8.354 Å at x = 0.0 to be 8.403 Å at x = 0.3, while crystallite size deteriorated from 20.1 nm at x = 0.0 to be around 3 nm at x = 0.3. In addition, the surface roughness average was influenced by the dopant content, where it decreased from 35.6 nm at the pure magnetite to be 25.87 nm at x = 0.3. The ICP examination indicated that the measured contents of Ru ions through competitions were around 41 ppm and increased to 190 ppm comparing with 43 and 199 ppm as a theoretical value both x = 0.1 and 0.5. Regarding magnetic properties, the coercivity raised from 40.11 Oe and raised 44.66 Oe for x = 0.0 and 0.5, respectively. This manipulated behavior of magnetite due to dopant suggests that desired properties could be achieved via the dopant strategy to be used for several applications.
... 1.75, 1.9, and 2.1 to 2.2 mm) (60) are not detected in the OVIRS spectra. Magnetite and organics are optically opaque and unlikely to have the normal albedos of 10 to at least 19% observed for the bright features and veins (23,61). Thus, although the 3.4-mm feature in some OVIRS spectra could be attributable to organics rather than carbonates (Fig. 2), the low albedo of organic-rich material means that it is not a plausible candidate for the dominant composition of bright veins on Bennu. ...
The complex history of Bennu's surface
The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020.
Science , this issue p. eabc3660 , p. eabc3522 , p. eabc3557
... This suggests a key role of magnetite in determining spectral shape in the UV-VIS region for serpentines. Indeed, recent laboratory spectra of magnetite show that it is has a significant blue slope in the 0.2-to 0.5-μm region (Izawa et al., 2019). Magnetite has been discussed in the literature as playing a role in the VNIR blue spectral shape of some carbonaceous chondrites and serpentines (e.g., Clark et al., 2011;Cloutis, Hudon, et al., 2011). ...
Space weathering effects on the rocky S‐class asteroids are well understood. However, on the low‐albedo C‐complex asteroids, such as spacecraft targets Bennu and Ryugu, the situation is more complicated, especially due to a lack of spectral features throughout the visible‐near infrared spectral region. Here we show, through a combination of observational data and laboratory data of carbonaceous chondrites, phyllosilicates, and mixtures, that the UV‐visible spectral region is a diagnostic regime for studying space weathering effects on C‐complex asteroids. We show that space‐weathering‐produced opaque constituents, such as graphitized carbons, darken mixtures with phyllosilicates and produce a bluing the UV‐visible spectrum, consistent with what is seen on the asteroids compared with carbonaceous chondrites. Furthermore, we demonstrate that the UV upturns in the spectra of Bennu and Ryugu are consistent with the presence of graphitized carbon on those surfaces, the result of surface processing.
... Lauretta et al. 12 have identified a candidate magnetite feature at 0.55 µm (ref. 13 ) in the darkest materials imaged by the MapCam instrument; however, as of yet, no such feature has been observed in OVIRS spectra that would confirm this detection or its assignment to magnetite. Such a feature may become evident in the higher-spatial-resolution OVIRS data that will be collected later in the mission. ...
Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth.
During the first 2934 sols of the Curiosity rover’s mission 33,468 passive visible/near-infrared reflectance spectra were taken of the surface by the mast-mounted ChemCam instrument on a range of target types. ChemCam spectra of bedrock targets from the Murray and Carolyn Shoemaker formations on Mt. Sharp were investigated using principal component analysis (PCA) and various spectral parameters including the band depth at 535 nm and the slope between 840 nm and 750 nm. Four endmember spectra were identified. Passive spectra were compared to Laser Induced Breakdown Spectroscopy (LIBS) data to search for correlations between spectral properties and elemental abundances. The correlation coefficient between FeOT reported by LIBS and BD535 from passive spectra was used to search for regions where iron may have been added to the bedrock through oxidation of ferrous-bearing fluids, but no correlations were found. Rocks in the Blunts Point-Sutton Island transition that have unique spectral properties compared to surrounding rocks, that is flat near-infrared (NIR) slopes and weak 535 nm absorptions, are associated with higher Mn and Mg in the LIBS spectra of bedrock. Additionally, calcium-sulfate cements, previously identified by Ca and S enrichments in the LIBS spectra of bedrock, were also shown to be associated with spectral trends seen in Blunts Point. A shift towards steeper near-infrared slope is seen in the Hutton interval, indicative of changing depositional conditions or increased diagenesis.
Context. Spectrophotometry data of asteroids obtained in the 1980s showed that there are large variations in their near-ultraviolet (NUV) reflectance spectra. Reflectance spectra at NUV wavelengths are important because they help detect the presence of hydrated minerals and organics on the asteroid surfaces. However, the NUV wavelength region has not been fully investigated yet using spectroscopic data.
Aims. The aim of our study is to obtain the near-ultraviolet to visible (NUV-VIS, 0.35–0.95 μm) reflectance spectra of primitive asteroids with a focus on members of the Themis and Polana-Eulalia complex families. This characterization allows us to discuss the origin of two recent sample return mission target asteroids, (162173) Ryugu and (101955) Bennu.
Methods. We obtain low-resolution visible spectra of target asteroids down to 0.35 μm using the telescopes located at the Roque de los Muchachos Observatory (La Palma, Spain) and revisit spectroscopic data that have already been published. Using new spectroscopic and already published spectrophotometric and spectroscopic data, we study the characteristics of the NUV-VIS reflectance spectra of primitive asteroids, focusing on data of the Themis family and the Polana-Eulalia family complex. Finally, we compare the NUV characteristics of these families with (162173) Ryugu and (101955) Bennu. In this work, we also study systematic effects due to the use of the five commonly used stars in Landolt’s catalog as solar analogs to obtain the asteroid reflectance in the NUV wavelength range. We compare the spectra of five G-stars in Landolt’s catalog with the spectrum of the well-studied solar analog Hyades 64, also observed on the same nights.
Results. We find that many widely used Landolt’s G-type stars are not solar analogs in the NUV wavelength spectral region and thus are not suitable for obtaining the reflectance spectra of asteroids. We also find that, even though the Themis family and the PolanaEulalia family complex show a similar blueness at visible wavelengths, the NUV absorption of the Themis family is much deeper than that of the Polana-Eulalia family complex. We did not find significant differences between the New Polana and Eulalia families in terms of the NUV-VIS slope. (162173) Ryugu’s and (101955) Bennu’s spectral characteristics in the NUV-VIS overlaps with those of the Polana-Eulalia family complex which implies that it is the most likely origin of these two near-Earth asteroids.
Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-m-long elliptical crater. This exposed material that is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500–700 kg per cubic meter, about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 g of material, which will be delivered to Earth in 2023.
Two of the instruments onboard the OSIRIS-REx spacecraft, the MapCam color imager and the OVIRS visible and infrared spectrometer, observed the surface of asteroid (101955) Bennu in partially overlapping wavelengths. Significant scientific advances have been enabled by using data from these two instruments in tandem, but a robust statistical understanding of their relationship is needed for future analyses to cross-compare their data as accurately and sensitively as possible. Here we present a cross-instrument comparison of data acquired by MapCam and OVIRS, including methods and results for all global and site-specific observation campaigns in which both instruments were active. In our analysis, we consider both the absolute radiometric offset and the relative (normalized) variation between the two instruments; we find that both depend strongly on the photometric and instrumental conditions during the observation. The two instruments have a large absolute offset (>15%) due to their independent radiometric calibrations. However, they are very consistent (relative offset as low as 1%) when each instrument’s response is normalized at a single wavelength, particularly at low phase angles where shadows on Bennu’s rough surface are minimized. We recommend using the global datasets acquired at 12:30 pm local solar time for cross-comparisons; data acquired at higher phase angles have larger uncertainties.
L'étude de la formation et de l'évolution de notre Système solaire se fait selon plusieurs approches : i/ l'analyse de matière extraterrestre primitive retrouvée sur Terre en laboratoire, telles que les IDPs (Interplanetary Dust Particles), les météorites et les micrométéorites provenant en grande majorité des surfaces primitives du Système solaire (astéroïdes, comètes). ii/ l'observation des objets du Système solaire (planètes, astéroïdes, comètes) et d'autres systèmes planétaires en formation. iii/ les missions spatiales qui effectuent sur ces corps des analyses in-situ (Rosetta, Hayabusa2, OSIRIS-REx) et des retours d'échantillons (Stardust, Hayabusa 1&2, OSIRIS-REx). Les analyses de laboratoire sur des matériaux extraterrestres récupérés sur Terre permettent d’obtenir des informations très précises sur la chimie et les assemblages de la matière extraterrestre à différentes échelles. Cependant, pour replacer ces analyses dans un contexte astrophysique il est nécessaire d’identifier le corps parent de l’objet analysé (astéroïde ou comète). Ce lien n’est pas toujours évident à établir et peut se faire suivant différents critères, dont certains spectroscopiques (pentes, albédo, bandes d’absorptions) notamment dans la gamme du Vis-NIR (0,4 - 2,5 µm) souvent utilisée pour l’observation de ces corps. Les météorites constituent les objets les plus communément étudiés en laboratoire mais ne permettent pas aujourd’hui de représenter la diversité des petits corps du Système solaire. Les IDPs représentent une grande quantité de la matière extraterrestre accrétée chaque année par la Terre et sont étudiés en laboratoire depuis 1981. Mais peu d’études ont tenté jusqu’à présent de faire le lien entre les IDPs et les surfaces primitives du Système solaire avec des mesures de réflectances Vis-NIR. Dans ce contexte, nous avons analysé 15 IDPs fournis par la NASA. Après avoir développé un dispositif de mesure de réflectance Vis-NIR (0,45 - 1,0 µm) en salle blanche, un protocole de mesure a été mis en place pour obtenir le spectre de réflectance d’un grain isolé de taille micrométrique et un spectre Vis-NIR de chaque IDP a été obtenu. Ces mêmes IDPs ont été ensuite analysés en IR au synchrotron SOLEIL et en Raman permettant une analyse multi-longueur d’onde de ces échantillons. Les analyses IR et Raman permettent d’obtenir des informations précises sur la chimie (minéraux et organiques) des IDPs et de la matière extraterrestre en général. Tandis que la gamme du Visible permet une comparaison avec les spectres observationnels des surfaces primitives du Système solaire. Pour la première fois nous avons fait le lien entre un IDP hydraté et les astéroïdes de types Ch/Cgh. Les données IR pourront être également utilisées pour l'interprétation des données d’observation qui seront fournies par le futur télescope JWST (instrument MIRI) qui sera lancé en novembre 2021 et qui sondera la poussière dans les disques protoplanétaires et dans le milieu interstellaire et qui apportera de nouvelles observations des petits corps du Système solaire.
We present rotationally resolved spectroscopy of asteroid (93) Minerva using the Lowell Discovery Telescope with the Near-Infrared High Throughput Spectrograph (NIHTS). We obtained spectroscopy over ~34% of the asteroid's rotation period. Minerva has been shown to be spectrally similar to primitive carbonaceous chondrites (e.g., McAdam et al., 2018, Icarus 306, 32–49) indicating it has amorphous materials on its surface. The extent to which these materials appear over Minerva's surface could provide constraints on the asteroid's formation time and/or directly relate the asteroid to a chemical group of carbonaceous chondrite meteorites. Parent asteroids are thought to preserve primitive meteorites in either an outer shell of material or by avoiding parent body processing (e.g., accreting after the peak heat flux of ²⁶Al or before the introduction of exogenous ²⁶Al to the Solar System). These two scenarios are expected to have different properties: the no processing scenario produces an asteroid with a compositionally homogenous surface and interior while the outer shell scenario would have compositionally distinct surface and interior. Over the observed region, we report that Minerva's surface appears to have amorphous materials, potentially indicating a homogeneous surface. However, more data are needed to determine Minerva's compositional uniformity and which formation scenario is most appropriate.
The natural chemical and physical variations that occur within volcanic rocks (petrology) provide critical insights into mantle and crust conditions on terrestrial bodies. Visible/near-infrared (VNIR; 0.3–2.5 μm) and thermal infrared (TIR; 5–50 μm) spectroscopy are the main tools available to remotely characterize these materials from satellites in orbit. However, the accuracy of petrologic information that can be gained from spectra when rocks exhibit complex variations in mineralogy, crystallinity, and microtexture occurring together in natural settings is not well constrained. Here, we compare the spectra of a suite of volcanic planetary analog rocks from the Three Sisters Volcanic Complex, Oregon to their mineralogy, chemistry, and microtexture from X-ray diffraction, X-ray fluorescence, and electron microprobe analysis. Our results indicate that TIR spectroscopy is an effective petrologic tool in such rocks for modeling bulk mineralogy, crystallinity, and mineral chemistry. Given a library with appropriate glass endmembers, TIR modeling can derive glass abundance with similar accuracy as other major mineral groups and provide first-order estimates of glass wt.% SiO2 in glass-rich samples, but cannot effectively detect variations in microtexture and minor oxide minerals. In contrast, VNIR spectra often yield non-unique mineralogic interpretations due to overlapping absorption bands from olivine, glass, and Fe-bearing plagioclase. In addition, we find that sub-micron oxides hosted in transparent matrix material that are common in fine-grained extrusive rocks can lower albedo and suppress mafic absorption bands, leading to very different VNIR spectra in rocks with the same mineralogy and chemistry. Mineralogical interpretations from VNIR spectra should not be treated as rigorous petrologic indicators, but can supplement TIR-based petrology by providing unique constraints on oxide minerals, microtexture, and alteration processes.
Knowledge of the occurrence of water in the solar system provides key information concerning the formation and evolution of the solar system and lifeforms. In recent years, multiple remote-sensing observations have suggested the existence of water ice in permanently shadowed regions on some inner solar system bodies; however, the exact amount of water ice is highly uncertain. To test the performance of ice detection equipment for future lunar polar exploration missions, we constructed an apparatus to produce minute amounts of water ice (0.1–2 wt%) on lunar regolith analog minerals and measured their near-infrared spectra. The relationship between the strength of water absorption and the water content was quantified using the absorption at 1.5 μm in the reflectance spectra. The results show that the detectability of water ice attached to mineral grains depends on the mineral species. Laboratory reflectance spectra were compared to Hapke model spectra, and the observed spectral feature similarities indicate that the Hapke model can be effectively used when the ice is mixed in the form of spherical grains.
Context. The NASA Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission has obtained thousands of spectra of asteroid (101955) Bennu with the OSIRIS-REx Visible and InfraRed Spectrometer.
Aims. We present a spectral search for minor absorption bands and determine compositional variations on the surface of Bennu.
Methods. Reflectance spectra with low and high spatial resolutions were analyzed for evidence of weak absorption bands. Spectra were also divided by a global average spectrum to isolate unique spectral features, and variations in the strongest band depths were mapped on a surface shape model. The global visible to near-IR spectrum of Bennu shows evidence of several weak absorption bands with depths of a few percent.
Results. Several observed bands are consistent with phyllosilicates, and their distribution correlates with the stronger 2.74- μ m hydration band. A 0.55- μ m band is consistent with iron oxides and is deepest in the spectrally reddest areas on Bennu. The presence of hydrated phyllosilicates and iron oxides indicates substantial aqueous alteration in Bennu’s past.
Conclusions. Bennu’s spectra are not identical to a limited set of carbonaceous chondrite spectra, possibly due to compositional properties and spatial scale differences; however, returned samples should contain a mixture of common chondrite materials.
Multispectral sensors, along with common and advanced algorithms, have become efficient tools for routine lithological discrimination and mineral potential mapping. It is with this paradigm in mind that this paper sought to evaluate and discuss the detection and mapping of magnetite on the Eastern Limb of the Bushveld Complex, using high spectral resolution multispectral remote sensing imagery and GIS techniques. Despite the wide distribution of magnetite, its economic importance, and its potential as an indicator of many important geological processes, not many studies had looked at the detection and exploration of magnetite using remote sensing in this region. The Maximum Likelihood and Support Vector Machine classification algorithms were assessed for their respective ability to detect and map magnetite using the PlanetScope Analytic data. A K-fold cross-validation analysis was used to measure the performance of the training as well as the test data. For each classification algorithm, a thematic landcover map was created and an error matrix, depicting the user’s and producer’s accuracies as well as kappa statistics, was derived. A pairwise comparison test of the image classification algorithms was conducted to determine whether the two classification algorithms were significantly different from each other. The Maximum Likelihood Classifier significantly outperformed the Support Vector Machine algorithm, achieving an overall classification accuracy of 84.58% and an overall kappa value of 0.79. Magnetite was accurately discriminated from the other thematic landcover classes with a user’s accuracy of 76.41% and a producer’s accuracy of 88.66%. The overall results of this study illustrated that remote sensing techniques are effective instruments for geological mapping and mineral investigation, especially iron oxide mineralization in the Eastern Limb of the Bushveld Complex.
The complex history of Bennu's surface
The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020.
Science , this issue p. eabc3660 , p. eabc3522 , p. eabc3557
Structural absorption is widely accepted as an effective way to produce super black materials, which show matte black appearance and extremely broadband low reflectance. We have developed an ultrafast pulsed laser deposition (PLD) technology to deposit thin super black film by tuning the surface microstructure. Various coatings made by Fe, Al and Ti all showed broadband absorption, confirming that the PLD method is suitable for fabricating super back materials. At the wide range of wavelength from 250 nm to 2000 nm, the total integrated reflectance of iron coating is ∼1.2%. The super black iron surface coating has low volume density composed of separated tree-like aggregates of nanoparticles. We have demonstrated that low effective refractive index and multi-layered iron oxide structures are the two contributing factors for the broadband ultralow reflectance. The broadband high-performance film was employed to effectively convert sunlight to heat. The steady-state temperature of the super black surface is 76℃ under solar irradiation.
MFe2O4 nanoparticles with M = Fe, Co and Ni were prepared by three different methods (coprecipitation, sol-gel and hydrothermal synthesis). XRD patterns reveal an increase in the unit cell parameter associated to the presence of Fe²⁺. More intense diffraction maxima are observed for samples prepared by sol-gel. Particle size appears to be larger for Co ferrites and sol-gel samples. By contrast, smaller grain sizes are observed for NiFe2O4 and hydrothermal samples. The highest Ms appears in the Fe3O4 obtained by coprecipitation. An increment in the saturation magnetization values is found in the cobalt and nickel samples prepared by hydrothermal method. Iron and nickel samples behave as superparamagnetic materials with low blocking temperatures while CoFe2O4 samples exhibit a characteristic ferrimagnetism. Variations observed in the ZFC/FC curves of coprecipitation-prepared samples are related to both the different particle size distribution and the values of magnetic anisotropy constant.
Many studies have shown how pigments and internal nanostructures generate color in nature. External surface structures can also influence appearance, such as by causing multiple scattering of light (structural absorption) to produce a velvety, super black appearance. Here we show that feathers from five species of birds of paradise (Aves: Paradisaeidae) structurally absorb incident light to produce extremely low-reflectance, super black plumages. Directional reflectance of these feathers (0.05-0.31%) approaches that of man-made ultra-absorbent materials. SEM, nano-CT, and ray-tracing simulations show that super black feathers have titled arrays of highly modified barbules, which cause more multiple scattering, resulting in more structural absorption, than normal black feathers. Super black feathers have an extreme directional reflectance bias and appear darkest when viewed from the distal direction. We hypothesize that structurally absorbing, super black plumage evolved through sensory bias to enhance the perceived brilliance of adjacent color patches during courtship display.
Asteroids that are targets of spacecraft missions are interesting because they present us with an opportunity to validate ground-based spectral observations. One such object is near-Earth asteroid (NEA) (162173) Ryugu, which is the target of the Japanese Space Agency's (JAXA) Hayabusa2 sample return mission. We observed Ryugu using the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, on July 13, 2016 to constrain the object's surface composition, meteorite analogs, and link to other asteroids in the main belt and NEA populations. We also modeled its photometric properties using archival data. Using the Lommel-Seeliger model we computed the predicted flux for Ryugu at a wide range of viewing geometries as well as albedo quantities such as geometric albedo, phase integral, and spherical Bond albedo. Our computed albedo quantities are consistent with results from Ishiguro et al. (2014). Our spectral analysis has found a near-perfect match between our spectrum of Ryugu and those of NEA (85275) 1994 LY and Mars-crossing asteroid (316720) 1998 BE7, suggesting that their surface regoliths have similar composition. We compared Ryugu's spectrum with that of main belt asteroid (302) Clarissa, the largest asteroid in the Clarissa asteroid family, suggested as a possible source of Ryugu by Campins et al. (2013). We found that the spectrum of Clarissa shows significant differences with our spectrum of Ryugu, but it is similar to the spectrum obtained by Moskovitz et al. (2013). The best possible meteorite analogs for our spectrum of Ryugu are two CM2 carbonaceous chondrites, Mighei and ALH83100.
The mineralogy of comet 81P/Wild 2 particles, collected in aerogel by the Stardust mission, has been determined using synchrotron Fe-K X-ray absorption spectroscopy with in situ transmission XRD and X-ray fluorescence, plus complementary microRaman analyses. Our investigation focuses on the terminal grains of eight Stardust tracks: C2112,4,170,0,0; C2045,2,176,0,0; C2045,3,177,0,0; C2045,4,178,0,0; C2065,4,187,0,0; C2098,4,188,0,0; C2119,4,189,0,0; and C2119,5,190,0,0. Three terminal grains have been identified as near pure magnetite Fe3O4. The presence of magnetite shows affinities between the Wild 2 mineral assemblage and carbonaceous chondrites, and probably resulted from hydrothermal alteration of the coexisting FeNi and ferromagnesian silicates in the cometary parent body. In order to further explore this hypothesis, powdered material from a CR2 meteorite (NWA 10256) was shot into the aerogel at 6.1 km s−1, using a light-gas gun, and keystones were then prepared in the same way as the Stardust keystones. Using similar analysis techniques to the eight Stardust tracks, a CR2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel.
The Hayabusa2 mission journeys to C-type near-Earth asteroid (162173) Ryugu (1999 JU3) to observe and explore the 900 m-sized object, as well as return samples collected from the surface layer. The Haybusa2 spacecraft developed by Japan Aerospace Exploration Agency (JAXA) was successfully launched on December 3, 2014 by an H-IIA launch vehicle and performed an Earth swing-by on December 3, 2015 to set it on a course toward its target Ryugu. Hayabusa2 aims at increasing our knowledge of the early history and transfer processes of the solar system through deciphering memories recorded on Ryugu, especially about the origin of water and organic materials transferred to the Earth’s region. Hayabusa2 carries four remote-sensing instruments, a telescopic optical camera with seven colors (ONC-T), a laser altimeter (LIDAR), a near-infrared spectrometer covering the 3-μm absorption band (NIRS3), and a thermal infrared imager (TIR). It also has three small rovers of MINERVA-II and a small lander MASCOT (Mobile Asteroid Surface Scout) developed by German Aerospace Center (DLR) in cooperation with French space agency CNES. MASCOT has a wide angle imager (MasCam), a 6-band thermal radiator (MARA), a 3-axis magnetometer (MasMag), and a hyperspectral infrared microscope (MicrOmega). Further, Hayabusa2 has a sampling device (SMP), and impact experiment devices which consist of a small carry-on impactor (SCI) and a deployable camera (DCAM3). The interdisciplinary research using the data from these onboard and lander’s instruments and the analyses of returned samples are the key to success of the mission.
Nanophase iron (oxyhydr)oxides are ubiquitous on Earth, globally distributed on Mars, and likely present on numerous other rocky solar system bodies. They are often structurally and, therefore, spectrally distinct from iron (oxyhydr)oxide bulk phases. Because their spectra vary with grain size, they can be difficult to identify or distinguish unless multiple analysis techniques are used in tandem. Yet, most literature reports fail to use multiple techniques or adequately parameterize sample morphology, making it difficult to understand how morphology affects spectral characteristics across techniques. Here, we present transmission electron microscopy, Raman, visible and near-infrared, and mid-infrared attenuated total reflectance data on synthetic, nanophase akaganéite, lepidocrocite, goethite, hematite, ferrihydrite, magnetite, and maghemite. Feature positions are tabulated and compared to those for bulk (oxyhydr)oxides and other nanophase iron (oxyhydr)oxides from the literature. The utility and limitations of each technique in analyzing nanophase iron (oxyhydr)oxides are discussed. Raman, mid-infrared, and visible near-infrared spectra show broadening, loss of some spectral features, and shifted positions compared to bulk phases. Raman and mid-infrared spectroscopies are useful in identifying and distinguishing akaganéite, lepidocrocite, goethite, and hematite, though ferrihydrite, magnetite, and maghemite have overlapped band positions. Visible near-infrared spectroscopy can identify and distinguish among ferrihydrite, magnetite, and maghemite in pure spectra, though akaganéite, lepidocrocite, and goethite can have overlapping bands. It is clear from this work that further understanding of variable spectral features in nanophase iron (oxyhydr)oxides must await additional studies to robustly assess effects of morphology. This study establishes a template for future work.
In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on Jan. 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in August 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennus resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.
The putative occurrence of methane in the Martian atmosphere has had a major influence on the exploration of Mars, especially by the implication of active biology. The occurrence has not been borne out by measurements of atmosphere by the MSL rover Curiosity but, as on Earth, methane on Mars is most likely in the subsurface of the crust. Serpentinization of olivine-bearing rocks, to yield hydrogen that may further react with carbon-bearing species, has been widely invoked as a source of methane on Mars, but this possibility has not hitherto been tested. Here we show that some Martian meteorites, representing basic igneous rocks, liberate a methane-rich volatile component on crushing. The occurrence of methane in Martian rock samples adds strong weight to models whereby any life on Mars is/was likely to be resident in a subsurface habitat, where methane could be a source of energy and carbon for microbial activity.
Magnetite crystals produced by terrestrial magnetotactic bacterium MV-1 are elongated on a [111] crystallographic axis, in a so-called "truncated hexa-octahedral" shape. This morphology has been proposed to constitute a biomarker (i.e., formed only in biogenic processes). A subpopulation of magnetite crystals associated with carbonate globules in Martian meteorite ALH84001 is reported to have this morphology, and the observation has been taken as evidence for biological activity on Mars. In this study, we present evidence for the exclusively inorganic origin of [111]-elongated magnetite crystals in ALH84001. We report three-dimensional (3-D) morphologies for ∼ 1000 magnetite crystals extracted from: (1) thermal decomposition products of Fe-rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments: (2) carbonate globules in Martian meteorite ALH84001; and (3) cells of magnetotactic bacterial strain MV-1. The 3-D morphologies were derived by fitting 3-D shape models to two-dimensional bright-field transmission-electron microscope (TEM) images obtained at a series of viewing angles. The view down the {110} axes closest to the [111] elongation axis of magnetite crystals ([111]·{110} ≠0) provides a 2-D projection that uniquely discriminates among the three [111]-elongated magnetite morphologies found in these samples: [111]-elongated truncated hexa-octahedron ([111]-THO), [111]-elongated cubo-octahedron ([111]-ECO), and [111]-elongated simple octahedron ([111]-ESO). All [111]-elongated morphologies are present in the three types of sample, but in different proportions. In the ALH84001 Martian meteorite and in our inorganic laboratory products, the most common [111]-elongated magnetite crystal morphology is [111]-ECO. In contrast, the most common morphology for magnetotactic bacterial strain MV-1 is [111]-THO. These results show that: (1) the morphology of [111]-elongated magnetite crystals associated with the carbonate globules in Martian meteorite ALH84001 is replicated by an inorganic process; and (2) the most common crystal morphology for biogenic (MV-1) magnetite is distinctly different from that in both ALH84001 and our inorganic laboratory products. Therefore, [111]-elongated magnetite crystals in ALH84001 do not constitute, as previously claimed, a "robust biosignature" and, in fact, an exclusively inorganic origin for the magnetite is fully consistent with our results. Furthermore, the inorganic synthesis method, i.e., the thermal decomposition of hydrothermally precipitated Fe-rich carbonate, is a process analogue for formation of the magnetite on Mars. Namely, precipitation of carbonate globules from carbonate-rich hydrothermal solutions followed at some later time by a thermal pulse, perhaps in association with meteoritic impact or volcanic processes on the Martian surface.
Wüstite is a complex nonstoichiometric oxide of iron with the general structural formula VI[Fe2+1−3xFe3+2x−t □x+t]IVFe3+tO where 0.04 < x < 0.12 and 2.0 < (x + t)/t < 4.5. Wüstites of a given bulk composition exist in a variety of structure types characterized by differing ratios of octahedral to tetrahedral ferric iron and by differing degrees of long- and short-range order of clustered defects. In addition, most, if not all, wüstites have exsolved magnetite or iron lamellae on the scale of several tens of unit cells. Though wüstite is compositionally intermediate between pure FeO and magnetite, wüstite is not structurally intermediate and thus does not represent a solid solution between the two stoichiometric iron oxides. Physical properties of wüstite, including cation diffusion, electrical conductivity, and magnetic properties, are dependent on defect structure type and exsolution, as well as degree of nonstoichiometry. Simple linear relationships between stoichiometry and molar volume or between stoichiometry and bulk modulus, for example, do not obtain. Experimental studies on wüstite should include data on conditions of synthesis, as well as details on the structural state before and also after the experiments.
TEM observations of phases showing different types of aqueous alteration
in two CM2 chondrites. In Murchison, some of these phases ehibit
magnetite. In Maribo, a texturally unique phase reveals at least three
different stages of aqueous alteration.
Hydrogen is commonly produced during the high-temperature hydration of
mafic and ultramafic rocks, owing to the oxidation of reduced iron
present in the minerals. Hydrothermal hydrogen is known to sustain
microbial communities in submarine vent and terrestrial hot-spring
systems. However, the rates and mechanisms of hydrogen generation below
temperatures of 150°C are poorly constrained. As such, the existence
and extent of hydrogen-fuelled ecosystems in subsurface terrestrial and
oceanic aquifers has remained uncertain. Here, we report results from
laboratory experiments in which we reacted ground ultramafic and mafic
rocks and minerals--specifically peridotite, pyroxene, olivine and
magnetite--with anoxic fluids at 55 and 100°C, and monitored
hydrogen gas production. We used synchrotron-based micro-X-ray
fluorescence and X-ray absorption near-edge structure spectroscopy to
identify changes in the speciation of iron in the materials. We report a
strong correlation between molecular hydrogen generation and the
presence of spinel phases--oxide minerals with the general formula
[M2+M23+]O4 and a cubic
crystal structure--in the reactants. We also identify
Fe(III)-(hydr)oxide reaction products localized on the surface of the
spinel phases, indicative of iron oxidation. We propose that the
transfer of electrons between Fe(II) and water adsorbed to the spinel
surfaces promotes molecular hydrogen generation at low temperatures. We
suggest that these localized sites of hydrogen generation in ultramafic
aquifers in the oceanic and terrestrial crust could support
hydrogen-based microbial life.
The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. Various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence.
Carbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of volatiles and organic material to the early Earth. Presumed CC-like asteroids are also the targets of two current sample return missions: OSIRIS-REx to asteroid Bennu and Hayabusa-2 to asteroid Ryugu, and the Dawn orbital mission at asteroid Ceres. To improve our ability to identify and characterize CM2 CC-type parent bodies, we have examined how factors such as particle size, particle packing, and viewing geometry affect reflectance spectra of the Murchison CM2 CC. The derived relationships have implications for disc-resolved examinations of dark asteroids and sampleability. It has been found that reflectance spectra of slabs are more blue-sloped (reflectance decreasing toward longer wavelengths as measured by the 1.8/0.6 µm reflectance ratio), and generally darker, than powdered sample spectra. Decreasing the maximum grain size of a powdered sample results in progressively brighter and more red-sloped spectra. Decreasing the average grain size of a powdered sample results in a decrease in diagnostic absorption band depths, and redder and brighter spectra. Decreasing porosity of powders and variations in surface texture result in spectral changes that may be different as a function of viewing geometry. Increasing thickness of loose dust on a denser powdered substrate leads to a decrease in absorption band depths. Changes in viewing geometry lead to different changes in spectral metrics depending on whether the spectra are acquired in backscatter or forward-scatter geometries. In backscattered geometry, increasing phase angle leads to an initial increase and then decrease in spectral slope, and a general decrease in visible region reflectance and absorption band depths, and frequent decreases in absorption band minima positions. In forward scattering geometry, increasing phase angle leads to small non-systematic changes in spectral slope, and general decreases in visible region reflectance, and absorption band depths. The highest albedos and larger band depths are generally seen in the lowest phase angle backscattering geometry spectra. The reddest spectra are generally seen in the lowest phase angle backscatter geometry spectra. For the same phase angle, spectra acquired in forward scatter geometry are generally redder and darker and have shallower absorption bands than those acquired in backscatter geometry. Overall, backscatter geometry-acquired spectra are flatter, brighter, and have deeper 0.7 µm region absorption band depths than forward scatter geometry-acquired spectra. It was also found that the 0.7, 0.9, and 1.1 µm absorption bands in Murchison spectra, which are attributable to various Fe electronic processes, are ubiquitous and can be used to recognize CM2 chondrites regardless of the physical properties of the meteorite and viewing geometry.
Ceres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground based telescopic observations, because of its low albedo, flat to red-sloped spectra in the visible and near-infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 µm). Areas exhibiting slopes < –10 %/µm constitute only ~3 % of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥ –10 %/µm occur on more than 90 % of the cerean surface; in the following denoted as Ceres’ background material. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 µm absorption, as well as their reflectance at 0.653 µm. This resulted in a good match to heated CI Ivuna (heated to 200–300°C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres.
A number of planetary spacecraft missions have carried instruments with sensors covering the ultraviolet (UV) wavelength range. However, there exists a general lack of relevant UV reflectance laboratory data to compare against these planetary surface remote sensing observations in order to make confident mate- rial identifications. To address this need, we have systematically analyzed reflectance spectra of carbona- ceous materials in the 20 0–50 0 nm spectral range, and found spectral-compositional-structural relation- ships that suggest this wavelength region could distinguish between otherwise difficult-to-identify carbon phases. In particular (and by analogy with the infrared spectral region), large changes over short wave- length intervals in the refractive indices associated with the trigonal sp 2 π–π∗transition of carbon can lead to Fresnel peaks and Christiansen-like features in reflectance. Previous studies extending to shorter wavelengths also show that anomalous dispersion caused by the σ–σ∗transition associated with both the trigonal sp 2 and tetrahedral sp 3 sites causes these features below λ= 200 nm. The peak wavelength positions and shapes of π–π∗and σ–σ∗features contain information on sp 3 /sp 2 , structure, crystallinity, and powder grain size. A brief comparison with existing observational data indicates that the carbon frac- tion of the surface of Mercury is likely amorphous and submicroscopic, as is that on the surface of the martian satellites Phobos and Deimos, and possibly comet 67P/Churyumov–Gerasimenko, while further coordinated observations and laboratory experiments should refine these feature assignments and com- positional hypotheses. The new laboratory diffuse reflectance data reported here provide an important new resource for interpreting UV reflectance measurements from planetary surfaces throughout the solar system, and confirm that the UV can be rich in important spectral information.
Significance
Abiotic methane discharged from serpentinizing rocks supplies metabolic energy to chemosynthetic microbial communities and may have done so since the earliest lifeforms evolved on Earth. Several recent reports have claimed observation of abiotic formation of methane during low-temperature serpentinization of olivine-rich rocks during laboratory experiments. However, using ¹³ C-labeled carbon sources, this study shows that the methane observed in such experiments is predominantly derived from background sources rather than abiotic synthesis. Conversely, more rapid production of methane is observed when an H 2 -rich vapor phase is present within the reaction vessel. Overall, the results indicate that in situ abiotic synthesis may contribute less methane to near-surface serpentinites than some recent studies have suggested.
A remarkably complex electronic order of Fe2+/Fe3+ charges, Fe2+ orbital states, and weakly metal–metal bonded Fe3 units known as trimerons, was recently discovered in stoichiometric magnetite (Fe3O4) below the 125 K Verwey transition. Here, the low temperature crystal structure of a natural magnetite from a mineral sample has been determined using the same microcrystal synchrotron X-ray diffraction method. Structure refinement demonstrates that the natural sample has the same complex electronic order as pure synthetic magnetite, with only minor reductions of orbital and trimeron distortions. Chemical analysis shows that the natural sample contains dopants such as Al, Si, Mg and Mn at comparable concentrations to extraterrestrial magnetites, for example, as reported in the Tagish Lake meteorite. Much extraterrestrial magnetite exists at temperatures below the Verwey transition and hence our study demonstrates that the low temperature phase of magnetite represents the most complex long-range electronic order known to occur naturally.
Structures with Fe^(3+) shared through oxo- or hydroxyl-groups have antiferromagnetic
interactions. Such interactions result in enhanced intensity of the Fe^(3+) optical absorption bands
which in some systems can be as great as a factor of 100 compared to isolated, octahedrally
coordinated Fe^(3+) ions. A comparison is presented between the intensity of the lowest energy crystal-field
band of Fe^(3+) minerals and their magnetic moments which demonstrates the dependence of
optical absorption intensity and antiferromagnetic interactions in the host phase. Hematite, which is
usually responsible for the red color of geological materials, owes its intense color to these magnetic
interactions.
The stoichiometry of titanomagnetite spinel in the martian basaltic meteorites is assessed using quantitative analysis of
oxygen measured by electron microprobe and electron energy loss spectroscopy in the transmission electron microscope. The
spinels are stoichiometric within the errors of the techniques, enabling the calculation of oxygen fugacity with confidence.
The oxygen fugacity is calculated using the Ghiorso-Sack and Ca-QUIlF models, which also yield estimates of temperature. The
oxygen fugacity of the martian basalts increases from 3 log units below the QFM buffer for QUE 94201 to QFM − 1.8 for EETA
79001 (both lithologies), to QFM − 1.0 for Shergotty, Zagami, and Los Angeles. Dar al Gani 476 spinels contain significant
MgAl2O4 and FeCr2O4 components, complicating the use of Fe-Ti oxide models. The oxygen fugacity of Dar al Gani 476 is estimated to be 1.5 log
units below QFM, on the basis of the Ghiorso-Sack model. The absolute error on the oxygen fugacity estimates is ± 0.5 log
units; however, a consistent electron microprobe analytical routine was applied to all of the basalts, and the relative uncertainty
is closer to 0.2 log units. Oxyexsolution has occurred in QUE 94201, but reconstruction of pre-exsolution titanomagnetite
compositions permits the calculation of oxygen fugacity. Subsolidus reactions involving oxides and adjacent Fe-rich silicates
are documented and the use of the Ca-QUIlF model for calculation of oxygen fugacity from these phases is discussed.
We show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes. Chemically zoned carbonate globules and Fe sulfides were rapidly precipitated under low-temperature (150 °C), hydrothermal, and non-equilibrium conditions from multiple fluxes of Ca-Mg-Fe-CO2-S-H2O solutions that have different compositions. Chemically pure, single-domain, defect-free magnetite crystals were formed by subsequent decomposition of previously precipitated Fe-rich carbonates by brief heating to 470 °C. The sequence of hydrothermal precipitation of carbonates from flowing CO2-rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules, magnetite, and Fe sulfides in ALH84001. In separate experiments, kinetically controlled 13C enrichment was observed in synthetic carbonates that is similar in magnitude to the 13C enrichment in ALH84001 carbonates.
We report the first microstructural confirmation of circumstellar magnetite, identified in a petrographic thin section of the LaPaz Icefield 031117 CO3.0 chondrite. The O-isotopic composition of the grain indicates an origin in a low-mass (∼2.2 Mo.), approximately solar metallicity red/asymptotic giant branch (RGB/AGB) star undergoing first dredge-up. The magnetite is a single crystal measuring 750 × 670 nm, is free of defects, and is stoichiometric Fe3O4. We hypothesize that the magnetite formed via oxidation of previously condensed Fe dust within the circumstellar envelope of its progenitor star. Using an empirically derived rate constant for this reaction, we calculate that such oxidation could have occurred over timescales ranging from approximately ∼9000-500,000 years. This timescale is within the lifetime of estimates for dust condensation within RGB/AGB stars. © 2015. The American Astronomical Society. All rights reserved..
The rock-forming minerals which are abundant at the Earth’s surface are chiefly translucent silicates and carbonates. Opaque minerals occur as minor components in many rocks and the ore minerals, raw materials for the world supplies of metals, are mostly opaque phases. The major categories of opaque minerals are: (i) native metals, semimetals and alloys; (ii) metal sulphides and other chalcogenides (compounds of metals with Se, Te, As, Sb, Bi); (iii) oxides of certain elements, notably the transition metals: (iv) certain hydroxides and oxysalts.
Olivine-rich asteroids appear to be common in the main asteroid belt as well as present in the near-Earth asteroid population. There are a number of meteorite classes that are dominated by olivine ± metal. To determine whether relationships exist between these asteroids and meteorites, we spectrally character-ized a number of olivine + meteoritic metal powder intimate and areal mixtures, pallasite slabs, and olivine powders on a metal slab. Our goal is to understand the spectral characteristics of olivine + metal assem-blages and develop spectral metrics that can be used to analyze reflectance spectra of olivine-dominated asteroids. We found that the major olivine absorption band in the 1 lm region is resolvable in intimate mixtures for metal abundances as high as $90 wt.%. The wavelength position of the 1 lm region olivine
Abstract– We have carried out a sample-correlated spectroscopic and mineralogical investigation of samples from seven different collection sites of the Tagish Lake C2 chondrite. Rietveld refinement of high-resolution powder X-ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Thermal infrared (400–4500 cm−1, 2.2–25.0 μm) spectra of the same samples were obtained using diffuse (biconical) reflectance infrared Fourier transform spectroscopy (DRIFTS). Our results are in good agreement with previous studies of the mineralogy of the Tagish Lake meteorite; we find however that Tagish Lake is more varied in major mineralogy than has previously been reported. In particular, we observed two new distinct lithologies, an inclusion-poor magnetite- and sulfide-rich lithology, and a carbonate-rich, siderite-dominated lithology in addition to the previously documented carbonate-rich and carbonate-poor lithologies. Grain density for each Tagish Lake sample was calculated from the measured mineral modal abundances and known mineral densities. For powders from three originally intact inclusion-rich samples, the calculated grain density is 2.77 ± 0.05 g cm−3, in excellent agreement with those reported in the literature for other intact inclusion-rich Tagish Lake samples. Tagish Lake disaggregated samples have a significantly higher calculated grain density due to their lower saponite-serpentine content, likely a result of mineral separation in the meltwater holes from which they were collected; the disaggregated samples may not therefore adequately represent bulk samples of the Tagish Lake meteorite. The predominance of very fine-grained material in the Tagish Lake samples investigated in this study is expected to produce infrared spectra representative of asteroidal regolith. Gypsum and talc have been found by XRD in powders from the inclusion-rich, intact Tagish Lake samples in this study, and may have been present in the parent body; if present, these hydrous sulfates would complicate the interpretation of possible hydrated mineral features in asteroid infrared spectra.
We present the magnetic properties of the Noachian Martian breccia NWA 7034. Among the 25 unpaired Martian meteorites studied to date, NWA 7034 has a unique magnetic mineralogy. It contains about 15 wt. % of iron oxides as magnetite that has experienced cation substitution and partial alteration to maghemite, with about a quarter of the oxides being pure maghemite. It also contains oxyhydroxides in the form of superparamagnetic goethite. The presence of maghemite and goethite makes NWA 7034 the most oxidized Martian meteorite. The overall magnetic assemblage is partly linked to near-surface hydrothermal alteration. The high concentration of magnetic phases with high laboratory unblocking temperatures makes NWA 7034 a plausible analogue source lithology for the strong magnetization of the Martian Noachian crust. Near surface hydrothermal alteration can enhance the remanence of Martian rocks and account for local, high magnetic anomalies of shallow source.
Micron to submicron-sized magnetites with distinctive morphologies are
present in many chondritic meteorites. Determination of the origin of
any magnetite type could allow important constraints to be placed on
nebular or parent body processes.
The optical and magneto-optical spectra of charge-ordered magnetite
(Fe3O4) below the Verwey transition and
Mg2+- and Al3+-substituted
Fe3O4 are investigated theoretically from first
principles, using the fully relativistic Dirac linear muffin-tin orbital
band structure method. The electronic structure is obtained with the
local spin-density approximation (LSDA), as well as with the so-called
LSDA+U approach for which the charge ordering is found to be a stable
solution with an energy gap value of 0.19 eV (the experimental value is
0.14 eV) in contrast to a metallic state given by LSDA. The origin of
the Kerr rotation realized in the compounds is examined.
Fayalite, hedenbergite, and magnetite in Y-81020, EET 90043 and MAC
88107 resulted from aqueous alteration of the CO chondrite parent
asteroid at low water/rock ratio.
X-ray Absorption Fine Structure techniques have been used on Comet Wild2/81P tracks from the Stardust mission. Fe-XANES and EXAFS have been performed on aerogel sections from Tracks 41 and 162 as well as the mid and terminal positions of Track 134. This is the first use of EXAFS in the study of early Solar System materials. With EXAFS, we have measured Fe–O and Fe–S bond lengths and thus, together with complementary XANES measurements, identified Fe-rich phases. In particular, we show that ferric-rich phases in 2 Tracks (41, 162) have Fe–O bond 1st shell bond lengths of 1.99–2.01 Å and Fe K absorption edge and pre edge centroid positions consistent with being hematite-dominated grains. These iron oxides can be clearly distinguished from a magnetite grain, present in Track 134. We also demonstrate the identification of the Mg-rich end member olivine using EXAFS with XANES in Track 162. The terminal grain of Track 134 is pyrrhotite, its first atomic shell has an Fe–S structure, with 4 nearest neighbouring S atoms at a distance of 2.29 ± 0.05 Å.Our XANES results show the presence of Fe3+-bearing grains along the Stardust tracks and suggest either flash-cooling of an Fe–S–SiO–O2 gas during capture or the presence of a Fe–Ni–S–O melt along the cometary tracks during impact capture in the aerogel, rather than the capture process being solely associated with reduction of cometary phases. Accurate determination of Comet Wild2 redox conditions requires the identification of phases, in particular terminal grains, which have not experienced this melting. For instance, the larger hematite-rich grains (>10 μm) are more likely to be cometary in origin. EXAFS provides a valuable new analytical technique to study fine-grained early Solar System materials.
It is reported about measurements of the equatorial magneto-optic Kerr effect of Fe3O4 in the near infrared at temperatures 95 K and 293 K. Out of these measurements real and imaginary parts of the corresponding nondiagonal component of the dialectric tensor were determined. The results show a broad magneto-optical absorption maximum centered at a photon energy of 0.55 eV. On cooling the material, the position of the maximum remains essentially unchanged.
From reflectivity measurements, the optical constants n and k of magnetite have been determined in the photon energy region between 0.1 and 3 eV for temperatures between 77 and 300 K. The results show a broad band (width 1.2 eV), which splits at the Verwey temperature with a gap of about 0.11 eV.ZusammenfassungMit Hilfe von Reflexionsmessungen wurden die optischen Konstanten n und k von Magnetit für Photonenenergieen zwischen 0.1 und 3 eV und Temperaturen zwischen 77 und 300 K bestimmt. Die Ergebnisse zeigen ein breites Band (Bandbreite 1.2 eV) und eine Aufspaltung dieses Bandes bei der Verweytemperatur mit einer Energielücke von 0.11 eV.
The optical and magneto-optical spectra of charge-ordered magnetite Fe3O4 below the Verwey transition and Mg2+ and Al3+ substituted Fe3O4 are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA) as well as with the so called LSDA+U approach for which the charge ordering is found to be a stable solution with an energy gap value of 0.19 eV (the experimental value is 0.14 eV) in contrast to a metallic state given by LSDA. The origin of the Kerr rotation realized in the compounds is examined.
We present reflectance spectra from 0.4 to 2.4μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H2O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).
Spinet single crystals of 19 compositions along the magnetite-ulvospinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations. the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mossbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate. and tetrahedral bond length increase with increasing ulvospinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation. the Akimoto model (T)(Fe(1-X)(3+)Fe(X)(2+))(M)(Fe(2+)Fe(1-X)(3+)Ti(X))O(4) describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction (T)Fe(2+) + (M)Fe(3+) = (T)Fe(3+) + (M)Fe(2+), which causes (M)Fe(2+) not equal 1 and (T)Fe(2+)/Ti not equal 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti approximate to 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the (T)Fe(3+) + (M)Fe(3+) = (T)Fe(2+) + (M)Ti(4+) chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvospinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.
The reliability and utility of diffuse reflectance spectra are briefly but critically reviewed. The results of measurements of diffuse reflectance over the wavelength range 200 < λ < 2500 nm are reported for wüstite, magnetite, hematite, maghemite, ilmenite, ulvöspinel, and α -FeO · OH (goethite), β -FeO 7sd OH, γ -FeO · OH (lepidocrocite), and δ -FeO · OH. The spectra have been assigned by reference to simplified molecular-orbital energy-level diagrams derived from recent SCF-X α calculations.
The specular reflectances reported in the Quantitative Data File (Henry, 1977) are related to the diffuse reflectance spectra in a rational way. Minerals that absorb strongly throughout the visible display little dispersion of specular reflectance, and their powders are dark (wüstite, magnetite, ilmenite, ulvöspinel); those that absorb much more strongly in the near ultraviolet than in the visible have specular reflectances that decrease monotonically from blue to red according to a simple dispersion relation derived by combining the Sellmeier dispersion and Fresnel reflexion equations; their powders are strongly coloured (hematite, maghemite, lepidocrocite, goethite) and their optical anisotropy is closely related to crystal structure.
The spectral properties (0.35-2.20 microns) of submicron powders of hematite, maghemite, magnetite, goethite, and lepidocrocite are determined. Other physicochemical data are obtained for the powders in order to determine if deviations from stoichiometry occur due to their small particle size, to determine their state of chemical and phase purity, and to determine the physical characteristics of the individual powders. The physicochemical data obtained include mean particle diameter, discrete particle shape, chemical composition, crystallographic phase, magnetic parameters, and Moessbauer parameters. The positions of the spectral features for the hematite, maghemite, and magnetite powders are independent of temperature over the interval between about +20 and -110 C. For the goethite and lepidocrocite powders, a small shift of about 0.02 micron to shorter wavelengths is observed for some of the features after cooling to about -110 C. The spectral properties of the iron oxides and oxyhydroxides are important not only for understanding the basic physics and chemistry of the compounds but also for applications such as the remote sensing of the earth and Mars.