首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Lunar highland region and associated craters are mostly composed of anorthosite. In the present study, we studied the reflectance spectra of terrestrial anorthosites collected from Sittampundi Anorthosites Complex, which is considered as equivalent (simulant) of lunar highland anorthosites. The objective of the study is to interpret diagnostic spectral features of analog anorthosite for remotely exploring lunar highland region. Reflectance spectra of anorthosites were measured under two different environments, such as controlled field and laboratory conditions. In these two procedures, the laboratory spectra give clear, diagnostic spectral information in the present study. Reflectance spectra captured under 350-2500 nm covering UV, Visible, NIR, and SWIR part of the electromagnetic spectrum. The spectral characteristics of anorthosites measured under various parts of electromagnetic spectrum have diagnostic absorption features at 380-387, 700-740, 930-1100, 1160-1200, 1415, 1920, 2200 and 2330 nm correspondingly due to plagioclase UV absorption, Fe3+ electron transition absorption, Fe2+ pyroxene and olivine absorption, OH/Mn3+ crystal transition absorption, pyroxene absorption, Al-OH absorption and Mg-OH absorption. Mineralogical and chemical analyses were carried out for four anorthosites and compared with the results of chemical component of lunar anorthosite. The percentage of plagioclase content, relative abundance of low and high calcium pyroxene and olivine in different anorthosite samples are correlated with the albedo range, absorption shape, absorption centers and band depth. The similarity in the diagnostic spectral features of the anolog anorthosite with lunar anorthosites could be effectively utilized for remotely mapping the lunar highland region.  相似文献   

2.
Abstract— The lunar soil characterization consortium, a group of lunar‐sample and remote‐sensing scientists, has undertaken the extensive task of characterization of the finest fractions of lunar soils, with respect to their mineralogical and chemical makeup. These compositional data form the basis for integration and modeling with the reflectance spectra of these same soil fractions. This endeavor is aimed at deciphering the effects of space weathering of soils on airless bodies with quantification of the links between remotely sensed reflectance spectra and composition. A beneficial byproduct is an understanding of the complexities involved in the formation of lunar soil. Several significant findings have been documented in the study of the <45 μm size fractions of selected Apollo 17 mare soils. As grain size decreases, the abundance of agglutinitic glass increases, as does the plagioclase, whereas the other minerals decrease. The composition of the agglutinitic glass is relatively constant for all size fractions, being more feldspathic than any of the bulk compositions; notably, TiO2 is substantially depleted in the agglutinitic glass. However, as grain size decreases, the bulk composition of each size fraction continuously changes, becoming more Al‐rich and Fe‐poor, and approaches the composition of the agglutinitic glasses. Between the smallest grain sizes (10–20 and < 10 μm), the IS/FeO values (amount of total iron present as nanophase Fe0) increase by greater than 100% (>2x), whereas the abundance of agglutinitic glass increases by only 10–15%. This is evidence for a large contribution from surface‐correlated nanophase Fe0 to the IS/FeO values, particularly in the <10 μm size fraction. The surface nanophase Fe0 is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils, and to a lesser degree for the immature soils (Keller et al., 1999a). It is reasoned that the vapor‐deposited patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.  相似文献   

3.
Abstract— Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry, Ranging) and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the “space weathering” process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low‐Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine ultraviolet‐visible (UV‐vis) image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (>90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth‐based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied “1 μm” mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low‐Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron impurity absorption at 1.25 μm. Detection of this diagnostic band may be possible in fresh crater deposits.  相似文献   

4.
Reliable quantitative mapping of minerals exposed on Vesta's surface is crucial for understanding the crustal composition, petrologic evolution, and surface modification of the howardite, eucrite, and diogenite (HED) parent body. However, mineral abundance estimates derived from visible–near infrared (VIS–NIR) reflectance spectra are complicated by multiple scattering, particle size, and nonlinear mixing effects. Radiative transfer models can be employed to accommodate these issues, and here we assess the utility of such models to accurately and efficiently determine modal mineralogy for a suite of eucrite and olivine‐bearing (harzburgitic) diogenite meteorites. Hapke and Shkuratov radiative transfer models were implemented to simultaneously estimate mineral abundances and particle size from VIS–NIR reflectance spectra of these samples. The models were tested and compared for laboratory‐made binary (pyroxene–plagioclase) and ternary mixtures (pyroxene–olivine–plagioclase) as well as eucrite and diogenite meteorite samples. Results for both models show that the derived mineral abundances are commonly within 5–10% of modal values and the estimated particle sizes are within the expected ranges. Results for the Hapke model suggest a lower detection limit for olivine in HEDs when compared with the Shkuratov model (5% versus 15%). Our current implementation yields lower uncertainties in mineral abundance (commonly <5%) for the Hapke model, though both models have an advantage over typically used parameters such as band depth, position, and shape in that they provide quantitative information on mineral abundance and particle size. These results indicate that both the Hapke and Shkuratov models may be applied to Dawn VIR data in a computationally efficient manner to quantify the spatial distribution of pyroxene, plagioclase, and olivine on the surface of Vesta.  相似文献   

5.
Abstract– Diagnostic mineral absorption features for pyroxene(s), olivine, phyllosilicates, and hydroxides have been detected in the near‐infrared (NIR: approximately 0.75–2.50 μm) spectra for 60% of the Tholen‐classified ( Tholen 1984, 1989 ) M‐/X‐asteroids observed in this study. Nineteen asteroids (42%) exhibit weak Band I (approximately 0.9 μm) ± Band II (approximately 1.9 μm) absorptions, three asteroids (7%) exhibit a weak Band I (approximately 1.05–1.08 μm) olivine absorption, four asteroids (9%) display multiple absorptions suggesting phyllosilicate ± oxide/hydroxide minerals, one (1) asteroid exhibits an S‐asteroid type NIR spectrum, and 18 asteroids (40%) are spectrally featureless in the NIR, but have widely varying slopes. Tholen M‐asteroids are defined as asteroids exhibiting featureless visible‐wavelength (λ) spectra with moderate albedos ( Tholen 1989 ). Tholen X‐asteroids are also defined using the same spectral criterion, but without albedo information. Previous work has suggested spectral and mineralogical diversity in the M‐asteroid population ( Rivkin et al. 1995, 2000 ; Busarev 2002 ; Clark et al. 2004 ; Hardersen et al. 2005 ; Birlan et al. 2007 ; Ockert‐Bell et al. 2008, 2010 ; Shepard et al. 2008, 2010 ; Fornasier et al. 2010 ). The pyroxene‐bearing asteroids are dominated by orthopyroxene with several likely to include higher‐Ca clinopyroxene components. Potential meteorite analogs include mesosiderites, CB/CH chondrites, and silicate‐bearing NiFe meteorites. The Eos family, olivine‐bearing asteroids are most consistent with a CO chondrite analog. The aqueously altered asteroids display multiple, weak absorptions (0.85, 0.92, 0.97, 1.10, 1.40, and 2.30–2.50 μm) indicative of phyllosilicate ± hydroxide minerals. The spectrally featureless asteroids range from metal‐rich to metal‐poor with meteorite analogs including NiFe meteorites, enstatite chondrites, and stony‐iron meteorites.  相似文献   

6.
Abstract— We present combined multi‐spectral imager (MSI) (0.95 μm) and near‐infrared spectrometer (NIS) (0.8–2.4 μm) observations of Psyche crater on S‐type asteroid 433 Eros obtained by the Near‐Earth Asteroid Rendezvous (NEAR)—Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material. At spatial scales of 620 m/ spectrum, Psyche crater wall materials exhibit albedo contrasts of 32–40% at 0.946 μm. Associated spectral variations occur at a much lower level of 4–8% (±2–4%). We report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation. We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to variations in grain size, olivine, pyroxene or lunar‐like optical maturation. A grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed. Olivine and pyroxene variations would produce strong band‐correlated variations that are not observed. A simple lunar‐like optical maturation effect would produce strong reddening that is not observed. The contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of shock) and lunar‐like optical maturation. These results suggest that space weathering processes may affect the spectral properties of Eros materials, causing surface exposures to differ optically from subsurface bedrock. However, there are significant spectral differences between Eros' proposed analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures of subsurface bright materials. After accounting for all differences in the measurement units of our reflectance comparisons, we have found that the bright materials on Eros have reflectance values at 0.946 μm consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 μm. Most importantly, we calculate that average Eros surface materials are 30–40% darker than meteorites.  相似文献   

7.
Abstract— The fine fraction of lunar soils (<45 μm) dominates the optical properties of the bulk soil. Definite trends can be seen in optical properties of size separates with decreasing particle size: diminished spectral contrast and a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fraction (defined here as the <10 μm fraction) appears to be enriched in weathering products relative to the larger size fractions, as would be expected for surface correlated processes. This <10 μm fraction tends to exhibit very little spectral contrast, often with no distinguishable ferrous iron absorption bands. Additionally, the finest fractions of highland soils are observed to have very different spectral properties than the equivalent fraction of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whereas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to differences in the total amount of nanophase iron that accumulates in space weathering products. Such ground‐truth information derived from the <10 μm fraction of lunar soils provides valuable insight into optical properties to be expected in other space weathering environments such as the asteroids and Mercury.  相似文献   

8.
Abstract— We investigate the possibility that Mercury's crust is very reduced with FeO concentrations of less than ?0.1 wt%. We believe that such a surface could have a composition of enstatite, plagioclase, diopside, and sulfide, similar to the mineral assemblages found in aubritic meteorites. To test this hypothesis, we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of Mercury. We found that some sulfides have distinctive absorption features in their spectra shortwards of ?0.6 μm that may be apparent in the spectrum of such an object. Determination of the surface composition of Mercury using orbital x‐ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of Al, Mg, S, and Fe. The strongest argument against Mercury having an enstatite basalt composition is its extreme spectral redness. Significant reddening of the surface of an object (such as Mercury) is believed to require reduction of FeO to nanophase iron, thus requiring a few percent FeO in the material prior to alteration.  相似文献   

9.
Abstract— Concentrations of adsorbed water in single mineral grains of Antarctic lunar meteorites were determined with micro infrared (IR) spectroscopy. A relationship was found between the mineral's ability to adsorb water and the extent of Ce anomaly in rare earth element (REE) patterns precisely determined by the isotope dilution method using a thermal ionization mass spectrometer. Asuka (A) 881757, a lunar meteorite from the mare basalt without Ce anomaly, showed no trace of IR absorption due to adsorbed water. On the contrary, Yamato (Y) 791197-109, Y-86032-98, Y-86032-95, Y-791197-115 and Y-82192-55A from the lunar highland exhibiting positive Ce anomaly showed IR absorption due to adsorbed water in some of their minerals. The detected water would be of terrestrial origin, because it was not structurally bound and easy to exchange judging from the spectral band shape. The contrast in concentration of adsorbed water between the lunar highland and the mare basalt is derived from a difference in the density of microfractures in mineral grains. Average concentrations of adsorbed water in the lunar highland meteorites were 3.8 mg/cm3 for pyroxene and olivine, and 1.7 mg/cm3 for plagioclase. This contrast between minerals is noteworthy because it has been known that Ce anomaly of pyroxene and olivine is larger than that of plagioclase both for Antarctic lunar meteorites and some lunar rocks. Furthermore, more adsorbed water was detected for minerals in meteorites that exhibit larger Ce anomaly. The present observations demonstrate that the extent of Ce anomaly correlated with the concentration of adsorbed water, which suggests that active mineral surface resulting in adsorption of water could be a trace of interaction forming Ce anomaly. Terrestrial weathering on Antarctica and REE fractionation on the Moon are discussed for possible origins of the Ce anomaly.  相似文献   

10.
Reflectance Spectral Characteristics of Lunar Surface Materials   总被引：2，自引：0，他引：2  
Based on a comprehensive analysis of the mineral composition of major lunar rocks (highland anorthosite, lunar mare basalt and KREEP rock), we investigate the reflectance spectral characteristics of the lunar rock-forming minerals, including feldspar, pyroxene and olivine. The affecting factors, the variation of the intensity of solar radiation with wavelength and the reflectance spectra of the lunar rocks are studied. We also calculate the reflectivity of lunar mare basalt and highland anorthosite at 300 nm, 415 nm, 750 nm, 900 nm, 950 nm and 1000 nm. It is considered that the difference in composition between lunar mare basalt and highland anorthosite is so large that separate analyses are needed in the study of the reflectivity of lunar surface materials in the two regions covered by mare basalt and highland anorthosite, and especially in the region with high Th contents, which may be the KREEP-distributed region.  相似文献   

11.
The goal of this study is to develop an efficient and accurate model for using visible–near infrared reflectance spectra to estimate the abundance of minerals on the lunar surface. Previous studies using partial least squares (PLS) and genetic algorithm–partial least squares (GA–PLS) models for this purpose revealed several drawbacks. PLS has two limitations: (1) redundant spectral bands cannot be removed effectively and (2) nonlinear spectral mixing (i.e., intimate mixtures) cannot be accommodated. Incorporating GA into the model is an effective way for selecting a set of spectral bands that are the most sensitive to variations in the presence/abundance of lunar minerals and to some extent overcomes the first limitation. Given the fact that GA–PLS is still subject to the effect of nonlinearity, here we develop and test a hybrid partial least squares–back propagation neural network (PLS–BPNN) model to determine the effectiveness of BPNN for overcoming the two limitations simultaneously. BPNN takes nonlinearity into account with sigmoid functions, and the weights of redundant spectral bands are significantly decreased through the back propagation learning process. PLS, GA–PLS and PLS–BPNN are tested with the Lunar Soil Characterization Consortium dataset (LSCC), which includes VIS–NIR reflectance spectra and mineralogy for various soil size fractions and the accuracy of the models are assessed based on R2 and root mean square error values. The PLS–BPNN model is further tested with 12 additional Apollo soil samples. The results indicate that: (1) PLS–BPNN exhibits the best performance compared with PLS and GA–PLS for retrieving abundances of minerals that are dominant on the lunar surface; (2) PLS–BPNN can overcome the two limitations of PLS; (3) PLS–BPNN has the capability to accommodate spectral effects resulting from variations in particle size. By analyzing PLS beta coefficients, spectral bands selected by GA, and the loading curve of the latent variable with the largest weight in PLS–BPNN, we conclude that spectral information incorporated into the three models is directly derived from the diagnostic absorption bands associated with the individual minerals. It is concluded that the PLS–BPNN model should be applicable to both Clementine UV–VIS–NIRs and Moon Mineralogy Mapper (M3) data.  相似文献   

12.
M-type asteroids, as defined in the Tholen taxonomy (Tholen, D.J. [1984]. Asteroid Taxonomy from Cluster Analysis of Photometry. Ph.D. Dissertation, University of Arizona, Tucson), are medium albedo bodies supposed to have a metallic composition and to be the progenitors both of differentiated iron–nickel meteorites and enstatite chondrites. We carried out a spectroscopic survey in the visible and near infrared wavelength range (0.4–2.5 μm) of 30 asteroids chosen from the population of asteroids initially classified as Tholen M-types, aiming to investigate their surface composition. The data were obtained during several observing runs during the years 2004–2007 at the TNG, NTT, and IRTF telescopes. We computed the spectral slopes in several wavelength ranges for each observed asteroid, and we searched for diagnostic spectral features. We confirm a large variety of spectral behaviors for these objects as their spectra are extended into the near infrared, including the identification of weak absorption bands, mainly of the 0.9 μm band tentatively attributed to orthopyroxene, and of the 0.43 μm band that may be associated to chlorites and Mg-rich serpentines or pyroxene minerals such us pigeonite or augite. A comparison with previously published data indicates that the surfaces of several asteroids belonging to the M-class may vary significantly.We attempt to constrain the asteroid surface compositions of our sample by looking for meteorite spectral analogs in the RELAB database and by modeling with geographical mixtures of selected meteorites/minerals. We confirm that iron meteorites, pallasites, and enstatite chondrites are the best matches to most objects in our sample, as suggested for M-type asteroids. For 22 Kalliope, we demonstrate that a synthetic mixture obtained enriching a pallasite meteorite with small amounts (1–2%) of silicates well reproduce the spectral behavior including the observed 0.9 μm feature.The presence of subtle absorption features on several asteroids confirms that not all objects defined by the Tholen M-class have a pure metallic composition.A statistical analysis of spectral slope distribution vs. orbital parameters shows that our sample originally defined as Tholen M-types tend to be dark in albedo and red in slope for increasing value of the semi-major axis. However, we note that our sample is statistically limited by our number of objects (30) and slightly varying results are found for different subsets. If confirmed, the albedo and slope trends could be due to a difference in composition of objects belonging to the outer main belt, or alternatively to a combination of surface composition, grain size and space weathering effects.  相似文献   

13.
New spectral reflectance measurements of asteroid 4 Vesta were obtained using a silicon vidicon spectrometer with a resolution of 0.002–0.004 μm. The major absorption band in the near infrared has a minimum at 0.924 ± 0.004 μm with a bandwidth of 0.18 μm full width at half power (fwhp). The band represents a 30% absorption relative to peak reflectance at 0.75 μm. The absorption band has been interpreted to be due to electronic absorptions in ferrous iron in sixfold coordination in the pyroxene, pigeonite. The increased spectral resolution of these observations compared to earlier spectrophotometry enables us to refine the pyroxene composition, from the position of the Fe2+ absorption band, and arrive at a relative calcium content [Ca/(Mg + Fe + Ca)] of 10–12%. The absorption band is symmetric about its center, implying the presence of little or no olivine. The existence of the 2.0-μm pyroxene band which was verified by Larson and Fink (1975) confirms the interpretation based on the 1.0-μm band.  相似文献   

14.
Abstract— Based on recent progress in simulating space weathering on asteroids using pulse‐laser irradiation onto olivine and orthopyroxene samples, detailed analyses of two of the A and R type asteroid reflectance spectra have been performed using reflectance spectra of laser‐treated samples. The visible‐near‐infrared spectrum of olivine is more altered than that of pyroxene at the same pulse‐laser energy, suggesting that olivine weathers more rapidly than orthopyroxene in space. The same trend can be detected from reflectance spectra of the asteroids, where the more olivine an asteroid has, the redder its 1 μm band continuum can become. Comparison of the 1 μm band continuum slope and the 2/1 μm band area ratio between the asteroids and olivine and pyroxene samples (including the laser‐treated ones) suggests that asteroids may be limited in the degree of space weathering they can exhibit, possibly due to the short life of their surface regolith. Their pyroxenes may also have a limited chemical composition range. Fitting the visible continuum shape and other parts of the spectra (especially the 2μm part) has been impossible with any combination of common rock‐forming minerals such as silicates and metallic irons. However, this study shows, for the first time, excellent fits of reflectance spectra of an A asteroid (Aeternitas) and an R asteroid (Dembowska), including their visible spectral curves, band depths and shapes, and overall continuum shapes. Our results provide estimates that Aeternitas consists of 2% fresh olivine, 93% space‐weathered olivine, 1% space‐weathered orthopyroxene, and 4% chromite, and that Dembowska consists of 1% fresh olivine, 55% space‐weathered olivine, and 44% space‐weathered orthopyroxene. These results suggest that space weathering effects maybe important to the interpretation of asteroid reflectance spectra, even those with deep silicate absorption bands. Modified Gaussian model deconvolutions of the laser‐irradiated olivine samples show that their identity as olivine remained. The most recent submicroscopic mineralogical analyses have revealed that the laser‐irradiated olivine samples contain nanophase iron particles similar to those in space‐weathered lunar samples.  相似文献   

15.
《Icarus》1987,70(1):146-152
We measured emissionless infrared diffuse reflectances of some meteorites by using a Fourier transform infrared spectrophotometer to obtain additional information on identification of asteroidal surface materials. Although the spectral features of diffuse reflectances are apparently different from those of transmission spectra, the absorption bands of constituent minerals can be detected. C2 carbonaceous chondrite materials can be distinguished from C3 materials by the depth of their hydration bands around 3 and 6 μm. These hydration bands do not lose contrast by pulverization of the sample. Acid dissolution experiment shows that the 6.8-μm band in the spectra of the Murchison (C2) meteorite is probably due to carbonates. The enstatite meteorite examined (Norton County) shows absorption bands around 10 μm caused by pyroxene unlike the iron meteorite examined (Mundrabilla). Because these latter two meteorite types give similar spectra without strong absorption bands in the UV-Visible-near IR region, middle infrared spectra should be helpful in interpreting asteroidal surface materials when combined with the UV-Visible-near IR spectra.  相似文献   

16.
We performed ion irradiation of mineral samples with 50 keV He+, aimed to investigate ion irradiation effects on diagnostic spectral features. Reflectance spectra of samples in 0.375–2.5 μm are measured before and after ion irradiation. Silicates, including Luobusha olivine, plagioclase and basaltic glass, have shown reddening and darkening of reflectance spectra at the VIS–NIR range. Olivine is more sensitive to ion irradiation than plagioclase and basaltic glass. Irradiated Panzhihua ilmenite exhibits higher reflectance and stronger absorption features, which is totally different from lunar soil and analog silicate materials in other experiments. Using continuum removal and MGM fit, we extracted and compared absorption features of olivine spectra before and after irradiation. Ion irradiation can induce band strength decrease of olivine but negligible band centers shift. We estimate band centers shift caused by ion irradiation are quite limited, even less than variations due to chemical composition in silicates. It provides one possible explanation for no systematic shift in band positions in lunar soil. Irradiated Luobusha olivine spectrum matches spectra of olivine-dominated asteroids. Our results suggest space weathering should be new clues to explain the subtle difference between A-type asteroid spectra and laboratory spectra of olivine.  相似文献   

17.
Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr3+ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.  相似文献   

18.
The distribution of minerals on the lunar surface is information which could contribute to studying lunar origin and evolution. In this paper, the distribution of clinopyroxene, orthopyroxene, olivine, ilmenite, and plagioclase on the lunar surface has been mapped based on Hapke radiative transfer model and linear unmixing of spectra with Clementine UVVIS/NIR data. The results have been validated on the basis of minerals modal abundance data of the Apollo samples, and problems in the minerals abundance mapping have been analyzed. The validation based on analysis data of Apollo samples indicates that plagioclase mapped in this paper represents the total abundance of plagioclase and agglutinitic glass. The minerals mapping results show that the lunar surface is mainly composed of pyroxene, plagioclase, agglutinitic glass, and ilmenite. Basalt in the lunar mare is mainly composed of clinopyroxene and ilmenite, and lunar highland is mainly composed of plagioclase and agglutinitic glass. Orthopyroxene is mainly distributed on the north of Mare Imbrium, on the south of Maria and Aitken Basin. According to our results, there is probably no large area of olivine distribution on the lunar surface which is different from earlier published results. Therefore, emphasis should be put on the olivine distribution in the minerals mapping using hyperspectral data such as M3 of Chandrayaan-1 and IIM of ChangE-1.  相似文献   

19.
The spectral reflectance (0.32–1.06 μm) of Mercury was measured during two elongations, September–October 1974 and March 1975. The spectra are much improved over the previously available measurements. The reflectance increases almost linearly with wavelength and no electronic transition absorption bands are evident to the precision of the data. The new spectra agree very well with the reflectance of mature lunar mare and upland soils and are dissimilar to those of any other observed solar system object. This suggests that the surface materials and the weathering processes controlling their optical properties are similar for both the Moon and Mercury. Differences between the spectra for the two observation periods may indicate differences in the average composition for the different regions of Mercury observed.  相似文献   

20.
Abstract— Sayh al Uhaymir (SaU) 300 comprises a microcrystalline igneous matrix (grain size <10 μm), dominated by plagioclase, pyroxene, and olivine. Pyroxene geothermometry indicates that the matrix crystallized at ?1100 °C. The matrix encloses mineral and lithic clasts that record the effects of variable levels of shock. Mineral clasts include plagioclase, low‐ and high‐Ca pyroxene, pigeonite, and olivine. Minor amounts of ilmenite, FeNi metal, chromite, and a silica phase are also present. A variety of lithic clast types are observed, including glassy impact melts, impact‐melt breccias, and metamorphosed impact melts. One clast of granulitic breccia was also noted. A lunar origin for SaU 300 is supported by the composition of the plagioclase (average An95), the high Cr content in olivine, the lack of hydrous phases, and the Fe/Mn ratio of mafic minerals. Both matrix and clasts have been locally overprinted by shock veins and melt pockets. SaU 300 has previously been described as an anorthositic regolith breccia with basaltic components and a granulitic matrix, but we here interpret it to be a polymict crystalline impact‐melt breccia with an olivine‐rich anorthositic norite bulk composition. The varying shock states of the mineral and lithic clasts suggest that they were shocked to between 5–28 GPa (shock stages S1–S2) by impact events in target rocks prior to their inclusion in the matrix. Formation of the igneous matrix requires a minimum shock pressure of 60 GPa (shock stage >S4). The association of maskelynite with melt pockets and shock veins indicates a subsequent, local 28–45 GPa (shock stage S2–S3) excursion, which was probably responsible for lofting the sample from the lunar surface. Subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号