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1.
In order to gain further insight into their surface compositions and relationships with meteorites, we have obtained spectra for 17 C and X complex asteroids using NASA’s Infrared Telescope Facility and SpeX infrared spectrometer. We augment these spectra with data in the visible region taken from the on-line databases. Only one of the 17 asteroids showed the three features usually associated with water, the UV slope, a 0.7 μm feature and a 3 μm feature, while five show no evidence for water and 11 had one or two of these features. According to DeMeo et al. (2009), whose asteroid classification scheme we use here, 88% of the variance in asteroid spectra is explained by continuum slope so that asteroids can also be characterized by the slopes of their continua. We thus plot the slope of the continuum between 1.8 and 2.5 μm against slope between 1.0 and 1.75 μm, the break at ∼1.8 μm chosen since phyllosilicates show numerous water-related features beyond this wavelength. On such plots, the C complex fields match those of phyllosilicates kaolinite and montmorillonite that have been heated to about 700 °C, while the X complex fields match the fields for phyllosilicates montmorillonite and serpentine that have been similarly heated. We thus suggest that the surface of the C complex asteroids consist of decomposition products of kaolinite or montmorillonite while for the X complex we suggest that surfaces consist of decomposition products of montmorillonite or serpentine. On the basis of overlapping in fields on the continuum plots we suggest that the CI chondrites are linked with the Cgh asteroids, individual CV and CR chondrites are linked with Xc asteroids, a CK chondrite is linked with the Ch or Cgh asteroids, a number of unusual CI/CM meteorites are linked with C asteroids, and the CM chondrites are linked with the Xk asteroids. The associations are in reasonable agreement with chondrite mineralogy and albedo data.  相似文献   

2.
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   

3.
Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine-pyroxene-phyllosilicate-sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel-phyllosilicate-diopside calcium-aluminum-rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine-grained rims of material resembling matrix. The opaque, fine-grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material. The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix-supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low-Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca-carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine-grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly-crystalline material with the same bulk composition as matrix phyllosilicate. There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C. We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM-like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type-2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.  相似文献   

4.
The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water‐rich asteroids. We show that CM1 chondrites contain abundant (84–91 vol%) phyllosilicate, plus olivine (4–8 vol%), magnetite (2–3 vol%), Fe‐sulfide (<5 vol%), and calcite (<2 vol%). The CM1/2 chondrites contain phyllosilicate (71–88 vol%), olivine (4–20 vol%), enstatite (2–6 vol%), magnetite (2–3 vol%), Fe‐sulfides (1–2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg‐serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg‐rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120 °C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that “CM1‐like” asteroids, as has been proposed for Bennu—target for the OSIRIS‐REx mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.  相似文献   

5.
J.M. Carvano  T. Mothé-Diniz 《Icarus》2003,161(2):356-382
We present an analysis of 460 featureless asteroid spectra in the range 0.5-0.92 μm obtained in the Small Solar System Objects Spectroscopic Survey. The spectra are described in terms of the continuum steepness (cSlope), its concavity (RRE), and the blue wing of drop in the UV reflectance (BD). Comparison with meteorite spectra confirms the link between CM meteorites and asteroids with asteroids with 0.7 μm band. Also, it is found that asteroids with extreme negative slope values may be related to CK chondrites and that asteroids with pronounced concave-down curvature are related to CO chondrites. An analysis of the distribution of the spectral parameters with semimajor axis, diameter, and albedo is performed.  相似文献   

6.
Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High-quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally-metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B-7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating-temperature ranges. These correspond to (1) <400 °C: phyllosilicate-rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late-stage heating events.  相似文献   

7.
Based on the high abundance of fine‐grained material and its dark appearance, NWA 11024 was recognized as a CM chondrite, which is also confirmed by oxygen isotope measurements. But contrary to known CM chondrites, the typical phases indicating aqueous alteration (e.g., phyllosilicates, carbonates) are missing. Using multiple analytical techniques, this study reveals the differences and similarities to known CM chondrites and will discuss the possibility that NWA 11024 is the first type 3 CM chondrite. During the investigation, two texturally apparent tochilinite–cronstedtite intergrowths were identified within two thin sections. However, the former phyllosilicates were recrystallized to Fe‐rich olivine during a heating event without changing the textural appearance. A peak temperature of 400–600 °C is estimated, which is not high enough to destroy or recrystallize calcite grains. Thus, calcites were never constituents of the mineral paragenesis. Another remarkable feature of NWA 11024 is the occurrence of unknown clot‐like inclusions (UCLIs) within fine‐grained rims, which are unique in this clarity. Their density and S concentration are significantly higher than of the surrounding fine‐grained rim and UCLIs can be seen as primary objects that were not formed by secondary alteration processes inside the rims. Similarities to chondritic and cometary interplanetary dust particles suggest an ice‐rich first‐generation planetesimal for their origin. In the earliest evolution, NWA 11024 experienced the lowest degree of aqueous alteration of all known CM chondrites and subsequently, a heating event dehydrated the sample. We suggest to classify the meteorite NWA 11024 as the first type 3 CM chondrite similar to the classification of CV3 chondrites (like Allende) that could also have lost their matrix phyllosilicates by thermal dehydration.  相似文献   

8.
Based upon our characterization of three separate stones by electron and X‐ray beam analyses, computed X‐ray microtomography, Raman microspectrometry, and visible‐IR spectrometry, Sutter's Mill is a unique regolith breccia consisting mainly of various CM lithologies. Most samples resemble existing available CM2 chondrites, consisting of chondrules and calcium‐aluminum‐rich inclusion (CAI) set within phyllosilicate‐dominated matrix (mainly serpentine), pyrrhotite, pentlandite, tochilinite, and variable amounts of Ca‐Mg‐Fe carbonates. Some lithologies have witnessed sufficient thermal metamorphism to transform phyllosilicates into fine‐grained olivine, tochilinite into troilite, and destroy carbonates. One finely comminuted lithology contains xenolithic materials (enstatite, Fe‐Cr phosphides) suggesting impact of a reduced asteroid (E or M class) onto the main Sutter's Mill parent asteroid, which was probably a C class asteroid. One can use Sutter's Mill to help predict what will be found on the surfaces of C class asteroids such as Ceres and the target asteroids of the OSIRIS‐REx and Hayabusa 2 sample return missions (which will visit predominantly primitive asteroids). C class asteroid regolith may well contain a mixture of hydrated and thermally dehydrated indigenous materials as well as a significant admixture of exogenous material would be essential to the successful interpretation of mineralogical and bulk compositional data.  相似文献   

9.
We investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our underlying hypothesis is that the position and shape of the 3 μm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10?8 to 10?7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. “Group 1,” the least altered group as determined from various alteration indices, is characterized by 3 μm band centers at longer wavelengths, and is consistent with cronstedtite (Fe‐serpentine). “Group 3,” the most altered group, is characterized by 3 μm band centers at shorter wavelengths and is consistent with antigorite (serpentine). “Group 2” is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High‐quality ground‐based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.  相似文献   

10.
H2O and OH are readily detected in hydrated minerals in CM chondrites via reflectance spectroscopy due to their characteristic vibration absorptions at infrared wavelengths. Previous spectroscopic work on bulk powdered CM chondrites has shown that spectral parameters, like the wavelength position of the “3 μm absorption feature,” vary systematically with the extent to which the samples have been aqueously altered. However, it is yet unclear how these spectral features may vary across an intact meteorite chip when measured at spatial scales smaller than that of the individual components of the meteorite. Here, we explore the spatial variability of this spectral feature and others on intact CM2 chips which, unlike powders, retain their petrologic and textural characteristics. We also model the modal mineralogy of the bulk meteorite powders and correlate this with key spectral features, demonstrating that microscope Fourier transform infrared spectroscopic mapping provides a powerful, rapid, and non-destructive technique for assessing compositional diversity and variations in water–rock interactions in chondritic planetary materials. In all CM2 chondrites studied here, we find that variations in the position, shape, and strength of the 3 μm absorption feature reveal a single chondrite can exhibit as much spectral variation as the entire suite of CM2 chondrites. The observed variations in the position and shape of the 3 μm feature within individual CM2 chondrite chips suggest a range of alteration products (e.g., Mg-rich to Fe-rich phyllosilicates) are present and record sub-mm scale variations in the amount and/or chemistry of the altering fluids. The samples having experienced the most progressive aqueous alteration show the least amount of variability in features like the 3 μm absorption band minimum position, whereas the least altered samples exhibit the most variability. We also find that the bulk spectral signatures in the least altered samples appear to be biased toward the spectral signatures of clasts versus matrix. By extension, asteroid reflectance spectra exhibiting 3 μm absorption features consistent with those measured here may be interpreted in a similar framework in which the spectrum of what may appear to be the least altered asteroids represents an average that belies the true diversity of mineralogy and chemistry of the body.  相似文献   

11.
Abstract— I review the dynamical and compositional evidence for possibly linking CM chondrites and asteroids having G-class taxonomic designations. Three G asteroids have been identified through previous theoretical studies as being likely meteorite source bodies due to their locations near resonances. Two of these objects, 19 Fortuna and 13 Egeria, have spectral properties that are consistent with such a linkage with CM chondrites. Fortuna has a similar strength 0.7 μm absorption feature and near-infrared spectral slope to CM chondrites but a weaker ultraviolet feature. Egeria also has the characteristic 0.7 μm feature of CM chondrite spectra but does not match as well in the near-infrared. However, since the 0.7 μm feature is apparent in the spectra of approximately one-half of measured C-type asteroids, no definitive statement about any linkages can be made. Ceres is spectrally different from known meteorites in the 3 μm wavelength region and cannot be convincingly linked with any meteorite group.  相似文献   

12.
We present narrowband spectrophotometry between 2.3 and 3.5 μm for 14 main-belt C asteroids greater than 100 km in diameter. Absorption features at 3 μm due to water of hydration are present in the spectra of 9 of the asteroids, with intensities ranging from 6 to 23%. The other 5 asteroids have no 3-μm absorption greater than 2% in intensity. The strength of the “water” feature in the spectra correlates positively with the strength of the UV absorption feature shortward of 0.4 μm, and negatively with the slope of the continuum between 1.2 and 2.2 μm. These correlations are the same as those seen in laboratory spectra of carbonaceous chondrites, whose silicate compositions range from hydrated phyllosilicates to anhydrous olivine. We find no correlation between composition and semimajor axis for C asteroids as a class. The present C-asteroid population may be fragments of larger parent bodies with anhydrous C3-like cores and hydrated C11- or C2M-like mantles.  相似文献   

13.
Abstract— We describe the petrologic and trace element characteristics of the Yamato 86029 (Y‐86029) meteorite. Y‐86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse‐ and fine‐grained phyllosilicates, Fe‐Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine‐rich grains are present within a very fine‐grained phyllosilicate‐rich matrix. Analyses of 14 thermally mobile trace elements suggest that Y‐86029 experienced moderate, open‐system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500–600°C for Y‐86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re‐crystallized olivine. This transformation appears to proceed through ‘intermediate’ highly‐disordered ‘poorly crystalline’ phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg‐rich carbonate precursors. Y‐86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe‐rich clasts (300–500 μm). Fine‐grained carbonates with uniform texture are also present as small (10–15 μm in diameter), rounded to sub‐rounded ‘shells’ of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y‐86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock‐induced melt re‐crystallization. Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y‐86029 are quite similar to those of Y‐82162, the only other heated CI‐like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.  相似文献   

14.
We have conducted hydration–dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between CM2s and metamorphosed carbonaceous chondrites (MCCs). Two experiments were undertaken. In the first experiment, serpentine was obtained by hydrating terrestrial olivine (Fo90.9) in the laboratory. During this experiment, olivine was reacted with isotopically heavy water (δ18O 21.5‰) at T = 300 °C,  = 300 bar, for 100 days. The oxygen isotopic composition of the experimental serpentine was enriched in 18O (by 10 ‰ in δ18O) due to exchange of oxygen isotopes between olivine and the 18O‐rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T = 930 °C, for 1 h. The oxygen isotopic composition of the dehydrated serpentine was enriched in 18O by a further 7 ‰. The net result of the hydration–dehydration process was an enrichment of 18O in the final material by approximately 17‰. The new experimental results suggest that the oxygen isotopic compositions of MCCs of the Belgica‐like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM2 chondrites via several cycles of hydration–dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.  相似文献   

15.
Jbilet Winselwan is one of the largest CM carbonaceous chondrites available for study. Its light, major, and trace elemental compositions are within the range of other CM chondrites. Chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. Calcium‐ and aluminum‐rich inclusions (CAIs) are present at ≤1 vol% and at least one contains melilite. Jbilet Winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. In most lithologies, the chondrules and CAIs are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7–2.4 on the Rubin et al. ( 2007 ) scale. However, chondrules and CAIs in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. Following hydration, some lithologies suffered thermal metamorphism at 400–500 °C. Bulk X‐ray diffraction shows that Jbilet Winselwan consists of a highly disordered and/or very fine‐grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). Some aliquots of Jbilet Winselwan also show significant depletions in volatile elements such as He and Cd. The heating was probably short‐lived and caused by impacts. Jbilet Winselwan samples a mixture of hydrated and dehydrated materials from a primitive water‐rich asteroid. It may therefore be a good analog for the types of materials that will be encountered by the Hayabusa‐2 and OSIRIS‐REx asteroid sample‐return missions.  相似文献   

16.
High resolution spectroscopic observations of asteroid 2 Pallas from 1.7-3.5 μm are reported. These data are combined with previous measurements from 0.4-1.7 μm to interpret Pallas' surface mineralogy. Evidence is found for low-Fe2+ hydrated silicates, opaque components, and low-Fe2+ anhydrous silicates. This assemblage is very similar to carbonaceous chondrite matrix material such as is found in type CI and CM meteorites, but it has been subjected to substantial aqueous alteration and there is a major extraneous anhydrous silicate component. This composition is compared to that of asteroid 1 Ceres. Although there are substantial differences in their broad band spectral reflectances, it appears that both asteroids are genetically related to known carbonaceous chondrites.  相似文献   

17.
Abstract— Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.  相似文献   

18.
E.A. Cloutis  P. Hudon  T. Hiroi 《Icarus》2011,216(1):309-346
We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56 μm and 2.4 μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra - both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3-2.5 μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56 μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1 μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4-0.5, 0.65, 2.2 μm). The depths of the 0.7, 0.9, and 1.1 μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7 μm region (due to Fe3+-Fe2+ charge transfers), and the 0.9-1.2 μm region (due to Fe2+ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7 μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65 μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral-compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9 μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.  相似文献   

19.
Abstract– Micrometeoroids with 100 and 200 μm size dominate the zodiacal cloud dust. Such samples can be studied as micrometeorites, after their passage through the Earth atmosphere, or as microxenoliths, i.e., submillimetric meteorite inclusions. Microxenoliths are samples of the zodiacal cloud dust present in the asteroid Main Belt hundreds of millions years ago. Carbonaceous microxenoliths represent the majority of observed microxenoliths. They have been studied in detail in howardites and H chondrites. We investigate the role of carbonaceous asteroids and Jupiter‐family comets as carbonaceous microxenolith parent bodies. The probability of low velocity collisions of asteroidal and cometary micrometeoroids with selected asteroids is computed, starting from the micrometeoroid steady‐state orbital distributions obtained by dynamical simulations. We selected possible parent bodies of howardites (Vesta) and H chondrites (Hebe, Flora, Eunomia, Koronis, Maria) as target asteroids. Estimates of the asteroidal and cometary micrometeoroid mass between 2 and 4 AU from the Sun are used to compute the micrometeoroid mass influx on each target. The results show that all the target asteroids (except Koronis) receive the same amount (within the uncertainties) of asteroidal and cometary micrometeoroids. Therefore, both these populations should be observed among howardite and H chondrite carbonaceous microxenoliths. However, this is not the case: carbonaceous microxenoliths show differences similar to those existing among different groups of carbonaceous chondrites (e.g., CI, CM, CR) but two sharply distinct populations are not observed. Our results and the observations can be reconciled assuming the existence of a continuum of mineralogical and chemical properties between carbonaceous asteroids and comets.  相似文献   

20.
A crucial topic in planetology research is establishing links between primitive meteorites and their parent asteroids. In this study, we investigate the feasibility of a connection between asteroids similar to 21 Lutetia, encountered by the Rosetta mission in July 2010, and the CH3 carbonaceous chondrite Pecora Escarpment 91467 (PCA 91467). Several spectra of this meteorite were acquired in the ultraviolet to near‐infrared (0.3–2.2 μm) and in the midinfrared to thermal infrared (2.5–30.0 μm or 4000 to ~333 cm−1), and they are compared here to spectra from the asteroid 21 Lutetia. There are several similarities in absorption bands and overall spectral behavior between this CH3 meteorite and 21 Lutetia. Considering also that the bulk density of Lutetia is similar to that of CH chondrites, we suggest that this asteroid could be similar, or related to, the parent body of these meteorites, if not the parent body itself. However, the apparent surface diversity of Lutetia pointed out in previous studies indicates that it could simultaneously be related to other types of chondrites. Future discovery of additional unweathered CH chondrites could provide deeper insight in the possible connection between this family of metal‐rich carbonaceous chondrites and 21 Lutetia or other featureless, possibly hydrated high‐albedo asteroids.  相似文献   

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