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1.
Abstract— The near‐Earth asteroid rendezvous (NEAR) mission carried x‐ray/gamma‐ray spectrometers and multi‐spectral imager/near‐infrared spectrometer instrument packages which gave complementary information on the chemistry and mineralogy, respectively, of the target asteroid 433 Eros. Synthesis of these two data sets provides information not available from either alone, including the abundance of non‐mafic silicates, metal and sulfide minerals. We have utilized four techniques to synthesize these data sets. Venn diagrams, which examine overlapping features in two data sets, suggest that the best match for 433 Eros is an ordinary chondrite, altered at the surface of the asteroid, or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites. Normalized element distributions preclude FeO‐rich pyroxenes and suggest that the x‐ray and gamma‐ray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal. Normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium. Matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the x‐ray‐derived bulk composition, but seems to confirm the inconsistency between mineral compositions and orthopyroxene/clinopyroxene ratios. A unique solution does not seem possible in synthesizing these multiple data sets. Future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the NEAR data.  相似文献   

2.
Lucy F. Lim  Larry R. Nittler 《Icarus》2009,200(1):129-146
We present a new calibration of the elemental-abundance data for Asteroid 433 Eros taken by the X-ray spectrometer (XRS) aboard the NEAR-Shoemaker spacecraft. (NEAR is an acronym for “Near-Earth Asteroid Rendezvous.”) Quantification of the asteroid surface elemental abundance ratios depends critically on accurate knowledge of the incident solar X-ray spectrum, which was monitored simultaneously with asteroid observations. Previously published results suffered from incompletely characterized systematic uncertainties due to an imperfect ground calibration of the NEAR gas solar monitor. The solar monitor response function and associated uncertainties have now been characterized by cross-calibration of a large sample of NEAR solar monitor flight data against contemporary broadband solar X-ray data from the Earth-orbiting GOES-8 (Geostationary Operational Environmental Satellite). The results have been used to analyze XRS spectra acquired from Eros during eight major solar flares (including three that have not previously been reported). The end product of this analysis is a revised set of Eros surface elemental abundance ratios with new error estimates that more accurately reflect the remaining uncertainties in the solar flare spectra: Mg/Si=0.753+0.078/−0.055, Al/Si=0.069±0.055, S/Si=0.005±0.008, Ca/Si=0.060+0.023/−0.024, and Fe/Si=1.678+0.338/−0.320. These revised abundance ratios are consistent within cited uncertainties with the results of Nittler et al. [Nittler, L.R., and 14 colleagues, 2001. Meteorit. Planet. Sci. 36, 1673-1695] and thus support the prior conclusions that 433 Eros has a major-element composition similar to ordinary chondrites with the exception of a strong depletion in sulfur, most likely caused by space weathering.  相似文献   

3.
Abstract— Elemental composition and composition ratios derived from gamma‐ray measurements collected by the NEAR‐Shoemaker spacecraft while on the surface of 433 Eros are reported. Performance of the gamma‐ray spectrometer (GRS) during cruise and orbit is reviewed. The best gamma‐ray data were collected on the surface of Eros after the spacecraft's controlled descent on 2001 February 12. Methods used in spectral analysis, to convert peak areas to incident photons, and photons to elemental composition are described in some detail. The elemental abundance of K and the Mg/Si, Fe/Si, Si/O and Fe/O abundance ratios were determined. The Mg/Si and Si/O ratios and the K abundance are roughly chondritic, but the Fe/Si and Fe/O ratios are low compared to expected chondritic values. Three possible explanations for the apparent Fe depletion are considered.  相似文献   

4.
A reanalysis of NEAR X‐ray/gamma‐ray spectrometer (XGRS) data provides robust evidence that the elemental composition of the near‐Earth asteroid 433 Eros is consistent with the L and LL ordinary chondrites. These results facilitated the use of the gamma‐ray measurements to produce the first in situ measurement of hydrogen concentrations on an asteroid. The measured value,  ppm, is consistent with hydrogen concentrations measured in L and LL chondrite meteorite falls. Gamma‐ray derived abundances of hydrogen and potassium show no evidence for depletion of volatiles relative to ordinary chondrites, suggesting that the sulfur depletion observed in X‐ray data is a surficial effect, consistent with a space‐weathering origin. The newfound agreement between the X‐ray, gamma‐ray, and spectral data suggests that the NEAR landing site, a ponded regolith deposit, has an elemental composition that is indistinguishable from the mean surface. This observation argues against a pond formation process that segregates metals from silicates, and instead suggests that the differences observed in reflectance spectra between the ponds and bulk Eros are due to grain size differences resulting from granular sorting of ponded material.  相似文献   

5.
Abstract— The near‐Earth asteroid rendezvous (NEAR)‐Shoemaker remote‐sensing x‐ray/gamma‐ray spectrometer (XGRS) completed more than a year of operation in orbit and on the surface of 433 Eros. Elemental compositions for a number of regions on the surface of Eros have been derived from analyses of the characteristic x‐ray and gamma‐ray emission spectra. The NEAR XGRS detection system was included as part of the interplanetary network (IPN) for the detection and localization of gamma‐ray bursts (GRBs). Preliminary results for both the elemental composition of the surface of Eros and the detection of GRBs have been obtained. In addition to the science results, the design and operation of the NEAR XGRS is considered. Significant information important for the design of future remote sensing XGRS systems has been obtained and evaluated. We focus on four factors that became particularly critical during NEAR: (1) overall spacecraft design, (2) selection of materials, (3) increase of the signal‐to‐noise ratio and (4) knowledge of the incident solar x‐ray spectrum.  相似文献   

6.
Abstract— Data from the X‐ray and γ‐ray spectrometers onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft were used to constrain the chemical and mineralogical composition of asteroid 433 Eros (McCoy et al. 2001). The bulk composition appears to be consistent with that of L to H chondrites (Nittler et al. 2001). However, there appeared to be a marked depletion relative to ordinary chondritic composition in the S/Si ratio (0.014 ± 0.017). We investigate space weathering mechanisms to determine the extent to which sulfur can be preferentially lost from the surface regolith. The two processes considered are impact vaporization by the interplanetary meteoroid population and ion sputtering by the solar wind. Using impact data for Al projectiles onto enstatite, we find that the vaporization rate for troilite (FeS) is nine times as fast as that for the bulk of the regolith. If 20% of the iron is in the form of troilite, then the net vaporization rate, normalized to bulk composition, is 2.8 times faster for sulfur than for iron. Sputtering is equally efficient at removing sulfur as impact vaporization.  相似文献   

7.
Data obtained by the near-infrared spectrometer carried by the NEAR-Shoemaker spacecraft show that the spectral properties of the asteroid Eros vary with temperature. The manner in which they vary demonstrates that the mineral olivine is a major constituent of the surface. The near-IR temperature-dependent spectral properties of Eros in the northern hemisphere, and for two individual regions on the surface, show clear evidence of the presence of the mineral olivine and are a close match to the temperature-spectral behavior of LL-type ordinary chondrite meteorites. While the presence of other olivine-rich meteorites cannot be excluded, H-type ordinary chondrites are clearly too pyroxene-rich to be permitted as a major surface component of Eros. The results of the thermal-spectral analysis are consistent with results from analysis of conventional reflectance spectra of the asteroid and contribute unambiguous detection of olivine to the understanding of the surface composition of Eros.  相似文献   

8.
The X-ray spectrometer of the Near-Earth Asteroid Rendezvous (NEAR) mission discovered a low abundance of sulfur on the surface of asteroid Eros, which is seemingly inconsistent with the match of the overall surface composition to that of ordinary chondrites. Since troilite, FeS, is the primary sulfur-bearing mineral in ordinary chondrites, we investigated the hypothesis that sulfur loss from surface FeS could result from ‘space weathering’ by impact of solar wind ions and micrometeorites. We performed laboratory studies on the chemical alteration of FeS by 4 keV ions simulating exposure to the solar wind and by nanosecond laser pulses simulating pulsed heating by micrometeorite impact. We found that the combination of laser irradiation followed by ion impact lowers the S:Fe atomic ratio on the surface by a factor of up to 2.5, which is consistent with the value of at least 1.5 deduced from the NEAR measurements. Thus, our results support the hypothesis that the low abundance of sulfur at the surface of Eros is caused by space weathering.  相似文献   

9.
The NEAR mission to 433 Eros provided detailed data on the geology, mineralogy, and chemistry of this S-class asteroid [McCoy, T.J., Robinson, M.S., Nittler, L.R., Burbine, T.H., 2002. Chem. Erde 62, 89-121; Cheng, A.F., 1997. Space Sci. Rev. 82, 3-29] with a key science goal of understanding the relationship between asteroids and meteorites [Cheng, A.F., 1997. Space Sci. Rev. 82, 3-29; Gaffey, M.J., Burbine, T.H., Piatek, J.L., Reed, K.L., Chaky, D.A., Bell, J.F., Brown, R.H., 1993a. Icarus 106, 573-602]. Previously reported major element data revealed a bulk surface similar to that of ordinary chondrites, with the notable exception of sulfur, which was highly depleted [Trombka, J.I., and 23 colleagues, 2000. Science 289, 2101-2105; Nittler, L.R., and 14 colleagues, 2001. Meteorit. Planet. Sci. 36, 1673-1695]. The origin of this sulfur deficiency, and hence the fundamental nature of the asteroid's surface, has remained controversial. We report a new analysis of NEAR X-ray spectrometer data, indicating that Eros has Cr/Fe, Mn/Fe, and Ni/Fe ratios similar to ordinary chondrite meteorites of type LL or L. Chondritic levels of Cr, Mn, and Ni argue strongly against a partial melting explanation for the sulfur depletion. Instead, our results provide definitive evidence that Eros is a primitive body with composition and mineralogy similar to ordinary chondrites, but with a surface heavily modified by interactions with the solar wind and micrometeorites, processes collectively termed space weathering.  相似文献   

10.
Abstract— The bulk chemical composition of Orgueil and 25 other carbonaceous chondrites was determined by x‐ray fluorescence analysis. The sample sizes of the analyzed meteorites were in all cases 120 mg. The abundances of P and Ti in Orgueil and Ivuna were precisely determined by the standard addition method. The new P CI abundance is 926 ± 65 ppm. Excluding the low P of Ivuna and one Orgueil sample with unusual chemistry gives a CI P content of 930 ± 23 ppm. A CI abundance of 926 ppm corresponds to a P/Si wt ratio of 8.66 times 10?3 (atomic ratio 7.85 times 10?3). For Ti a CI content of 458 ± 18 ppm and a Ti/Si wtratio of 4.28 times 10?3 (atomic ratio 2.51 times 10?3) were found. A Si content of 10.69% was obtained for average CI. The new P CI abundance is 20 to 30% below earlier estimates, while the Ti CI abundance is in agreement with earlier determinations. From the results of the analyses of bulk carbonaceous chondrites it is concluded: (1) Refractory element/Mg ratios increase from CI through CM and C3O to C3V, but ratios among Al, Ca and Ti are constant, except for low Ca/Al ratios in the reduced subgroup of C3V. (2) The Si/Mg ratios are constant in all groups of carbonaceous chondrites. (3) There is a volatility related depletion of Cr and Fe, but the Cr/Fe ratios are constant. (4) The sequence of volatility related depletions of the moderately volatile elements P, Au, As, Mn, and Zn follows condensation temperatures (except for As), if in condensation calculations non‐ideal solid solution in the host phase is considered.  相似文献   

11.
Abstract— From April 24 to May 14, 2000, the Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission's near infrared spectrometer (NIS) obtained its highest resolution data of 433 Eros. High signal‐to‐noise ratio NIS reflectance spectra cover a wavelength range of 800–2400 nm, with footprint sizes from 213 times 427 m to 394 times 788 m. This paper describes improvement in instrument calibration by remediation of internally scattered light; derivation of a “pseudo channel” for NIS at 754 nm using Multispectral Imager (MSI) Eros approach maps at 951 and 754 nm; synthesis of a 3127‐spectrum high‐resolution data set with the improved calibration and expanded wavelength coverage; and investigation of global and localized spectral variation with respect to mineralogy, composition, and space weathering of Eros, comparing the findings with previous analyses. Scattered light removal reduces the “red” slope of Eros spectra, though not to the level seen by telescopic observations. The pseudo channel completes sampling of Eros' 1 micron (Band I) absorption feature, enabling direct comparison of NIS data with other asteroid and meteorite spectra without additional scaling or correction. Following scattered light removal and wavelength range extension, the spectral parameters of average Eros plot well inside the S(IV) field of Gaffey et al. (1993) and are consistent with the L6 chondrite meteorite fields of Gaffey and Gilbert (1998). Although Eros shows no evidence of mineralogical heterogeneity, modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid. Eros bright‐to‐dark spectral ratios are largely consistent with laboratory “space weathering” experiment results and modeling of space weathering effects. Eros brightness variation unaccompanied by significant spectral variation departs from “lunar‐type”—where band depths, slopes, and albedoes all correlate—and “Ida‐type”—where significant spectral variation is unaccompanied by corresponding brightness variation. The brightest areas on Eros—steep crater walls—have lesser spectral slope and deeper Band I, consistent with exposure of “fresher,” less space weathered materials. Bright crater slope materials have opx/(opx + olv) of 0.24–0.29 and may be more representative of the subsurface mineralogy than “average” Eros, which is probably affected by space weathering. The floors of the large craters Psyche and Himeros have lower albedo and contain the most degraded or altered looking materials. NIS spectra retain a “red” spectral slope at greater than 2 microns. The recalibrated and expanded NIS spectra show better agreements with mixing models based on space weathering of chondritic mixtures.  相似文献   

12.
Two silicate grains (RB‐QD04‐0049 and RA‐QD02‐0064, whose estimated masses are 0.050 μg and 0.048 μg, respectively) recovered from the asteroid Itokawa by the Hayabusa spacecraft were studied for their mineralogical characteristics by synchrotron X‐ray diffraction and synchrotron X‐ray microtomography and further analyzed for their bulk chemical compositions by instrumental neutron activation analysis (INAA). According to X‐ray tomography, RB‐QD004‐0049 is composed of olivine, high‐Ca pyroxene, plagioclase, Ca‐phosphate, and troilite, whereas RA‐QD002‐0064 entirely consists of olivine. INAA data are consistent with these mineral compositions except for rare earth elements (REEs). Although the grain RB‐QD004‐0049 contains measurable REEs, which seems to be consistent with the presence of Ca‐phosphate, their abundances are anomalously high. Very low abundance of Co implies less than 0.1 mass% of metals in these two grains by calculation, which is in contrast to the result for the previously analyzed grain RA‐QD02‐0049 (Ebihara et al., 2011). FeO/Sc ratios of the grains fall within the range of those for ordinary chondrite olivines, implying that these grains are extraterrestrial in origin. FeO/MnO ratios also confirm this conclusion and further suggest that the Hayabusa grains analyzed in this study are similar to material found in LL chondrites rather than CK chondrites although olivines from LL and CK chondrites have similar Fa# (molar% of Fe relative to [Fe+Mg] in olivine) (~30) to those of the Hayabusa grains including the two grains analyzed in this study.  相似文献   

13.
Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H2O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.  相似文献   

14.
Abstract— We present the results of irradiation experiments aimed at understanding the structural and chemical evolution of silicate grains in the interstellar medium. A series of He+ irradiation experiments have been performed on ultra‐thin olivine, (Mg,Fe)2SiO4, samples having a high surface/volume (S/V) ratio, comparable to the expected S/V ratio of interstellar dust. The energies and fluences of the helium ions used in this study have been chosen to simulate the irradiation of interstellar dust grains in supernovae shock waves. The samples were mainly studied using analytical transmission electron microscopy. Our results show that olivine is amorphized by low‐energy ion irradiation. Changes in composition are also observed. In particular, irradiation leads to a decrease of the atomic ratios O/Si and Mg/Si as determined by x‐ray photoelectron spectroscopy and by x‐ray energy dispersive spectroscopy. This chemical evolution is due to the differential sputtering of atoms near the surfaces. We also observe a reduction process resulting in the formation of metallic iron. The use of very thin samples emphasizes the role of surface/volume ratio and thus the importance of the particle size in the irradiation‐induced effects. These results allow us to account qualitatively for the observed properties of interstellar grains in different environments, that is, at different stages of their evolution: chemical and structural evolution in the interstellar medium, from olivine to pyroxene‐type and from crystalline to amorphous silicates, porosity of cometary grains as well as the formation of metallic inclusions in silicates.  相似文献   

15.
Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe0.9S*5(Fe,Mg)(OH)2. H2O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high 18O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites.  相似文献   

16.
Ordinary chondrite meteorites contain silicates, Fe,Ni‐metal grains, and troilite (FeS). Conjoined metal‐troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910–960 °C (the Fe,Ni‐FeS eutectic). On the basis of two‐pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni‐FeS eutectic and thus conjoined metal‐troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal‐troilite grains in ordinary chondrites using high‐resolution X‐ray computed tomography. The models show that metal‐troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal‐troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences.  相似文献   

17.
Bulk chemical and mineral analyses of five L6 chondrites of shock facies d to f bring the number of L6 falls analyzed by Jarosewich to 20 and enable us: 1) to examine the chemical effects of shock melting in chondrites of the same petrologic type that presumably sample a limited stratigraphic range in their parent body; and 2) to seek depth-related chemical variations by comparing the compositions of L3 and melt-free L6 chondrites. The mean Fe/Mg, Si/Mg, S/Mg and Ni/Mg ratios of melt-free L6 chondrites (shock facies a to c) are virtually identical to those of L3 chondrites, suggesting that L-group material had the same bulk composition early (L6) and late (L3) in the accretion of the parent body. Wider variations of S/Mg and Ni/Mg in L6 chondrites may signify that L6 material was less well mixed than L3, or that some mobilization of metal and troilite occurred at shock intensities (facies c) too low to melt silicates. L6 chondrites that experienced shock melting of silicates (facies d to f) show wide variations of Fe/Mg, Si/Mg, S/Mg and Ni/Mg. It appears that most of the major element variation in the L-group is tertiary (shock-related) rather than primary (nebular, accretionary) or secondary (metamorphic). There is some evidence that L-group chondrites comprise two subgroups with different Fe/S ratios, but these subgroups are now poorly defined and their significance is unknown.  相似文献   

18.
X‐ray microcomputed tomography and synchrotron X‐ray microcomputed tomography (μCT) are becoming popular tools for the reconnaissance imaging of chondrites. However, there are occasional concerns that the use of μCT may be detrimental to organic components of a chondrite. Soluble organic compounds represent ~2–10% of the total solvent extractable carbon in CI and CM carbonaceous chondrites and amino acids are among the most abundant compounds in the soluble organic fraction. We irradiated two samples of the Murchison CM2 carbonaceous chondrite under conditions slightly harsher (increased beam exposure time) than those typically used for x‐ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a nonexposed control sample occurred. After subjecting two meteorite portions to ionizing radiation dosages of 1.1 kiloGray (kGy) and 1.2 kGy with 48.6 and 46.6 keV monochromatic X‐rays, respectively, we analyzed the amino acid content of each sample. Within analytical errors, we found no differences in the amino acid abundances or enantiomeric ratios when comparing the control samples (nonexposed Murchison) and the irradiated samples. We show with calculations that any sample heating due to x‐ray exposure is negligible. We conclude that a monochromatic synchrotron X‐ray μCT experiment at beamline 13‐BM‐D of the Advanced Photon Source, which imparts ~1 kGy doses, has no detectable effect on the amino acid content of a carbonaceous chondrite. These results are important for the initial reconnaissance of returned samples from the OSIRIS‐REx and Hayabusa 2 asteroid sample return missions.  相似文献   

19.
Abstract— We document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high‐resolution synchrotron X‐ray microtomography (μCT) and helium pycnometry. We found total porosities ranging from ～10 to 20% within these chondrites, and with μCT we show that up to 64% of the void space is located within intergranular voids within the rock. Given the low (S1‐S2) shock stages of the samples and the large voids between mineral grains, we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history. With Fe metal and FeS metal abundances and grain size distributions, we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism. These materials were not “fluffed” on their parent body by impact‐related regolith gardening or events caused by seismic vibrations. Samples of all three chemical types of ordinary chondrites (LL, L, H) are represented in this study and we conclude that incomplete compaction is common within the asteroid belt.  相似文献   

20.
The spectral reflectance (0.33–1.07 μm) for the asteroid 433 Eros was determined as a function of rotational phase during January 28–30, and February 15, 1975. Interpretation of absorption features suggests Eros is composed of an undifferentiated assemblange of moderate to high temperature minerals (iron, pyroxene, and olivine, but no carbon). H-type ordinary chondrites are such assemblages, but it would be premature to conclude that Eros is like an H chondrite meteorite in composition until a better understanding is reached of possible physical differences between laboratory powders and asteroid regoliths for metal-bearing assemblages. There are no large-scale major compositional variations on the different sides of Eros.  相似文献   

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