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 937 毫秒
1.
Abstract— The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low‐blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed “fractionated solar” (FS), which could be implantation‐fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model‐dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated 21Ne/22Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 107 years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these 21Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.  相似文献   

2.
NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils. The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu, Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed (>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous, porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested by elemental composition measurements on cometary meteors.  相似文献   

3.
The chondritic‐porous subset of interplanetary dust particles (CP‐IDPs) are thought to have a cometary origin. Since the CP‐IDPs are anhydrous and unaltered by aqueous processes that are common to chondritic organic matter (OM), they represent the most pristine material of the solar system. However, the study of IDP OM might be hindered by their further alteration by flash heating during atmospheric entry, and we have limited understanding on how short‐term heating influences their organic content. In order to investigate this problem, five CP‐IDPs were studied for their OM contents, distributions, and isotopic compositions at the submicro‐ to nanoscale levels. The OM contained in the IDPs in this study spans the spectrum from primitive OM to that which has been significantly processed by heat. Similarities in the Raman D bands of the meteoritic and IDP OMs indicate that the overall gain in the sizes of crystalline domains in response to heating is similar. However, the Raman ΓG values of the OM in all of the five IDPs clearly deviate from those of chondritic OM that had been processed during a prolonged episode of parent body heating. Such disparity suggests that the nonaromatic contents of the OM are different. Short duration heating further increases the H/C ratio and reduces the δ13C and δD values of the IDP OM. Our findings suggest that IDP OM contains a significant proportion of disordered C with low H content, such as sp2 olefinic C=C, sp3 C–C, and/or carbonyl contents as bridging material.  相似文献   

4.
Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration. High 3He/4He ratios are detected for AMMs with high 20Ne/4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean 20Ne/22Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for 36Ar and 38Ar, the abundance of the Q component is comparable to that of the SEP component. 84Kr and 132Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.  相似文献   

5.
Abstract— We compiled a table of all major, minor, and trace-element abundances in 89 interplanetary dust particles (IDPs) that includes data obtained with proton-induced x-ray emission (PIXE), synchroton x-ray fluorescence (SXRF), and secondary ion mass spectrometry (SIMS). For the first time, the reliability of the trace-element abundances in IDPs is tested by various crosschecks. We also report on the results of cluster analyses that we performed on IDP compositions. Because of the incompleteness of the data set, we included only the elements Cr, Mn, Ni, Cu, and Zn, normalized to Fe and CI chondrite abundances, that are determined in 73 IDPs. The data arrange themselves in four rather poorly defined groups that we discuss in relation to CI chondrites following the assumption that on the average CI abundances are most probable. The largest group (chondritic), with 44 members, has close to CI abundances for many refractory and moderately refractory elements (Na, Al, Si, P, K, Sc, Ti, V, Cr, Co, Ge, Sr). It is slightly depleted in Fe and more in Ca and S, while the volatile elements (Cl, Cu, Zn, Ga, Se, Rb) are enriched by =1.7 × CI and Br by 21 × CI. The low-Zn group, with 12 members, is very similar to the chondritic group except for its Zn-depletion, stronger Ca-depletion and Fe-enrichment. The low-Ni group, with 11 members, has Ni/Fe = 0.03 × CI and almost CI-like Ca, but its extraterrestrial origin is not established. The last group (6 members) contains non-systematic particles of unknown origin. We found that Fe is inhomogeneously distributed on a micron scale. Furthermore, the abundances of elements that are measured near their limits of detection are easily overestimated. These biases involved, the incomplete data set and possible contaminating processes prevent us from obtaining information on the specific origin(s) of IDPs from elemental abundances.  相似文献   

6.
Abstract— Measurements of He isotopes in cluster interplanetary dust particles (IDPs) from stratospheric dust collector L2009 reveal anomalous 3He/4He ratios comparable to those seen earlier, up to ～40x the solar wind ratio, in particles from the companion collector L2011. These overabundances of 3He in the L2009 samples are masked by much higher 4He contents compared to the L2011 particles, and are visible only in minor gas fractions evolved by stepwise heating at high temperatures. Cosmic‐ray induced spallogenic reactions are efficient producers of 3He. The majority of this paper is devoted to a detailed assessment of the possible role of spallation in generating the 3He excesses in these and other cluster IDPs. A model of collisional erosion and fragmentation during inward transit through the interplanetary dust environment is used to estimate space lifetimes of particles from asteroidal and Edgeworth–Kuiper Belt sources. Results of the modeling indicate that Poynting–Robertson orbital evolution timescales of IDPs small enough to elude destruction on their way to Earth from either location are far shorter than the cosmic‐ray exposure ages required to account for observed 3He overabundances. Grains large enough to have sufficiently long space residence times are fragmented close to their sources. An alternative to long in‐space exposure could be prolonged irradiation of particles buried in parent body regoliths prior to their ejection as IDPs. A qualitative calculation suggests, however, that collisional erosion of asteroidal upper‐regolith materials is likely to occur on timescales shorter than the > 1 Ga burial times needed for accumulation of spallogenic 3He to the levels seen in several cluster particles. In contrast, regoliths on Edgeworth–Kuiper Belt objects may be stable enough to account for the 3He excesses, and delivery of heavily pre‐irradiated IDPs to the inner solar system by short‐period Edgeworth–Kuiper Belt comets remains a possibility. A potential problem is that the expected associated abundances of spallation‐produced 21Ne appear to be absent, although here the present IDP data base is too sparse and for the most part too imprecise to rule out a spallogenic origin. Relatively short periods of pre‐ejection residence in asteroidal regoliths may be responsible for the curiously broad exposure age distributions reported for micrometeorites extracted from Greenland and sea‐floor sediments.  相似文献   

7.
The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low‐Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)‐bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low‐Ca hypersthene grain, and a Ti‐oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe‐Ni‐S nanograins dispersed throughout the low‐Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine‐grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg‐Fe‐Ni‐S[Si‐O] compositions. To be specific, “Where are the GEMS”? The GEMS look‐alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.  相似文献   

8.
Abstract— Some fraction of Zn, Cu, Se, Ga and Ge in chondritic interplanetary dust particles (IDPs) collected in the lower stratosphere between 1981 May and 1984 June has a volcanic origin. I present a method to evaluate the extent of this unavoidable type of stratospheric contamination for individual particles. The mass-normalised abundances for Cu and Ge as a function of mass-normalised stratospheric residence time show their time-integrated stratospheric aerosol abundances. The Zn, Se and Ga abundances show a subdivision into two groups that span approximately two-year periods following the eruptions of the Mount St. Helens (1980 May) and El Chichón (1982 April) volcanos. Elemental abundances in particles collected at the end of each two-year period indicate low, but not necessarily ambient, volcanic stratospheric abundances. Using this time-integrated baseline, I calculate the stratospheric contaminant fractions in nine IDPs and show that Zn, Se and Ga abundances in chondritic IDPs derive in part from stratospheric aerosol contaminants. Post-entry elemental abundances (i.e., the amount that survived atmospheric entry heating of the IDP) show enrichments relative to the CI abundances but in a smaller number of particles than previously suggested.  相似文献   

9.
Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.  相似文献   

10.
Abstract— Petrological changes in Ni‐free and low‐Ni pyrrhotite, and much less in pentlandite, during atmospheric entry flash‐heating of the sulfide IDPs L2005E40, L2005C39, and L2006A28 support 1) ferrous sulfide oxidation with vacancy formation and Fe3+ ordering; and 2) Fe‐oxide formation and sulfur vapor loss through abundant vesicles. Melting of metastable chondritic aggregate materials at the IDP surface has occurred. All changes, e.g., formation of a continuous maghémite rim, proceeded as solid‐state reactions at a peak heating temperature of ?700 °C. This temperature in combination with particle size and density suggest a ?10 km/s?1 entry velocity. The IDPs probably belonged to cluster IDPs that entered the atmosphere with near‐Earth or Earth‐crossing asteroid velocities. They could be debris from extinct or dormant comet nuclei, which is consistent with shock comminution of pyrrhotite in these IDPs.  相似文献   

11.
Abstract– We report bulk and olivine compositions in 66 stony cosmic spherules (Na2O < 0.76 wt%), 200–800 μm in size, from the Transantarctic Mountains, Antarctica. In porphyritic cosmic spherules, relict olivines that survived atmospheric entry heating are always Ni‐poor and similar in composition to the olivines in carbonaceous or unequilibrated ordinary chondrites (18 spherules), and equilibrated ordinary chondrites (one spherule). This is consistent with selective survival of high temperature, Mg‐rich olivines during atmospheric entry. Olivines that crystallized from the melts produced during atmospheric entry have NiO contents that increase with increasing NiO in the bulk spherule, and that range from values similar to those observed in chondritic olivines (NiO generally <0.5 wt%) to values characteristic of olivines in meteoritic ablation spheres (NiO > 2 wt%). Thus, NiO content in olivine cannot be used alone to distinguish meteoritic ablation spheres from cosmic spherules, and the volatile element contents have to be considered. We propose that the variation in NiO contents in cosmic spherules and their olivines is the result of variable content of Fe, Ni metal in the precursor. NiO contents in olivines and in cosmic spherules can thus be used to discuss their parent body. Ni‐poor spherules can be derived from C‐rich and/or metal‐poor precursors, either related to CM, CI, CR chondrites or to chondritic fragments dominated by silicates, regardless of the parent body. Ni‐rich spherules (NiO > 0.7 wt%) that represent 55% of the 47 barred‐olivine spherules we studied, were derived from the melting of C‐poor, metal‐rich precursors, compatible with ordinary chondrite or CO, CV, CK carbonaceous chondrite parentages.  相似文献   

12.
Abstract— We report mass‐spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to >40x the solar‐wind value—‐and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high‐altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic‐ray‐induced spallation during estimated space residence times of IDPs, or by direct implantation of solar‐flare He. Minimum exposure ages inferred from the 3He excesses range from ～50 Ma to an impossible >10 Ga, compared to Poynting‐Robertson drag lifetimes for low‐density 20–30 μm particles on the order of ～0.1 Ma for an asteroidal source and ～10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar‐energetic‐particle (SEP) gases, to the virtual exclusion of solar‐wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ～1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low‐energy solar particle emission.  相似文献   

13.
The scoriaceous cosmic spherules (CSs) that make up to a few percent (for sizes >150 μm size) of total micrometeorite flux are ubiquitous and have remained enigmatic. The present work provides in-depth study of 81 scoriaceous CSs, from observed ~4000 CSs, collected from Antarctica (South Pole water well) and deep-sea sediments (Indian Ocean) that will allow us to analyze the nature of these particles. The fine-grained texture and the chemical composition of scoriaceous particles suggest that they are formed from matrix materials that are enriched in volatiles. The volatile components such as water, sulfide, Na, K, etc. have vanished due to partial evaporation and degassing during Earth's atmospheric entry leaving behind the vesicular features, yet largely preserving the elemental composition. The elemental ratios (Ca/Si, Mg/Si, Al/Si, Fe/Si, and Ni/Si) of interplanetary dust particles (IDPs) are compatible with the scoriaceous CSs, which in turn are indistinguishable from the matrices of CI and CM chondrites signifying similarities in the nature of the sources. Furthermore, the texture of cometary particles bears resemblance to the texture of the scoriaceous particles. The compilation of petrographic texture, chemical, and trace element composition of scoriaceous CSs presents a strong case for matrix components from hydrated and volatile-rich bodies, such as CI and CM chondrites, rather than chondrules. We conclude that the fine-grained scoriaceous CSs, the matrix materials of hydrated chondrites, IDPs, and cometary particles that overlap compositionally were widespread, indicating a dominant component in the early solar nebula.  相似文献   

14.
Comet 81P/Wild 2 dust, the first comet sample of known provenance, was widely expected to resemble anhydrous chondritic porous (CP) interplanetary dust particles (IDPs). GEMS, distinctly characteristic of CP IDPs, have yet to be unambiguously identified in the Stardust mission samples despite claims of likely candidates. One such candidate is Stardust impact track 57 “Febo” in aerogel, which contains fine‐grained objects texturally and compositionally similar to GEMS. Their position adjacent the terminal particle suggests that they may be indigenous, fine‐grained, cometary material, like that in CP IDPs, shielded by the terminal particle from damage during deceleration from hypervelocity. Dark‐field imaging and multidetector energy‐dispersive X‐ray mapping were used to compare GEMS‐like‐objects in the Febo terminal particle with GEMS in an anhydrous, chondritic IDP. GEMS in the IDP are within 3× CI (solar) abundances for major and minor elements. In the Febo GEMS‐like objects, Mg and Ca are systematically and strongly depleted relative to CI; S and Fe are somewhat enriched; and Au, a known aerogel contaminant, is present, consistent with ablation, melting, abrasion, and mixing of the SiOx aerogel with crystalline Fe‐sulfide and minor enstatite, high‐Ni sulfide, and augite identified by elemental mapping in the terminal particle. Thus, GEMS‐like objects in “caches” of fine‐grained debris abutting terminal particles are most likely deceleration debris packed in place during particle transit through the aerogel.  相似文献   

15.
Abstract Reflectance spectra were collected from chondritic interplanetary dust particles (IDPs), a polar micrometeorite, Allende (CV3) meteorite matrix, and mineral standards using a microscope spectrophotometer. Data were acquired over the 380–1100 nm wavelength range in darkfield mode using a halogen light source, particle aperturing diaphrams, and photomultiplier tube (PMT) detectors. Spectra collected from titanium oxide (Ti4O7), magnetite (Fe3O4), and Allende matrix establish that it is possible to measure indigenous reflectivities of micrometer-sized (>5 μm in diameter) particles over the visible (VIS) wavelength range 450–800 nm. Below 450 nm, small particle effects cause a fall-off in signal into the ultraviolet (UV). Near-infrared (IR) spectra collected from olivine and pyroxene standards suggest that the ～1 μm absorption features of Fe-bearing silicates in IDPs can be detected using microscope spectrophotometry. Chondritic IDPs are dark objects (<15% reflectivity) over the VIS 450–800 nm range. Large (>1 μm in diameter) embedded and adhering single mineral grains make IDPs significantly brighter, while surficial magnetite formed by frictional heating during atmospheric entry makes them darker. Most chondritic smooth (CS) IDPs, dominated by hydrated layer silicates, exhibit generally flat spectra with slight fall-off towards 800 nm, which is similar to type CI and CM meteorites and main-belt C-type asteroids. Most chondritic porous (CP) IDPs, dominated by anhydrous silicates (pyroxene and olivine), exhibit generally flat spectra with a slight rise towards 800 nm, which is similar to outer P and D asteroids. The most C-rich CP IDPs rise steeply towards 800 nm with a redness comparable to that of the outer asteroid object Pholus (Binzel, 1992). Chondritic porous IDPs are the first identified class of meteoritic materials exhibiting spectral reflectivities (between 450 and 800 nm) similar to those of P and D asteroids. Although large mineral grains, secondary magnetite, and small particle effects complicate interpretation of IDP reflectance spectra, microscope spectrophotometry appears to offer a rapid, nondestructive technique for probing the mineralogy of IDPs, comparing them with meteorites, investigating their parent body origins, and identifying IDPs that may have been strongly heated during atmospheric entry.  相似文献   

16.
Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al2O3) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.  相似文献   

17.
Abstract— On July 21, 2002, a meteorite fall occurred over the Thuathe plateau of western Lesotho. The well‐defined strewn field covers an area of 1.9 times 7.4 km. Many of the recovered specimens display a brecciated texture with leucocratic, angular to subrounded clasts in a somewhat darker groundmass. Mineralogical and chemical data, as well as oxygen isotopic analysis, indicate that Thuathe is an H4/5, S2/3 meteorite, with local H3 or H6 character. A number of anomalous features include somewhat high Co contents of kamacite and taenite relative to normal H‐group chondrites. Oxygen isotopic data plot at the edge of the normal H chondrite data field. Variable contents of metallic mineral phases and troilite result in a heterogeneous bulk composition (e.g., with regard to Si, Fe, and Mg), resulting in a spread of major element ratios that is not consistent with previously accepted H‐group composition. Trace element abundances are generally consistent with H chondritic composition, and Kr and Xe isotopic data agree with an H4 classification for this meteorite. Noble gas analysis gave U, Th‐4He gas retention and K‐Ar ages typical for H chondrites; no major thermal event affected this material since ～3.7 Ga. The exposure age for Thuathe is 5 Ma, somewhat lower than for other H chondrites. Cosmogenic nuclide analysis indicates a pre‐atmospheric radius of this meteorite between 35 and 40 cm. In the absence of evidence for solar gases, we classify Thuathe as a fragmental breccia. Numerous narrow, black veins cut across samples of Thuathe and are the result of a brittle deformation event that also caused local melting, especially in portions rich in sulfide. The formation of these veinlets is not the result of locally enhanced shock pressures (i.e., of shock melting) but rather of shearing under brittle conditions with local, friction‐related temperature excursions causing melting mostly of Fe‐sulfide and FeNi‐metal but also, locally, of silicate minerals. Frictional temperature excursions must have attained values in excess of 1500 °C to permit complete melting of forsteritic olivine.  相似文献   

18.
Abstract Fragments from 20 individual particles, collected in the Earth's stratosphere and believed to be interplanetary dust particles (IDPs), were obtained from NASA's Johnson Space Center collection and subjected to step-heating to see if differences in the release pattern for 4He could be observed which might provide clues to the origin of the particles. Comparisons were made to the release pattern for 18 individual lunar surface grains heated in the same manner. Twelve of the IDP fragments contained an appreciable amount of 4He, 50 percent of which was released by the time the particles were heated to approximately 630 °C. For the 18 individual lunar grains the corresponding average temperature was 660 °C. The 3He/4He ratios found for these fragments agreed well with those found for deep Pacific magnetic fines believed to be of extraterrestrial origin, and were comparable to those which have been observed for the solar wind and lunar surface soil grains. Four of the IDP fragments contained appreciably less 4He, and this was released at a higher temperature. The remaining four fragments had too little 4He to permit a determination. From Flynn's analyses of the problem of the heating of IDPs in their descent in the atmosphere, the present results suggest that the parent IDPs of the 12 particles which contained an appreciable amount of 4He suffered very little heating in their descent and are likely of asteroidal origin, although one cannot rule out the possibility that at least some of them had a cometary origin and entered the earth's atmosphere at a grazing angle. Mineralogical and morphological studies on fragments companion to those used in the present investigation are under way. When these are completed, a more definite picture should emerge.  相似文献   

19.
Diagnostic infrared spectra of individual nanogram-sized interplanetary dust particles (IDPs) collected in the Earth's stratosphere have been obtained. A mount containing three crushed “chondritic” IDPs shows features near 1000 and 500 cm?1, suggestive of crystalline pyroxene, and different from those of crystalline olivine, amorphous olivine, or meteoritic clay minerals. The structural diversity of chondritic IDPs and possible effects of atmospheric heating must be considered when comparing this spectrum with astrophysical spectra of interplanetary and cometary dust. Transmission electron microscope (TEM) and infrared observations are also reported on one member of the rare subset of IDPs which resemble hydrated carbonaceous chondrite matrix material. The infrared spectrum of this particle between 4000 and 400 cm?1 closely matches that of the C2 meteorite Murchison. TEM observations suggest that this class of particles might serve as a thermometer for the process of heating on atmospheric entry.  相似文献   

20.
Abstract— Fragments of 24 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were obtained from NASA's Johnson Space Center collection and subjected to pulse-heating sequences to extract He and Ne and to learn about the thermal history of the particles. A motivation for the investigation was to see if the procedure would help distinguish between IDPs of asteroidal and cometary origin. The use of a sequence of short-duration heat pulses to perform the extractions is an improvement over the employment of a step-heating sequence, as was used in a previous investigation. The particles studied were fragments of larger parent IDPs, other fragments of which, in coordinated experiments, are undergoing studies of elemental and mineralogical composition in other laboratories. While the present investigation will provide useful temperature history data for the particles, the relatively large size of the parent IDPs (～40 μm in diameter) resulted in high entry deceleration temperatures. This limited the usefulness of the study for distinguishing between particles of asteroidal and cometary origin.  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