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1.
Abstract— It appears that the mineralogy and chemical properties of type 3 enstatite chondrites could have been established by fractionation processes (removal of a refractory component, and depletion of water) in the solar nebula, and by equilibration with nebular gas at low‐to‐intermediate temperatures (approximately 700–950 K). We describe a model for the origin of type 3 enstatite chondrites that for the first time can simultaneously account for the mineral abundances, bulk‐chemistry, and phase compositions of these chondrites by the operation of plausible processes in the solar nebula. This model, which assumes a representative nebular gas pressure of 10?5 bar, entails three steps: (1) initial removal of 56% of the equilibrium condensed phases in a system of solar composition at 1270 K; (2) an average loss of 80–85% water vapor in the remaining gas; and (3) two different closure temperatures for the condensed phases. The first step involves a “refractory element fractionation” and is needed to account for the overall major element composition of enstatite chondrites, assuming an initial system with a solar composition. The second step, water‐vapor depletion, is needed to stabilize Si‐bearing metal, oldhamite, and niningerite, which are characteristic minerals of the enstatite chondrites. Variations in closure temperatures are suggested by the way in which the bulk chemistry and mineral assemblages of predicted condensates change with temperature, and how these parameters correlate with the observations of enstatite chondrites. In general, most phases in type 3 enstatite chondrites appear to have ceased equilibrating with nebular gas at approximately 900–950 K, except for Fe‐metal, which continued to partially react with nebular gas to temperatures as low as ～700 K.  相似文献   

2.
Atlanta is the fifth known brecciated enstatite chondrite. It contains a centimeter-sized troilite-rich clast, similar to those that occur in Blithfield. All of these clasts probably formed in the solar nebula under high pS2/pO2 conditions in a gas of non-cosmic composition. The absence of ordinary or carbonaceous chondrite clasts in any of the enstatite chondrite breccias and absence of enstatite chondrite clasts or materials formed at high pS2/pO2 ratios in ordinary and carbonaceous chondrite breccias support the model that enstatite chondrites were formed at a location distant from those of the other chondritic groups.  相似文献   

3.
John T. Wasson 《Icarus》2008,195(2):895-907
Studies of matrix in primitive chondrites provide our only detailed information about the fine fraction (diameter <2 μm) of solids in the solar nebula. A minor fraction of the fines, the presolar grains, offers information about the kinds of materials present in the molecular cloud that spawned the Solar System. Although some researchers have argued that chondritic matrix is relatively unaltered presolar matter, meteoritic chondrules bear witness to multiple high-temperature events each of which would have evaporated those fines that were inside the high-temperature fluid. Because heat is mainly transferred into the interior of chondrules by conduction, the surface temperatures of chondrules were probably at or above 2000 K. In contrast, the evaporation of mafic silicates in a canonical solar nebula occurs at around 1300 K and FeO-rich, amorphous, fine matrix evaporates at still lower temperatures, perhaps near 1200 K. Thus, during chondrule formation, the temperature of the placental bath was probably >700 K higher than the evaporation temperatures of nebular fines. The scale of chondrule forming events is not known. The currently popular shock models have typical scales of about 105 km. The scale of nebular lightning is less well defined, but is certainly much smaller, perhaps in the range 1 to 1000 m. In both cases the temperature pulses were long enough to evaporate submicrometer nebular fines. This interpretation disagrees with common views that meteoritic matrix is largely presolar in character and CI-chondrite-like in composition. It is inevitable that presolar grains (both those recognized by their anomalous isotopic compositions and those having solar-like compositions) that were within the hot fluid would also have evaporated. Chondrule formation appears to have continued down to the temperatures at which planetesimals formed, possibly around 250 K. At temperatures >600 K, the main form of C is gaseous CO. Although the conversion of CO to CH4 at lower temperatures is kinetically inhibited, radiation associated with chondrule formation would have accelerated the conversion. There is now evidence that an appreciable fraction of the nanodiamonds previously held to be presolar were actually formed in the solar nebula. Industrial condensation of diamonds from mixtures of CH4 and H2 implies that high nebular CH4/CO ratios favored nanodiamond formation. A large fraction of chondritic insoluble organic matter may have formed in related processes. At low nebular temperatures appreciable water should have been incorporated into the smoke that condensed following dust (and some chondrule) evaporation. If chondrule formation continued down to temperatures as low as 250 K this process could account for the water concentration observed in primitive chondrites such as LL3.0 and CO3.0 chondrites. Higher H2O contents in CM and CI chondrites may reflect asteroidal redistribution. In some chondrite groups (e.g., CR) the Mg/Si ratio of matrix material is appreciably (30%) lower than that of chondrules but the bulk Mg/Si ratio is roughly similar to the CI or solar ratio. This has been interpreted as a kind of closed-system behavior sometimes called “complementarity.” This leads to the conclusion that nebular fines were efficiently agglomerated. Its importance, however is obscured by the observation that bulk Mg/Si ratios in ordinary and enstatite chondrites are much lower than those in carbonaceous chondrites, and thus that complementarity did not hold throughout the solar nebula.  相似文献   

4.
Abstract– Compared with ordinary chondrites, there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies. In this study, we report 39Ar‐40Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous‐textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar‐Ar age spectra show unusual 39Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show 40Ar diffusion loss. Few additional Ar‐Ar ages for enstatite meteorites are available in the literature. When all available Ar‐Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50–4.54 Ga, whereas five other meteorites show only lower age limits over 4.35–4.46 Ga. Ar‐Ar ages of several enstatite chondrites are as old or older as the oldest Ar‐Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and/or shocked, and some of the younger Ar‐Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event.  相似文献   

5.
Abstract— Due to the discoveries in Antarctica, the number of known enstatite chondrites has doubled in the last few years, and many rare or previously unknown types have been collected, most notably many EL3 and EH3 chondrites. We have applied the five major enstatite chondrite thermometers to the new and previously known enstatite chondrites, the thermometers being: (1) kamacite-quartz-enstatite-oldhamite-troilite (KQEOT), (2) oldhamite, (3) alabandite-niningerite, (4) sphalerite, and (5) phosphide-metal. Measured temperatures based on the KQEOT and oldhamite systems are 800 °C-1000 °C with the type 3 enstatite chondrites having values similar to those of type 4–6. It seems likely that these temperatures relate to events prior to parent body metamorphism, such as nebula condensation or chondrule formation, and were not significantly reset by later events. Measured temperatures for alabandite-niningerite, metal-phosphide and sphalerite in EH chondrites increase from 300 °C-400 °C to 600 °C-800 °C with petrographic indications of increasing metamorphism. In contrast, measured temperatures for all EL chondrites, including the most heavily metamorphosed, are generally <400 °C. Apparently EL chondrites cooled more slowly than the EH chondrites regardless of metamorphism experienced. Measured temperatures for the alabandite-niningerite, metal-phosphide and sphalerite are actually closure temperatures for the last thermal event suffered by the meteorite, and the fast cooling rates indicated are most consistent with processes occurring in thick regoliths.  相似文献   

6.
Abstract– Enstatite chondrites (ECs) were subjected to noble gas analyses using stepped crushing and pyrolysis extraction methods. ECs can be classified into subsolar gas‐carrying and subsolar gas‐free ECs based on the 36Ar/84Kr/132Xe ratios. For subsolar gas‐free ECs, elemental ratios, and Xe isotopic compositions indicate that Q gas is the dominant trapped component, the Q gas concentration can be correlated with the petrologic type, reasonably explained by gas release from a common EC parental material during subsequent heating. Atmospheric Xe with sub‐Q elemental ratios is found in Antarctic E3s at 600–800 °C and through crushing. The 132Xe released in these fractions accounts for 30–60% of the bulk concentrations. Hence, the sub‐Q signature is generally due to contamination of elementally fractionated atmosphere. Subsolar gas is mainly released (up to 78% of the bulk 36Ar) at 1300–1600 °C and through crushing, suggesting that enstatite and friable phases are the host phases. Subsolar gas is isotopically identical to solar gas, but elementally fractionated. These observations are consistent with a previous study, which suggested that subsolar gas could be fractionated solar wind having been implanted into chondrule precursors ( Okazaki et al. 2001 ). Unlike subsolar gas‐free ECs, the primordial gas concentrations of subsolar gas‐carrying ECs are not simply correlated with the petrologic type. It is inferred that subsolar gas‐rich chondrules were heterogeneously distributed in the solar nebula and accreted to form subsolar gas‐carrying ECs. Subsequent metamorphic and impact‐shock heating events have affected noble gas compositions to various degrees.  相似文献   

7.
Abstract— We report noble gas, oxygen isotope, 14C and 10Be data of Itqiy as well as noble gas, 14C and 10Be results for Zak?odzie. Both samples have been recently classified as anomalous enstatite meteorites and have been compared in terms of their mineralogy and chemical composition. The composition of enstatite and kamacite and the occurrence of specific sulfide phases in Itqiy indicate it formed under similar reducing conditions to those postulated for enstatite chondrites. The new results now seem to point at a direct spatial link. The noble gas record of Itqiy exhibits the presence of a trapped subsolar component, which is diagnostic for petrologic types 4–6 among enstatite chondrites. The concentration of radiogenic 4He is very low in Itqiy and indicates a recent thermal event. Its 21Ne cosmic‐ray exposure age is 30.1 ± 3.0 Ma and matches the most common age range of enstatite chondrites (mostly EL6 chondrites) but not that of Zak?odzie. Itqiy's isotopic composition of oxygen is in good agreement with that observed in Zak?odzie as well as those found in enstatite meteorites suggesting an origin from a common oxygen pool. The noble gas results, on the other hand, give reason to believe that the origin and evolution of Itqiy and Zak?odzie are not directly connected. Itqiy's terrestrial age of 5800 ± 500 years sheds crucial light on the uncertain circumstances of its recovery and proves that Itqiy is not a modern fall, whereas the 14C results from Zak?odzie suggest it hit Earth only recently.  相似文献   

8.
Abstract— In this paper, we review the mineralogy and chemistry of calcium‐aluminum‐rich inclusions (CAIs), chondrules, FeNi‐metal, and fine‐grained materials of the CR chondrite clan, including CR, CH, and the metal‐rich CB chondrites Queen Alexandra Range 94411, Hammadah al Hamra 237, Bencubbin, Gujba, and Weatherford. The members of the CR chondrite clan are among the most pristine early solar system materials, which largely escaped thermal processing in an asteroidal setting (Bencubbin, Weatherford, and Gujba may be exceptions) and provide important constraints on the solar nebula models. These constraints include (1) multiplicity of CAI formation; (2) formation of CAIs and chondrules in spatially separated nebular regions; (3) formation of CAIs in gaseous reservoir(s) having 16O‐rich isotopic compositions; chondrules appear to have formed in the presence of 16O‐poor nebular gas; (4) isolation of CAIs and chondrules from nebular gas at various ambient temperatures; (5) heterogeneous distribution of 26Al in the solar nebula; and (6) absence of matrix material in the regions of CAI and chondrule formation.  相似文献   

9.
Abstract— The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4–6 chondrites: 36Ar/132Xe = 582 ± 270 and 36Ar/84Kr = 242 ± 88). E3 chondrites usually contain Q gases, but also a composition with lower 36Ar/132Xe and 36Ar/84Kr ratios, which we call sub‐Q (36Ar/132Xe = 37.0 ± 18.0 and 36Ar/84Kr = 41.7 ± 18.1). The presence of either the subsolar or the sub‐Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites. Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.  相似文献   

10.
The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within 15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown.  相似文献   

11.
Abstract— Ningqiang is an anomalous CV chondrite (oxidized subgroup) containing a high abundance of aggregational inclusions (13.7 vol.%) and low abundances of refractory inclusions (1.0+1.0–0.5 vol.%) and bulk refractory lithophiles (～0.82 × CV). Ningqiang may have agglomerated after most refractory inclusions at the nebular midplane had already been incorporated into other objects. Coarse-grained rims surround only ～5% of Ningqiang chondrules, compared to ～50% in normal CV chondrites. Aggregational inclusions appear to have formed by incipient melting of fine-grained aggregates at relatively low temperatures in the solar nebula, possibly by the mechanism responsible for chondrule formation. Granoblastic porphyritic chondrules, which contain olivines forming 120° triple junctures and no mesostasis, probably formed in the solar nebula by incomplete melting of precursor materials that were olivine normative and had relatively low concentrations of Si, Ca, Al, Fe and Na.  相似文献   

12.
Abstract– The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine‐grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar δ15N and δ13C; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium‐heterogeneity and having mean H‐ and N‐isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer‐sized areas highly enriched in 15N (δ15N up to 1600‰). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.  相似文献   

13.
Abstract— We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö‐Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine‐grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ15N and δ13C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15N excesses are found, with δ15Nmax ～+955‰, confirming previous results. Excesses in 15N are also detected in fine‐grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N excesses are attributed to ion‐molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small‐ and large‐scale carbon isotopic variations in the solar nebula.  相似文献   

14.
Ca-Al rich refractory mineral inclusions (CAIs) found at 1-6% mass fraction in primitive chondrites appear to be 1-3 million years older than the dominant (chondrule) components which were accreted into the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the Sun. We reassess the situation in terms of a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can overcome inward drift, and prevent significant numbers of CAI-size particles from being lost into the Sun for times on the order of 106 years. CAIs can form early, when the inner nebula was hot, and persist in sufficient abundance to be incorporated into primitive planetesimals at a much later time. Small (?0.1 mm diameter) CAIs persist for longer times than large (?5 mm diameter) ones. To obtain a quantitative match to the observed volume fractions of CAIs in chondrites, another process must be allowed for: a substantial enhancement of the inner hot nebula in silicate-forming material, which we suggest was caused by rapid inward drift of meter-sized objects. This early in nebula history, the drifting rubble would have a carbon content probably an order of magnitude larger than even the most primitive (CI) carbonaceous chondrites. Abundant carbon in the evaporating material would help keep the nebula oxygen fugacity low, plausibly solar, as inferred for the formation environment of CAIs. The associated production of a larger than canonical amount of CO2 might also play a role in mass-independent fractionation of oxygen isotopes, leaving the gas rich in 16O as inferred from CAIs and other high temperature condensates.  相似文献   

15.
Abstract— It is proposed that the chondrules in enstatite chondrites formed near the Sun from rain‐like supercooled liquid silicate droplets and condensed Fe‐Ni alloys in thermodynamic equilibrium with a slowly cooling nebula. FeO formed and dissolved in the droplets in an initial stage when the nucleation of iron was blocked, and was later mostly reduced to unalloyed Fe. At high temperatures, the silicate droplets contained high concentrations of the less volatile components CaO and Al2O3. At somewhat lower temperatures the equilibrium MgO content of the droplets was relatively high. As cooling progressed, some droplets gravitated toward the Sun, and moved in other directions, depleting the region in CaO, Al2O3, and MgO and accounting for the relatively low observed CaO/SiO2, Al2O3/SiO2, and MgO/SiO2 ratios in enstatite chondrites. At approximately 1400 K, the remaining supercooled silicate droplets crystallized to form MgSiO3 (enstatite) with small amounts of olivine and a high‐SiO2 liquid phase which became the mesostases. The high enstatite content is the result of the supercooled chondrules crystallizing at a relatively low temperature and relatively high total pressure. Finally, FeS formed at temperatures below 680 K by reaction of the condensed Fe with H2S. All calculations were performed with the evaluated optimized thermodynamic databases of the FactSage thermodynamic computer system. The thermodynamic properties of compounds and solutions in these databases were optimized completely independently of any meteoritic data. Agreement of the model with observed bulk and phase compositions of enstatite chondrules is very good and is generally within experimental error limits for all components and phases.  相似文献   

16.
Abstract— The CBb chondrites are rare, primitive, metal‐rich meteorites that contain several features, including zoned metal, that have previously been interpreted as evidence for origins in the solar nebula. We have measured concentrations of Ni, Cu, Ga, Ru, Pd, Ir, and Au within both zoned and unzoned metal grains in the CBb chondrites Hammadah al Hamra (HaH) 237 and Queen Alexandra Range (QUE) 94627 using laser ablation inductively coupled plasma mass spectrometry. The refractory elements Ni, Ru, and Ir are enriched in the grain cores, relative to the rims, in the zoned metal. All refractory elements are uniform across the unzoned metal grains, at concentrations that are highly variable between grains. The volatile elements Cu, Ga, and Au are usually depleted relative to chondritic abundances and are most often uniform within the grains but are sometimes slightly elevated at the outermost rim. The Pd abundances are nearly uniform, at close to chondritic abundances, in all of the metal grains. A condensation origin is inferred for both types of metal. The data support a model in which the zoned metal formed at high temperatures, in a relatively rapidly cooling nebular gas, and the unzoned metal formed at lower temperatures and at a lower cooling rate. The CBb metal appears to have formed by a process very similar to that of the CH chondrites, but the CBb meteorite components experienced even less thermal alteration following their formation and are among the most primitive materials known to have formed in the solar nebula.  相似文献   

17.
Abstract Merrihueite (K,Na)2(Fe, Mg)5Si12O30 (na < 0.5, fe > 0.5, where na = Na/(Na + K), fe = Fe/(Fe + Mg) in atomic ratio) is a rare mineral described only in several chondrules and irregularly-shaped fragments in the Mezö-Madaras L3 chondrite (Dodd et al., 1965; Wood and Holmberg, 1994). Roedderite (Na,K)2(Mg, Fe)5Si12O30 (na > 0.5, fe < 0.5) has been found only in enstatite chondrites and in the reduced, subchondritic silicate inclusions in IAB irons (Fuchs, 1966; Rambaldi et al., 1984; Olsen, 1967). We describe silica-roedderite-bearing clasts in L/LL3.5 ALHA77011 and LL3.7 ALHA77278, a silica-roedderite-bearing chondrule in L3 Mezö-Madaras, and a silica-merrihueite-bearing chondrule in L/LL3.5 ALHA77115. The findings of merrihueite and roedderite in ALHA77011, ALHA77115, ALHA77278 and Mezö-Madaras fill the compositional gap between previously described roedderite in enstatite chondrites and silicate inclusions in IAB irons and merrihueite in Mezö-Madaras, suggesting that there is a complete solid solution of roedderite and merrihueite in meteorites. We infer that the silica- and merrihueite/roedderite-bearing chondrules and clasts experienced a complex formational history including: (a) fractional condensation in the solar nebula that produced Si-rich and Al-poor precursors, (b) melting of fractionated nebular solids resulting in the formation of silica-pyroxene chondrules, (c) in some cases, fragmentation in the nebula or on a parent body, (d) reaction of silica with alkali-rich gas that formed merrihueite/roedderite on a parent body, (e) formation of fayalitic olivine and ferrosilite-rich pyroxene due to reaction of silica with oxidized Fe on a parent body, and (f) minor thermal metamorphism, possibly generated by impacts.  相似文献   

18.
Models of the protosatellite accretion disk of Saturn are developed that satisfy cosmochemical constraints on the volatile abundances in the atmospheres of Saturn and Titan with due regard for the data obtained with the Cassini orbiter and the Huygens probe, which landed on Titan in January 2005. All basic sources of heating of the disk and protosatellite bodies are taken into account in the models, namely, dissipation of turbulence in the disk, accretion of gaseous and solid material onto the disk from the feeding zone of Saturn in the solar nebula, and heating by the radiation of young Saturn and thermal radiation of the surrounding region of the solar nebula. Two-dimensional (axisymmetric) temperature, pressure, and density distributions are calculated for the protosatellite disk. The distributions satisfy the cosmochemical constraints on the disk temperature, according to which the temperature at the stage of the satellite formation ranged from 60–65 K to 90–100 K at pressures from 10?7 to ?10?4 bar in the zone of Titan’s formation (according to estimates, r = 20–35R Sat). Variations of the basic input parameters (the accretion rate onto the protosatellite disk of Saturn from the feeding zone of the planet ?; the parameter α characterizing turbulent viscosity of the disk; and the mass concentration ratio in the solid/gas system) satisfying the aforementioned temperature constraint are found. The spectrum of models satisfying the cosmochemical constraints covers a considerable range of consistent parameters. A model with a rather small flux of ? = 10?8 M Sat/ yr and a tenfold depletion of Saturn’s disk in gas due to gas scattering from the solar nebula is at one side of this range. A model with a much higher flux of ? = 10?6 M Sat/yr and a hundredfold decrease in opacity of the disk matter owing to decreased concentration of dust particles and/or their agglomeration into large aggregates and sweeping up by planetesimals is at the other side of the range.  相似文献   

19.
Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped 20Ne and 36Ar, and the ratio (36Ar/20Ne)t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (36Ar/20Ne)t and trapped 36Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (36Ar/20Ne)t ratio, Ne‐isotopic composition, and excess of 4He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (21Ne/22Ne)c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.  相似文献   

20.
Abstract— We present a model for the thermal processing of particles in shock waves typical of the solar nebula. This shock model improves on existing models in that the dissociation and recombination of H2 and the evaporation of particles are accounted for in their effects on the mass, momentum and energy fluxes. Also, besides thermal exchange with the gas and gas‐drag heating, particles can be heated by absorbing the thermal radiation emitted by other particles. The flow of radiation is calculated using the equations of radiative transfer in a slab geometry. We compute the thermal histories of particles as they encounter and pass through the shock. We apply this shock model to the melting and cooling of chondrules in the solar nebula. We constrain the combinations of shock speed and gas density needed for chondrules to reach melting temperatures, and show that these are consistent with shock waves generated by gravitational instabilities in the protoplanetary disk. After their melting, cooling rates of chondrules in the range 10–1000 K h?1 are naturally reproduced by the shock model. Chondrules are kept warm by the reservoir of hot shocked gas, which cools only as fast as the dust grains and chondrules themselves can radiate away the gas's energy. We predict a positive correlation between the concentration of chondrules in a region and the cooling rates of chondrules in that region. This correlation is supported by the unusually high frequency of (rapidly cooled) barred chondrules among compound chondrules, which must have collided preferentially in regions of high chondrule density. We discuss these and other compelling consistencies between the meteoritic record and the shock wave model of chondrule formation.  相似文献   

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