Precondensed matter: Key to the early solar system

Chemical and isotopic anomalies in meteorites may be understandable in terms of the chemical fractionation routinely expected in the interstellar medium (ISM). Dust of distinct composition is idealized as being of three types: (1) thermal supernova condensates (SUNOCONS), (2) thermal condensation during other stellar mass-loss processes (STARDUST), and (3) nonthermal sticking processes in cold nebulae (NEBCONS). Great depletions in ISM of Ca Al Ti are due to SUNOCONS, although STARDUST is about twice as abundant. An abundance table of interstellar SUNOCONS is presented. Parent bodies in the solar system are accumulated directly from the ISM. No hot solar condensation sequence is assumed. Only relatively volatile elements within NEBOCONS are vaporized in the warm solar accretion disk. Variations in the relative amounts of these components during accumulation processes plus subsequent solid chemistry may have produced such chemical anomalies as the meteoritic fractionation patterns and the Ca Al-rich inclusions. Isotopic anomalies result from four processes that selectively site specific isotopes: (1) extinct radioactivities, (2) distinct supernova shells, (3) gas-dust separation, and (4) gas-dust age difference. Planetary accumulation will have been fingerprinted by the chemical state of the ISM if this picture is correct.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Grain motions in the solar nebula

Isotopic analyses of meteorites suggest the possibility that some interaction between supernova ejecta and grains occurred in the solar nebula. In particular, the dynamics of grain motions in the solar nebula can explain the observed mixing of nucleosynthetic components. The effect of a shock wave on the motions of grains are examined. A steady-state, plane shock propagating into a uniform region of gas and dust grains is followed by a zone of gas/grain slip, in which the grains are accelerated by drag forces from the pre-shock to the post-shock gas velocity, i.e. reducing the relative velocity between the gas and grains to zero. On the basis of these calculations, it is estimated that if grains carried the isotopic anomalies investigated by Lee, Papanastassoiu, and Wasserburg (1978), then those grains could be no bigger than 2×10^–4 cm in size. A scenario is suggested in which the sluggishness of grains provides a natural way to concentrate and mix the nucleosynthetic components carried by grains in the ejecta and in the solar nebula.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

恒星尘埃的实验室研究--实验天体物理学

原始球粒陨石含有来自恒星的微小固体颗粒(微米级),这些尘埃的同位素组成与太阳系物质截然不同,它们是目前唯一能直接获得的恒星固体样品．已发现的恒星尘埃有金刚石、石墨、碳化硅、刚玉、尖晶石、氮化物、和硅酸盐等,它们的母体恒星包括红巨星,AGB恒星、新星和超新星．对恒星尘埃的研究,使得更深入地了解星系的化学演化历史、恒星内部的核反应和湍流机制、恒星大气中尘埃的形成、星际介质物理现象等．恒星尘埃把天体物理领域延伸到了微观世界,它有机地结合了地球化学实验技术和天体物理理论,开辟了一门崭新的天文学分支实验天体物理学．     

The supernova trigger for formation of the solar system

It is suggested that the explosion of a Type II supernova triggered the collapse of a nearby interstellar cloud and led to the formation of the solar system. Estimates of the abundances resulting from nuclear processing of the supernova ejecta are presented. It appears promising that nucleosynthesis in this single supernova event can account for most isotopic anomalies and traces of extinct radioactivities in solar system material.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.     

Laboratory studies of stardust: Experimental astrophysics

Primitive meteorites contain microscopic pre-solar stardust grains that originated from stellar outflows and supernova ejecta. Identified phases include nano-diamond, graphite, silicon carbide, corundum, spinel, hibonite, nitride, and silicates. Their stellar origin was manifested by their enormous isotopic ratio variations compared to solar system materials. They are solid samples from various stellar sources, including red giant stars, AGB stars, novae, and supernovae. Laboratory isotopic analyses of these grains provide unique insights into stellar evolution, nucleosynthesis and mixing processes, dust formation in stellar envelopes, and galactic chemical evolution. Pre-solar grains open a new observational window for astrophysical researches.     

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous ¹⁶O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ¹⁸O = 5–10‰, which suggests that the domain was formed in a ¹⁶O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from ¹⁶O‐rich (δ¹⁸O = ?40‰) to ¹⁶O‐poor (δ¹⁸O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more ¹⁶O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.     

Solar system isotopic anomalies: Supernova neighbor or presolar carriers?

I evaluate several nuclear and chemical problems related both to the recent scenario suggesting that the known isotopic anomalies in the solar system have resulted from a supernova near the protosolar nebula and to the model of extinct presolar carriers. Major features include: (1) Large quantities of extinct ²⁴⁸Cm and ³⁶Cl are predicted from the Cameron-Truran model of a minor injection about 10⁶ yr before condensation; (2) an extinct-carrier model of ²⁶Mg is set forth in detail with a solid chemistry picture of the early solar system; (3) a major thermonuclear supernova responsible for ²⁶Al, ²⁴⁴Pu, and ⁴⁰K would have to have occurred several million years (3 m.y.) before condensation and contributed a large fraction of the major stable chemical elements; (4) carbon isotope families are to be expected if the oxygen isotope families are due to a late injection of ¹⁶O; (5) the Earth and E meteorites may have condensed primarily in a carbon-rich nebula existing before admixtures of a major late ¹⁶O-rich mixture; (6) the extinct-presolar-carrier model is the single best explanation of all anomalies.     

Refractory metal nuggets within presolar graphite: First condensates from a circumstellar environment

Transmission electron microscope (TEM) investigations have revealed Os, Ru, Mo‐rich refractory metal nuggets within four different presolar graphites, from both the high‐density (HD) Murchison (MUR) and low‐density (LD) Orgueil (ORG) fractions. Microstructural and chemical data suggest that these are direct condensates from the gas, rather than forming later by exsolution. The presolar refractory metal nugget (pRMN) compositions are variable (e.g., from 8 < Os atom% < 77), but follow the same chemical fractionation trends as isolated refractory metal nuggets (mRMNs) previously found in meteorites (Berg et al. 2009). From these compositions one can infer a temperature of last equilibration with the gas of 1405–1810 K (e.g., Berg et al. 2009 at approximately 100 dyne cm^?2 pressure), which implies that the host graphites form over roughly the same range (in agreement with predictions) and that the pRMNs are chemically isolated from the gas when captured by graphite. Further, the pRMN compositions give evidence that HD graphites form at a higher T than LD ones. Chemical and phase similarities with the isolated mRMNs suggest that the mRMNs also condense directly from a gas, although from the early solar nebula rather than a presolar environment. Although the pRMNs themselves are too small for detection of isotopic anomalies, NanoSIMS isotopic measurements of their host graphites confirm a presolar origin for the assemblages. The two pRMN‐containing LD graphites show evidence of a supernova (SN) origin, whereas the stellar origins of the pRMNs in HD graphite are unclear, because only less‐diagnostic ¹²C enrichments are detectable (as is commonly true for HD graphites).     

Composition and clues to the origin of refractory metal nuggets extracted from chondritic meteorites

Refractory metal nuggets (RMNs) contain elements, such as Os, Ir, Mo, and Ru, which are predicted to condense from a cooling gas of solar composition simultaneously with CAI‐minerals. Berg et al. ( 2009 ) identified a large number of RMNs in acid‐resistant residues of the Murchison meteorite and suggested that they are pristine condensates. In extending the work of these authors, we have improved the chemical extraction process to enrich the concentration of RMNs in the residue sample and prepared three additional RMN‐rich residues from the chondritic meteorites Murchison, Allende, and Leoville. The results show that, while their origin is clearly solar, the compositions in detail of RMNs from all three meteorites do not match well with a simple condensation model based on equilibrium thermodynamics and ideal solid solution of all metals. Thus, we find that a primary formation by direct condensation, as suggested previously, is unlikely for most of the studied grains and that alternative scenarios should be considered in future work. The results also show that several, but not all, alloys from Allende and Leoville have undergone processes, such as metamorphic oxidation and sulfidization in the meteoritic environment, in which they lost, e.g., W and Mo. For Murchison and several Leoville and Allende RMNs, we propose a pristine nature.     

Calcium-aluminum-rich inclusion found in the Ivuna CI chondrite: Are CI chondrites a good proxy for the bulk composition of the solar system?

Cosmochemists have relied on CI carbonaceous chondrites as proxies for chemical composition of the non-volatile elements in the solar system because these meteorites are fine-grained, chemically homogeneous, and have well-determined bulk compositions that agree with that of the solar photosphere, within uncertainties. Here we report the discovery of a calcium-aluminum-rich inclusion (CAI) in the Ivuna CI chondrite. CAIs are chemically highly fractionated compared to CI composition, consisting of refractory elements and having textures that either reflect condensation from nebular gas or melting in a nebular environment. The CAI we found is a compact type A CAI with typical ¹⁶O-rich oxygen. However, it shows no evidence of ²⁶Al, which was present when most CAIs formed. Finding a CAI in a CI chondrite raises serious questions about whether CI chondrites are a reliable proxy for the bulk composition of the solar system. Too much CAI material would show up as mismatches between the CI composition and the composition of the solar photosphere. Although small amounts of refractory material have previously been identified in CI chondrites, this material is not abundant enough to significantly perturb the bulk compositions of CI chondrites. The agreement between the composition of the solar photosphere and CI chondrites allows no more than ~0.5 atom% of CAI-like material to have been added to CI chondrites. As the compositions of CI chondrites, carbonaceous asteroids, and the solar photosphere are better determined, we will be able to reduce the uncertainties in our estimates of the composition of the solar system.     

On the problem of Solar System origin: The regularities of noble gas fractionation in shock waves

The effects of the last supernova explosion before the formation of the Solar System are considered using noble gases as examples. Acceleration of generated supernova matter in the explosive shock wave led to its initial fractionation and to the formation of small-scale isotopic heterogeneity of primordial matter. This is fixed as some isotopic anomalies in high-temperature phases of the earliest condensates of carbonaceous chondrites, as well as in the isotopic systems of noble gases, and is the basis of the supernova phenomenon. Two main manifestations of shock-wave acceleration in noble gases are investigated: the change in the isotopic ratios of their cosmogenic components due to the increasing hardness of the spectrum of nuclear-active particles and the fractionation of gases, namely, the enrichment of their isotopic systems with heavier isotopes. The reality of the processes under consideration is demonstrated through the example of noble gases of solar corpuscular radiation in lunar ilmenites. The absence of r-process products among extinct radionuclides in Ca-and Al-rich inclusions (CAI) of carbonaceous chondrites with a formation interval of less than or equal to 1 Ma supports the idea that the last supernova was an Ia-type supernova, which possibly played an important role in the origin of the Solar System.     

Supernovae interacting with a circumstellar medium: New observations with X‐shooter

In this article we present new data of a sample of core‐collapse supernovae showing unequivocal evidences of interaction between their ejecta and pre‐existing circumstellar matter (CSM). This CSM was produced by the progenitor star through major mass loss episodes during the final stages of its life. The study of the properties of the SN ejecta and the circumstellar environment are crucial to unveil the nature of the progenitors producing interacting supernovae. In this context, the instrumental combination Very Large Telescope (VLT) plus X‐shooter provides spectra covering simultaneously the optical and near‐infrared domains, and with adequate resolution to constrain the main properties of the supernova and its environment, including its chemical composition, the ejection velocity and the distribution of the emitting gas (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nebular thermal evolution and the properties of primitive planetary materials

Abstract— Models of the solar nebula are constructed to investigate the hypothesis that surviving planetary objects began to form as the nebula cooled from an early, hot epoch. The imprint of such an epoch might be retained in the spatial distribution of planetary material, the systematic deviations of its elemental composition from that of the Sun, chemical indicators of primordial oxidation state, and variations in oxygen and other isotopic compositions. Our method of investigation is to calculate the time‐dependent, two‐dimensional temperature distributions within model nebulas of prescribed dynamical evolution, and to deduce the consequences of the calculated thermal histories for coagulated solid material. The models are defined by parameters which characterize nebular initial states (mass and angular momentum), mass accretion histories, and coagulation rates and efficiencies. It is demonstrated that coagulation during the cooling of the nebula from a hot state is expected to produce systematic heterogeneities which affect the chemical and isotopic compositions of planetary material. The radial thermal gradient at the midplane results in delayed coagulation of the more volatile elements. Vertical thermal gradients isolate the most refractory material and concentrate evaporated heavy elements in the gas phase. It is concluded that these effects could be responsible for the distribution of terrestrial planetary masses, the systematic depletion patterns of the moderately volatile elements in chondritic meteorites and the Earth, the range of oxygen isotopic compositions exhibited by calcium‐aluminum‐rich inclusions (CAIs) and other refractory inclusions, and some geochemical evidence for a moderately enhanced oxidation state. However, nebular fractionations on a global scale are unlikely to account for the more oxidizing conditions inferred for some CAIs and chondritic silicates, which require dust enhancements greater than a few hundred. This conclusion, along with the well‐established evidence from studies of chondrules and CAIs for thermal excursions of short duration, make it likely that local environments, unrelated to nebular thermal evolution, were also important.     

Accretion of Jupiter’s atmosphere from a supernova-contaminated molecular cloud

If Jupiter and the Sun both formed directly from the same well-mixed proto-solar nebula, then their atmospheric compositions should be similar. However, direct sampling of Jupiter’s troposphere indicates that it is enriched in elements such as C, N, S, Ar, Kr, and Xe by 2-6× relative to the Sun (Wong, M.H., Lunine, J.I., Atreya, S.K., Johnson, T., Mahaffy, P.R., Owen, T.C., Encrenaz, T. [2008]. 219-246). Most existing models to explain this enrichment require an extremely cold proto-solar nebula which allows these heavy elements to condense, and cannot easily explain the observed variations between these species. We find that Jupiter’s atmospheric composition may be explained if the Solar System’s disk heterogeneously accretes small amounts of enriched material such as supernova ejecta from the interstellar medium during Jupiter’s formation. Our results are similar to, but substantially larger than, isotopic anomalies in terrestrial material that indicate the Solar System formed from multiple distinct reservoirs of material simultaneously with one or more nearby supernovas (Trinquier, A., Birck, J.-L., Allegre, C.J. [2007]. Astrophys. J. 655, 1179-1185). Such temporal and spatial heterogeneities could have been common at the time of the Solar System’s formation, rather than the cloud having a purely well-mixed ‘solar nebula’ composition.     

Li Be B Energetics and Cosmic Ray Origin

Three different models have been proposed for LiBeB production bycosmic rays: the CRI model in which the cosmic rays areaccelerated out of an ISM of solar composition scaled withmetallicity; the CRS model in which cosmic rays with compositionsimilar to that of the current epoch cosmic rays are acceleratedout of fresh supernova ejecta; and the LECR model in which adistinct low energy component coexists with the postulated cosmicrays of the CRI model. These models are usually distinguished bytheir predictions concerning the evolution of the Be and Babundances. Here we emphasize the energetics which favor the CRSmodel. This model is also favored by observations showing that thebulk (80 to 90%) of all supernovae occur in hot, low densitysuperbubbles, where supernova shocks can accelerate the cosmicrays from supernova ejecta enriched matter. This revised version was published online in July 2006 with corrections to the Cover Date.     

Penetration of nearby supernova dust in the inner solar system

We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live ⁶⁰Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of M_dust,Fe/M_tot,Fe ? 4  × 10^?4 for the supernova which apparently produced these species 2–3 Myr ago.     

The condensation with partial isolation (CWPI) model of condensation in the solar nebula

Abstract— We have developed a nebular condensation model and a computational routine that potentially can account for the unequilibrated mineral assemblages in chondritic meteorites. The model assumes that as condensation proceeds, a specified fraction (called the isolation degree, ξ) of the existing condensate is steadily withdrawn from reactive contact with the residual gas, presumably as a result of the growth and aggregation of condensed mineral grains. The isolated condensates may remain in the condensing system as coarse inert objects; whereas, the mineral grains that are still in reactive contact with residual nebular gases are in the form of fine dust. This paper describes the condensation with partial isolation (CWPI) model of condensation and uses it to study condensation in a nebula of solar composition at a total pressure of 10^?5 bar. The systematic isolation of condensates from residual nebular gases has profound effects on the condensation sequence. At ξ values <0.2%, the condensation sequence is essentially independent of the isolation degree and identical to the classic condensation sequence. At ξ values >2.5%, the condensation sequence is also independent of the isolation degree and closely resembles the “inhomogeneous accretion model” or “chemical disequilibrium model” of condensation. In the intermediate range of ξ values, the character of the condensation sequence is very sensitive to the degree of chemical fractionation caused by condensate isolation. The mineralogy of chondritic meteorites is not consistent with condensation sequences having ξ > 2.5; this is an upper limit on the ξ values that is characteristic of condensation in the solar nebula. The mineralogy and chemistry of carbonaceous and enstatite chondrites can be explained by accretion of isolated condensates formed at ξ values of ≤0.1% and 0.7–1.5%, respectively, providing that segregation of the inert coarse objects and fine reactive dust occurred in the nebula. Segregation of these two categories of condensate may have been responsible for the observed volatility-based chemical fractionations among chondritic meteorites.     

Grain formation in the ejecta of a supernova: the nucleation rate of grains heated by radiation

In the ejecta of a supernova, the temperature of the created grains differs from that of the gas due to the radiation from the star. We investigated the grain formation in the supernova using a developed new nucleation rate where the temperature difference between the gas and the grains is taken into account. If the temperature of the grains is higher than that of the gas, the nucleation process does not occur when the gaseous temperature attains the condensation temperature. As a result we found that the temperature difference between the gas and the grains in SN1987A is about 50–200K which leads that the nucleation is delayed for about 20–100 days.     

Vapor‐condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO₂‐poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high‐temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca‐rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO₂‐rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti³⁺/Ti⁴⁺ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron‐wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass‐fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al₂O₃) relative to more volatile (MgO+SiO₂) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti³⁺/Ti⁴⁺ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest‐temperature condensates in the form of ices but the highest‐temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron‐wüstite, more than 10⁵ times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe‐Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon‐bearing ices that would make the even lower.