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The chemical equilibrium distribution of 69 elements between gas and melt is modeled for bulk silicate Earth (BSE) material from 1000 - 4500 K and 1e-6 to 100 bar. The BSE melt is modeled as a non-ideal solution and the effects of different activity coefficients and ideal solution are studied. Results include 50% condensation temperatures, major gases of each element, and oxygen fugacity (fO2) of dry and wet BSE material. The dry BSE model excludes H, C, N, F, Cl, Br, I, S, Se, Te. The wet BSE model includes H and the other volatiles. Key conclusions are much higher condensation temperatures in silicate vapor than in solar composition gas at the same total P, a different condensation sequence in silicate vapor than in solar composition gas, good agreement between different activity coefficient models except for the alkalis, agreement, where overlap exists, with prior published work, condensation of Re, Mo, W, Ru, Os oxides instead of metals, a stability field for Ni-rich metal as reported by Lock et al. (2018), agreement between ideal solution (from this work and from Lock et al. 2018) and real solution condensation temperatures for elements with minor deviations from ideality in the oxide melt, similar 50% condensation temperatures, within a few degrees, in the dry and wet BSE models for the major elements Al, Ca, Fe, Mg, Si, and the minor elements Co, Cr, Li, Mn, Ti, V, and much lower 50 percent condensation temperatures for elements such as B, Cu, K, Na, Pb, Rb, which form halide, hydroxide, sulfide, selenide, telluride and oxyhalide gases. The latter results are preliminary because the poorly known solubilities and activities of volatile elements in silicate melts must be considered for the correct equilibrium distribution, condensation temperatures and mass balance of F, Cl, Br, I, H, S, Se and Te bearing species between melt and vapor (abridged).

Lithium’s story spans the history of the universe and is one that links to all its largest-scale processes: big bang nucleosyntheses, the evolution of stars, and galactic chemical evolution. Lithium was the only metal produced in the big bang, alongside the gases H and He. Stars destroy both stable isotopes of Li easily, yet we still have Li today, even after generations of stars have come and gone. Ongoing production of Li by galactic cosmic rays and by a limited number of Li-producing nuclear reactions and transport processes in some rare types of stars keeps lithium present in the universe.

The Moon is depleted in water and other volatiles compared to Earth and the bulk solar composition. Such depletion of volatile elements and the stable isotope fractionations of these elements can be used to better understand the origin and early differentiation history of the Moon. In this study, we focus on the moderately volatile element, potassium, and we report the K elemental abundances and isotopic compositions (δ41K relative to NIST SRM 3141a) for nineteen Apollo lunar rocks and lunar meteorites (twenty-two subsamples), spanning all major geochemical and petrologic types of lunar materials. The K isotopic compositions of low-Ti and high-Ti basalts are indistinguishable, providing a lunar basalt average δ41K of –0.07 ± 0.09‰ (2SD), which we also consider to be the best estimate of the lunar mantle and the bulk silicate Moon. The significant enrichment of K in its heavier isotopes in the bulk silicate Moon, compared with the bulk silicate Earth (δ41K = –0.48 ± 0.03‰), is consistent with previous analyses of K isotopes and other moderately volatile elements (e.g., Cl, Cu, Zn, Ga, and Rb). We also report analyses of K isotopes for lunar nonmare samples, which show large variations of K isotopic ratios compared to lunar basalts. We interpret this large K isotopic fractionation as the result of late-stage magma ocean degassing during urKREEP formation, which is also coupled with Cl isotope fractionation. Degassing of urKREEP likely triggered redistribution of K isotopes in the Moon, enriching the urKREEP reservoir in heavy K isotopes while implanting the light K isotopic signatures onto the lunar surface. This scenario suggests a heterogeneous distribution of K isotopes in the Moon as a consequence of its magmatic evolution.

Solar system materials are variably depleted in moderately volatile elements (MVEs) relative to the proto-solar composition. To address the origin of this MVE depletion, we conducted a systematic study of high-precision K isotopic composition on 16 carbonaceous chondrites (CCs) of types CM1-2, CO3, CV3, CR2, CK4-5 and CH3 and 28 ordinary chondrites (OCs) covering petrological types 3 to 6 and chemical groups H, L, and LL. We observed significant overall K isotope (delta41K) variations (-1.54 to 0.70 permil). The K isotope compositions of CCs are largely higher than the Bulk Silicate Earth (BSE) value, whereas OCs show typically lower values than BSE. Neither CCs nor OCs show resolvable correlations between K isotopes and chemical groups, petrological types, shock levels, exposure ages, fall or find occurrence, or terrestrial weathering. The lack of a clear trend between K isotopes and K content indicates that the K isotope fractionations were decoupled from the relative elemental K depletions. The range of K isotope variations in the CCs is consistent with a four-component (chondrule, refractory inclusion, matrix and water) mixing model that is able to explain the bulk elemental and isotopic compositions of the main CC groups, but requires a fractionation in K isotopic compositions in chondrules. We propose that the major control of the isotopic compositions of group averages is condensation or vaporization in nebular environments that is preserved in the compositional variation of chondrules. Parent-body processes (aqueous alteration, thermal metamorphism, and metasomatism) can mobilize K and affect the K isotopes in individual samples. In the case of the OCs, the full range of K isotopic variations can only be explained by the combined effects of the size and relative abundances of chondrules, parent-body aqueous and thermal alteration., Comment: 71 pages, 2 tables, 9 figures

The chemical and isotopic signatures of moderately volatile elements are useful for understanding processes of volatile depletion in planetary formation and differentiation. However, the fractionation factors between gas and melt phases during evaporation that are required to model these planetary volatile depletion processes are still sparse. In this study, twenty heating experiments were conducted in 1 atm gas-mixing furnaces to constrain the behavior of K, Cu, and Zn evaporation and isotopic fractionation from basaltic melts at high temperatures. The temperatures range from 1300 °C to 1400 °C, and durations are from 2 to 8 days. Oxygen fugacities (fO2) range from one log unit below to ten log units above that of the iron-wustite buffer (IW–1 to IW + 10, corresponding to logfO2 of –10.7 to –0.68 at 1400 °C). The conditions were selected to achieve an evaporation-dominated regime (where timescales of diffusion ≪ evaporation for trace elements) in order to avoid diffusion-limited evaporation. Our results show during evaporation Zn behaved as the most volatile, followed by Cu and then K, regardless of temperature and oxygen fugacity. Partitioning of Zn into spinel layers within experimental capsules, however, has been observed, which has substantial effects on the Zn isotope fractionation factor. Therefore, Zn results are presented but further discussion is excluded. Element loss depends on both temperature and oxygen fugacity, where higher temperatures and lower oxygen fugacities promote evaporation. However, with varying temperature and oxygen fugacity, the kinetic isotopic fractionation factors, α (where, R R 0 = f α - 1 ), for K and Cu remain constant, thus these factors can be applied to a wider range of conditions than those in this study. The experimentally determined fractionation factors for K, and Cu during evaporation from basaltic melts are 0.9944, and 0.9961, respectively. The fractionation factors for these elements with varying volatilities are significantly larger than the “apparent observed fractionation factors,” which approach one and are inferred from lunar basalts relative to the Bulk Silicate Earth. This observation suggests near-equilibrium conditions during volatile-element loss from the Moon as the “apparent observed fractionation factors” of lunar basalts are similar for all three elements.

We report new high-precision stable K isotope data for three martian meteorites, one lunar meteorite, one ordinary chondrite, four terrestrial igneous United States Geological Survey (USGS) reference materials, and twenty howardite–eucrite–diogenite [HED] meteorites. The three martian meteorites define a relatively narrow δ41K range with an average of −0.36 ± 0.12‰ (2 SD) that is slightly heavier than the Bulk Silicate Earth (BSE) K isotopic composition (−0.48 ± 0.03‰). Except for the four Northwest Africa samples which were terrestrially contaminated, all HED meteorites reveal substantial 41K enrichment compared to BSE, lunar samples, martian meteorites, and chondrites. We propose that the average δ41K (+0.36 ± 0.16‰) obtained from HED meteorites is representative of Bulk Silicate 4-Vesta. The coupled volatile depletion and heavy K isotope enrichment in 4-Vesta could be attributed to both nebula-scale processes and parent-body events. The asteroid 4-Vesta is likely to have accreted from planetary feedstocks that have been significantly volatile-depleted prior to the major phases of planetary accretion in the early Solar System, with secondary effects of K loss during accretionary growth and magma ocean degassing.

Tektites are mm to cm sized glassy objects generated through high-energy meteoroid impacts on the surface of the Earth under high temperature and pressure, and reducing conditions. They are the products of large-scale catastrophic events in Earth’s history and can be used to understand the behavior of moderately volatile elements (e.g., K and Zn) during impact vaporization events. Here, we report bulk K isotopic compositions of tektites from three different strewn fields and “in-situ” profile analysis of both K and Zn isotopes in one complete tektite. All tektites span a narrow range in their K isotopic compositions (δ41KBSE: −0.10 ± 0.03‰ to 0.16 ± 0.04‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) and upper continental crust materials, which is consistent with previous results. In contrast, Zn isotopes show a large variation (δ66Zn: −0.39 ± 0.02‰ to 2.38 ± 0.03‰) even within one specimen. In order to provide a coherent explanation for the different behavior of moderately volatile elements (K, Zn and Cu), we have conducted thermochemical calculations to compute the partial vapor pressures of Cu2O, K2O, and ZnO dissolved in silicate melts as a function of temperature, pressure, oxygen and chlorine fugacities. In a large range of the parameter space, the calculations show that Cu and Zn can be vaporized much easier than K and thus produce large isotopic fractionation. In contrast, the lithophile element K is more prone to remain in the silicate melt because of its very low activity coefficient in the melt, and thus the K isotopes remain unfractionated. This study provides new constraints on the formation of tektites and is consistent with a “bubble-stripping” model to explain the extreme water and volatiles depletion in tektites.

Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass-independent and mass-dependent isotope fractionations. Due to the analytical challenges to obtain high-precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high-precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from -2.34 permil to -0.18 permil). However, K isotopic compositions of nearly all enstatite meteorites scatter around the Bulk Silicate Earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (-0.47 +/- 0.57 permil) is indistinguishable from the BSE value (-0.48 +/- 0.03 permil), thus further corroborating the isotopic similarity between Earth' building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent body processing. Our sample of the main group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (delta 41K= -2.34 +/- 0.12 permil). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K-bearing sulfide mineral is predicted to be enriched in 39K during equilibrium exchange with silicates., Comment: 46 pages, 7 figures, 2 tables, published in Meteoritics & Planetary Science

This brief special communications article gives data for atomic abundances and mass fractions for the elemental and isotopic solar system composition, the atomic masses of the elements and their isotopes, the composition of the solar photosphere, and the compositions of the major chondritic meteorite groups. This additional material is relevant for researchers who are interested in this Topical Collection on planetary evolution.

The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent “Perseverance” and “Tianwen-1” missions, “follow the water,” and, more broadly, “volatiles,” has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average “bulk silicate” δ(41)K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na “volatility” ratio, and most notably, bulk planet H(2)O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H(2)O to enable habitability and plate tectonics, with mass exceeding that of Mars.

Solar elemental abundances, or solar system elemental abundances, refer to the complement of chemical elements in the entire Solar System. The Sun contains more than 99% of the mass in the solar system and therefore the composition of the Sun is a good proxy for the composition of the overall solar system. The solar system composition can be taken as the overall composition of the molecular cloud within the interstellar medium from which the solar system formed 4.567 billion years ago. Active research areas in astronomy and cosmochemistry model collapse of a molecular cloud of solar composition into a star with a planetary system and the physical and chemical fractionation of the elements during planetary formation and differentiation. The solar system composition is the initial composition from which all solar system objects (the Sun, terrestrial planets, gas giant planets, planetary satellites and moons, asteroids, Kuiper-belt objects, and comets) were derived. Other dwarf stars (with hydrostatic hydrogen-burning in their cores) like the Sun (type G2V dwarf star) within the solar neighborhood have compositions similar to the Sun and the solar system composition. In general, differential comparisons of stellar compositions provide insights about stellar evolution as functions of stellar mass and age and ongoing nucleosynthesis but also about galactic chemical evolution when elemental compositions of stellar populations across the Milky Way Galaxy is considered. Comparisons to solar composition can reveal element destruction (e.g., Li) in the Sun and in other dwarf stars. The comparisons also show element production of, for example, C, N, O, and the heavy elements made by the s-process in low to intermediate mass stars (3–7 solar masses) after these evolved from their dwarf-star stage into red giant stars (where hydrogen and helium burning can occur in shells around their cores). The solar system abundances are and have been a critical test composition for nucleosynthesis models and models of galactic chemical evolution, which aim ultimately to track the production of the elements heavier than hydrogen and helium in the generation of stars that came forth after the Big Bang 13.4 billion years ago.

International audience; Understanding the formation of our planetary system requires identification of the materials from which it originated and the accretion processes that produced the planets. The compositional evolution of the solar system can be constrained by synthesizing astronomical datasets and numerical models with elemental and isotopic compositions from objects that directly sampled the disk: meteorites and their constituents (chondrules, refractory inclusions, and matrix). This contribution reviews constraints on early solar system evolution provided by the so-called non-carbonaceous (NC) and carbonaceous chondrite (CC) groups and their relationship to the volatile element characteristics of chondritic meteorites. In previous work, the NC or CC character of a parent body was used to infer its accretion location in the protoplanetary disk. The NC groups purportedly originated in the inner disk, and the CC groups were derived from the outer disk, where the NC and CC regions of the disk may have been separated early on by proto-Jupiter, a pressure maximum, or a dust trap in the disk. The tenet that all CC parent bodies accreted in the outer disk is, in part, based on evidence that a handful of CC meteorites are enriched in volatile species compared to NC meteorites. Here, it is reviewed if and how the volatile element and nucleosynthetic isotope compositions of meteorites can be linked to accretion locations within the disk. The nucleosynthetic isotope compositions of whole rock meteorite samples contrast the trends found for their major volatile element compositions (i.e., C, N, and O). Although there may be an increase in volatile abundances when comparing some stony NC and CC meteorites and their inferred accretion locations within the disk, this is not necessarily a general rule. The difficulties with inferring parent body accretion locations are discussed. It is found that it cannot always be assumed that parent bodies which formed in the CC reservoir are "volatile-rich" relative to those that formed in the NC reservoir which are "volatilepoor". Consequently, tracing the origin of terrestrial volatiles using the NC-CC isotope dichotomy remains challenging.

Elemental abundances in CI-chondrites are compared to recent photospheric data. Resulting issues for solar system abundances are noted.

We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor - melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.

We studied 14 presolar SiC mainstream grains for C-, Si-, and S-isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite-normalized S/Si ratios between 2 × 10−5 and 2 × 10−4. All grains have S-isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low-mass asymptotic giant branch (AGB) stars. In a δ33S versus δ34S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess 32S found in some Type AB SiC grains are the result of in situ decay of radioactive 32Si from born-again AGB stars that condensed into AB grains.

We explore a method to utilize the stoichiometry of iron sulfide to determine the sulfur fugacity in experiments containing CO-CO2-SO2 gas mixtures. The Fe-S phase diagram shows that the stoichiometry of iron sulfide melts is closely related to the sulfur fugacity f S2 at a given temperature and ambient pressure. We derive equations that relate the sulfur fugacity to the mole fraction of sulfur XS in the iron sulfide from available literature data, and a solution model using the Redlich-Kister approximation for the excess Gibbs energy of mixing. We test the method by exposing iron sulfide to CO-CO2-SO2 gas mixtures and subsequently analyzing the “Fe-S monitor” for its stoichiometry. Most sulfur fugacities calculated from the equilibrium gas composition agree within 5–10% (1200 °C) and 1–4% (1400 °C) with those derived from the sulfur mole fraction in the monitor and literature data calibration, which is consistent with the spread observed in literature data. There were no suitable literature data for a full calibration at 1300 °C, so we combined the available literature and our data to find the sulfur fugacity as a function of mole fraction sulfur. Overall the Fe-S monitor technique is a convenient method to determine the sulfur fugacity in high-temperature experiments containing CO-CO2-SO2 gas mixtures as long as oxygen fugacities remain below that of the iron-wustite or iron-magnetite buffer.

Extensive experimental studies show all major rocky elements (Si, Mg, Fe, Ni, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases and via reaction with HF or HCl as volatile halide gases in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at the same temperature. We compute the extent of fractional vaporization by defining gas to magma partition coefficients and show Earth's sub-solar Si to Mg bulk elemental ratio may be due to loss of a primordial steam atmosphere. We conclude hot rocky exoplanets that are undergoing or have undergone escape of steam atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Ca, Al, Na, and K. This loss may modify their bulk composition, density, heat balance, and internal structure., Comment: In press in The Astrophysical Journal, 83 pages text and references, 6 tables, 30 figures

We continue a systematic study of chemical abundances of the Strontium Filament found in the ejecta of eta Carinae. To this end we interpret the emission spectrum of Sc II and Cr II using multilevel non-LTE models of these systems. Since the atomic data for these ions was previously unavailable, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. The observed spectrum is emitted from a mostly neutral region with electron density of the order of 10(exp 7) cm (exp -3) and a temperature between 6000 and 7000 K. These conditions are consistent with our previous diagnostics from [Ni II], [Ti II], amd [Sr II]. The observed spectrum indicates an abundance of Sc relative Ni that more than 40 times the solar values, while the Cr/Ni abundance ratio is roughly solar. Various scenarios of depletion and dust destruction are suggested to explain such abnormal abundances.

We present an analysis of the 0.95-14.5 micron spectral energy distributions of nine field ultracool dwarfs with spectral types ranging from L1 to T4.5. Effective temperatures, gravities, and condensate cloud sedimentation efficiencies are derived by comparing the data to synthetic spectra computed from atmospheric models that self-consistently include the formation of condensate clouds. Derived effective temperatures decrease steadily through the L1 to T4.5 spectral types and we confirm that the effective temperatures of ultracool dwarfs at the L/T transition are nearly constant, decreasing by only ~200 K from spectral types L7.5 to T4.5. The two objects in our sample with very red J-Ks colors are best fitted with synthetic spectra that have thick clouds which hints at a possible correlation between the near-infrared colors of L dwarfs and the condensate cloud properties. The fits to the two T dwarfs in our sample (T2 and T4.5) also suggest that the clouds become thinner in this spectral class, in agreement with previous studies. Restricting the fits to narrower wavelength ranges (i.e., individual photometric bands) almost always yields excellent agreement between the data and models. Limitations in our knowledge of the opacities of key absorbers such as FeH, VO, and CH4 at certain wavelengths remain obvious, however. The effective temperatures obtained by fitting the narrower wavelength ranges can show a large scatter compared to the values derived by fitting the full spectral energy distributions; deviations are typically ~200 K and in the worst cases, up to 700 K.

Opacities and chemical abundance data are crucial ingredients of ultracool dwarf and extrasolar giant planet atmosphere models. We report here on the detailed sources of molecular opacity data employed by our group for this application. We also present tables of Rosseland and Planck mean opacities which are of use in some studies of the atmospheres, interiors, and evolution of planets and brown dwarfs. For the tables presented here we have included the opacities of important atomic and molecular species, including the alkali elements, pressure induced absorption by hydrogen, and other significant opacity sources but neglect opacity from condensates. We report for each species how we have assembled molecular line data from a combination of public databases, laboratory data that is not yet in the public databases, and our own numerical calculations. We combine these opacities with abundances computed from a chemical equilibrium model using recently revised solar abundances to compute mean opacities. The chemical equilibrium calculation accounts for the settling of condensates in a gravitational field, and is applicable to ultracool dwarf and extrasolar planetary atmospheres, but not circumstellar disks. We find that the inclusion of alkali atomic opacity substantially increases the mean opacities over those currently in the literature at densities relevant to the atmospheres and interiors of giant planets and brown dwarfs. We provide our opacity tables for public use and discuss their limitations., 32 pages, 6 figures, accepted for publication in Astrphysical Journal Supplement, units revised, now includes on-line only data files (available in "source" package from "other formats" link)

We present the first analysis of an optical to mid-infrared spectrum of the T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown dwarf evolution sequences. We obtain precise values for the basic parameters of Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to -5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that can only be fitted by reducing the abundance of NH3 by about one order of magnitude from the value obtained with chemical equilibrium models. We model departures from chemical equilibrium in the atmosphere of Gl 570D by considering the kinetics of nitrogen and carbon chemistry in the presence of vertical mixing. The resulting model spectrum reproduces the data very well., Comment: Accepted for publication in the ApJ. 10 pages, including 3 figures

Pre-solar grains from supernova ejecta – silicon carbide of type X, Si3N4 and low-density graphite – are characterized by Si isotopic anomalies (mainly 28Si excesses), low 14N/15N, high 26Al/27 Al ratios, and occasionally by excesses in 44Ca (from 44Ti decay). Overall isotopic features of these SiC and graphite grains can be explained by mixing of inner Si-rich zones and the outer C-and He-rich zones, but supernova models require fine tuning to account for 14N/15N and 29Si/28Si ratios of the grains. Isotopic ratios of Zr, Mo and Ba in SiC X grains may be explained by a neutron burst model. Some of the pre-solar nanodiamonds require a supernova origin to explain measured xenon isotopic ratios. Only a few nova grain candidates, with low 12C/13C, 14N/15N, and high 26Al/27 Al ratios, have been identified.

We study the nature of the [TiII] and [NiII] emission from the so-called strontium filament found in the ejecta of eta Carinae. To this purpose we employ multilevel models of the TiII and NiII systems which are used to investigate the physical condition of the filament and the excitation mechanisms of the observed lines. For the TiII ion, for which no atomic data was previously available, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. It is found that the observed spectrum is consistent with the lines being excited in a mostly neutral region with an electron density of the order of $10^7$ cm$^{-3}$ and a temperature around 6000 K. In analyzing three observations with different slit orientations recorded between March~2000 and November~2001 we find line ratios that change among various observations, in a way consistent with changes of up to an order of magnitude in the strength of the continuum radiation field. These changes result from different samplings of the extended filament, due to the different slit orientations used for each observation, and yield clues on the spatial extent and optical depth of the filament. The observed emission indicates a large Ti/Ni abundance ratio relative to solar abundances. It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas fractionation processes and does not reflect the absolute Ti/Ni ratio in the ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within the filament and suggest that the observed gas phase overabundance of Ti

We investigate the atmosphere and interior of the new transiting planet HD 149026b, which appears to be very rich in heavy elements. We first compute model atmospheres at metallicities ranging from solar to ten times solar, and show how for cases with high metallicity or inefficient redistribution of energy from the day side, the planet may develop a hot stratosphere due to absorption of stellar flux by TiO and VO. The spectra predicted by these models are very different than cooler atmosphere models without stratospheres. The spectral effects are potentially detectable with the Spitzer Space Telescope. In addition the models with hot stratospheres lead to a large limb brightening, rather than darkening. We compare the atmosphere of HD 149026b to other well-known transiting planets, including the recently discovered HD 189733b, which we show have planet-to-star flux ratios twice that of HD 209458 and TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive indicator of atmospheric temperature and metallicity and can be constrained with Spitzer IRAC observations. We then turn to interior studies of HD 149026b and use a grid of self-consistent model atmospheres and high-pressure equations of state for all components to compute thermal evolution models of the planet. We estimate that the mass of heavy elements within the planet is in the range of 60 to 93 M_earth. Finally, we discuss trends in the radii of transiting planets with metallicity in light of this new member of the class., Comment: Accepted to the Astrophysical Journal. 18 pages, including 10 figures. New section on the atmosphere of planet HD 189733b. Enhanced discussion of atmospheric Ti chemistry and core mass for HD 149026b

Infrared and ultraviolet observations of nova light curves have confirmed grain formation in their expanding shells that are ejected into the interstellar medium by a thermonuclear runaway. In this paper, we present isotopic ratios of intermediate-mass elements up to silicon for the ejecta of CO and ONe novae, based on 20 hydrodynamic models of nova explosions. These theoretical estimates will help to properly identify nova grains in primitive meteorites. In addition, equilibrium condensation calculations are used to predict the types of grains that can be expected in the nova ejecta, providing some hints on the puzzling formation of C-rich dust in O>C environments. These results show that SiC grains can condense in ONe novae, in concert with an inferred (ONe) nova origin for several presolar SiC grains., 42 pages. Accepted for publication in The Astrophysical Journal

Elemental abundances in Jupiter determined from Galileo probe measurements are compared to recently revised solar system abundances. When normalized to the abundance of sulfur, the most abundant refractory rock-forming element reliably determined in Jupiter's atmosphere by the Galileo probe, abundances of argon, krypton, and xenon are 1 times solar, the observed oxygen is depleted by a factor of 4, and carbon is enriched 1.7 times. The fairly uncertain nitrogen abundance ranges from 1 to 3 times solar. The oxygen abundance in Jupiter derived from the observed atmospheric water abundance is only a lower limit to the total planetary oxygen because oxygen is also bound to rock-forming elements such as magnesium or silicon sequestered deep in the planet. The sulfur abundance constrains the amount of rock-forming elements on Jupiter. Considering the amount of oxygen bound to silicate rock, the total oxygen abundance on Jupiter of 0.47 times solar system indicates an overall oxygen depletion by about a factor of 2. The hydrogen and helium abundances in the Jovian atmosphere are depleted (0.48 and 0.39 times solar system, respectively). These relative depletions may indicate the extent of hydrogen and helium partitioning from the molecular envelope into Jupiter's metallic layer. A formation scenario for Jupiter is proposed to explain the relative oxygen depletion and, at the same time, the relative carbon enrichment. In essence, the model assumes that at the time of Jupiter's formation, abundant carbonaceous matter was present near 5.2 AU rather than abundant water ice, increasing the surface mass density of solids in the solar nebula accretion disk. Carbonaceous matter, which has high sticking probabilities, was the agent that sped up accumulation of solid matter of proto-Jupiter. This led to runaway accretion of the planet. Major consequences of this scenario are that the water ice condensation front (the snow line) typically placed near 5.2 AU in solar nebula models must be replaced by a carbonaceous condensation/evaporation front (the tar line) and that the snow line is located farther out in the solar nebula.

Computations were done to correct erroneous data tables given in the 4th edition of the NIST-JANAF Thermochemical Tables. Updated enthalpies of formation were included to compute the thermochemical tables for four ideal gases: mercapto (HS), disulfur monoxide (S2O), thiazyl (NS), and thioxophosphino (PS).

Solar photospheric and meteoritic CI chondrite abundance determinations for all elements are summarized and the best currently available photospheric abundances are selected. The meteoritic and solar abundances of a few elements (e.g., noble gases, beryllium, boron, phosphorous, sulfur) are discussed in detail. The photospheric abundances give mass fractions of hydrogen (X ¼ 0:7491), helium (Y ¼ 0:2377), and heavy elements (Z ¼ 0:0133), leading to Z=X ¼ 0:0177, which is lower than the widely used Z=X ¼ 0:0245 from previous compilations. Recent results from standard solar models considering helium and heavy-element settling imply that photospheric abundances and mass fractions are not equal to protosolar abundances (representative of solar system abundances). Protosolar elemental and isotopic abundances are derived from photospheric abundances by considering settling effects. Derived protosolar mass fractions are X0 ¼ 0:7110, Y0 ¼ 0:2741, and Z0 ¼ 0:0149. The solar system and photospheric abundance tables are used to compute self-consistent sets of condensation temperatures for all elements. Subject headings: astrochemistry — meteors, meteoroids — solar system: formation — Sun: abundances — Sun: photosphere

The equilibrium gas and condensation chemistry of titanium and vanadium in M, L, and T dwarf atmospheres is computed. The calcium titanates (Ca3Ti2O7 and Ca4Ti3O10) are identified for the first time as important Ti-bearing condensates in addition to perovskite (CaTiO3) and Ti oxides in dwarf atmospheres. The chemistry of Ti is intimately coupled to that of refractory condensates containing Ca and Al, whose chemistry is much more intricate than commonly assumed. The TiO gas abundances in equilibrium with Ca3Ti2O7 or Ca4Ti3O10 are lower than the TiO gas abundances in equilibrium with perovskite. Consequently, this implies that TiO opacities are lower than previously assumed at temperatures and pressures where the new Ca titanates are stable. Vanadium condenses into solid solution with Ti-bearing condensates (as observed in meteorites) and not as "pure" vanadium oxide, as commonly assumed. The calculated TiO (gas) and VO (gas) abundances are used to constrain temperatures at the M/L dwarf transition. Adopted temperatures are 2010 K (with an upper limit of 2245 K) for M8 and 1960 K for M9. Temperatures at the beginning of the L sequence are less than 1950 K and are tentatively placed at ~1910 K for L0.

The chemical species containing carbon, nitrogen, and oxygen in atmospheres of giant planets, brown dwarfs (T and L dwarfs), and low-mass stars (M dwarfs) are identified as part of a comprehensive set of thermochemical equilibrium and kinetic calculations for all elements. The calculations cover a wide temperature and pressure range in the upper portions of giant planetary and T-, L-, and M-dwarf atmospheres. Emphasis is placed on the major gases CH4, CO, NH3, N2, and H2O but other less abundant gases are included. The results presented are independent of particular model atmospheres, and can be used to constrain model atmosphere temperatures and pressures from observations of different gases. The influence of metallicity on the speciation of these key elements under pressure–temperature (P–T) conditions relevant to low-mass object atmospheres is discussed. The results of the thermochemical equilibrium computations indicate that several compounds may be useful to establish temperature or pressure scales for giant planet, brown dwarf, or dwarf star atmospheres. We find that ethane and methanol abundance are useful temperature probes in giant planets and methane dwarfs such as Gl 229B, and that CO2 can serve as a temperature probe in more massive objects. Imidogen (NH) abundances are a unique pressure-independent temperature probe for all objects. Total pressure probes for warmer brown dwarfs and M dwarfs are HCN, HCNO, and CH2O. No temperature-independent probes for the total pressure in giant planets or T-dwarf atmospheres are identified among the more abundant C, N, and O bearing gases investigated here.

We present new calculations of Rosseland and Planck gaseous mean opacities relevant to the atmospheres of giant planets and ultracool dwarfs. Such calculations are used in modeling the atmospheres, interiors, formation, and evolution of these objects. Our calculations are an expansion of those presented in Freedman et al. (2008) to include lower pressures, finer temperature resolution, and also the higher metallicities most relevant for giant planet atmospheres. Calculations span 1 microbar to 300 bar, and 75 K to 4000 K, in a nearly square grid. Opacities at metallicities from solar to 50 times solar abundances are calculated. We also provide an analytic fit to the Rosseland mean opacities over the grid in pressure, temperature, and metallicity. In addition to computing mean opacities at these local temperatures, we also calculate them with weighting functions up to 7000 K, to simulate the mean opacities for incident stellar intensities, rather than locally thermally emitted intensities. The chemical equilibrium calculations account for the settling of condensates in a gravitational field and are applicable to cloud-free giant planet and ultracool dwarf atmospheres, but not circumstellar disks. We provide our extensive opacity tables for public use., This version fixes a units description error that was only in the arxiv version, but is correct in the ApJS version. In equations 4 and 5 pressure is in dyne/cm2. Online tables available at the ApJS website: https://iopscience.iop.org/article/10.1088/0067-0049/214/2/25

The formation of clouds affects brown dwarf and planetary atmospheres of nearly all effective temperatures. Iron and silicate condense in L dwarf atmospheres and dissipate at the L/T transition. Minor species such as sulfides and salts condense in mid-late T dwarfs. For brown dwarfs below Teff=450 K, water condenses in the upper atmosphere to form ice clouds. Currently over a dozen objects in this temperature range have been discovered, and few previous theoretical studies have addressed the effect of water clouds on brown dwarf or exoplanetary spectra. Here we present a new grid of models that include the effect of water cloud opacity. We find that they become optically thick in objects below Teff=350-375 K. Unlike refractory cloud materials, water ice particles are significantly non-gray absorbers; they predominantly scatter at optical wavelengths through J band and absorb in the infrared with prominent features, the strongest of which is at 2.8 microns. H2O, NH3, CH4, and H2 CIA are dominant opacity sources; less abundant species such as may also be detectable, including the alkalis, H2S, and PH3. PH3, which has been detected in Jupiter, is expected to have a strong signature in the mid-infrared at 4.3 microns in Y dwarfs around Teff=450 K; if disequilibrium chemistry increases the abundance of PH3, it may be detectable over a wider effective temperature range than models predict. We show results incorporating disequilibrium nitrogen and carbon chemistry and predict signatures of low gravity in planetary- mass objects. Lastly, we make predictions for the observability of Y dwarfs and planets with existing and future instruments including the James Webb Space Telescope and Gemini Planet Imager., Comment: 23 pages, 20 figures, Revised for ApJ

A total of 124 presolar SiC grains of type A and B (defined as having 12C/13C O in the envelope. A special nucleosynthetic problem is posed by the 14N/15N ratios of the grains. High ratios can be explained by hot bottom burning and by cool bottom processing in thermally pulsing AGB stars. Another proposed scenario that possibly yields this signature is extensive mixing of He-burning material into the H-rich envelope during the core He flash. However, the spread of the 14N/15N ratios and lower-than-solar 14N/15N ratios remains unexplained. The isotopic and elemental compositions of A+B grains can provide new information about nucleosynthesis in their possible parent stars that cannot be obtained in any other way.

infer the presence of monovalent Cu 1 (“CuO 0.5”) in the silicate melt on the basis of the solubility of Cu as function of oxygen fugacity. Experiments containing iron yield a formal valence of ;0.5 for Cu at very low oxygen fugacities, which is not observed in Fe-free systems. The low formal valence is explained by redox reactions between iron and copper in the silicate melts. There is no evidence for sulfidic dissolution of Cu in the silicates but sulfur has indirect effects on Cu partitioning. Iron metal/silicate partition coefficients depend on oxygen fugacity and on sulfur fugacity. Sulfidic dissolution of iron and oxide-sulfide exchange reactions with Cu cause a small increase in Cu metal/silicate partition coefficients. We derive an activity coefficient (gCuO0.5 )o f 106 1 for liquid CuO0.5 at 1300°C for the silicate melts used here. A comparison with literature data shows that log gCuO0.5 increases in proportion to the mass percentages [CaO 1(Al2O3)/2] in silicate melts. We recommend the following equations for Cu metal/silicate and sulfide/silicate partitioning for geochemical and cosmochemical modeling if silicate composition and the activity of Cu in the metal or sulfide is known: log D met/sil 52 0.48 2 0.25 z log fO2 2 log gCumetal 1 0.02 z [CaO 1 (Al2O3)/2; wt%]silicate log D sul/sil 51 0.76 2 0.25 z log fO2 1 0.25 log fS2 2 log gCS0.5,sulfide1 0.02 z [CaO 1 Al2O3/2; wt%]silicate. The derived Cu metal/silicate and metal/sulfide partition coefficients are applied to core formation in the Earth and Mars. The observed Cu abundances in the Earth cannot be easily explained by simple core-mantle equilibrium, but the observed Cu abundances for Mars are consistent with core-mantle equilibrium at low pressure and temperatures. Copyright © 2001 Elsevier Science Ltd

Revised thermochemical tables for phosphinidene (PH), phosphine (PH3), phosphorus nitride (PN), and magnesium orthophosphate (Mg3P2O8) are computed. These computations were done because the P reference state was not adjusted to the white P reference state and/or because the tables of these compounds are printed erroneously in the 4th edition of the NIST-JANAF Thermochemical Tables.

Recent thermodynamic modeling shows that some micas might be stable on Venussurface. However, prior studies considered only pure micas and did not consider mica solidsolutions, which are commonly observed on Earth. Here we use chemical equilibriumcalculations to evaluate the stability of mica solid solutions on Venus surface as a function ofatmospheric chemistry (H 2 O and HF abundances, and redox state), and surface elevation. Ourprior calculations show that the end-member micas eastonite (KMg 2 Al 3 Si 2 O 10 (OH) 2 ) andfluorphlogopite (KMg 3 AlSi 3 O 10 F 2 ) are stable on Venus surface, while the end-member micasphlogopite (KMg 3 AlSi 3 O 10 (OH) 2 ), annite (KFe 3 AlSi 3 O 10 (OH) 2 ), and siderophyllite ( KFe 2+ 2 Al 3 Si 2 O 10 ( OH ) 2 ) are unstable. Based on these results and known petrologic phase relationships, weconsider binary solutions of eastonite with either phlogopite or siderophyllite, andfluorphlogopite with phlogopite. We calculate that micas along all three binaries are stable onVenus. Micas containing ∼20 mole% eastonite and ∼80% phlogopite are stable in the lowertemperature highlands, and very eastonite-rich micas are stable over Venus entire surface.Fluorphlogopite-rich micas are also stable over Venus surface, while fluorphlogopite-poor micasare stable at higher elevations. Iron-poor micas along the eastonite-siderophyllite join, containing>80 mole% eastonite, are stable in both the highlands and lowlands. Finally, we use thethermodynamic calculations, terrestrial geology, and petrologic phase equilibria to discussplausible geological settings where micas may be present on Venus. These suggestions areimportant for the design of geochemical experiments on future lander and automated balloonmissions to Venus.

The kinetics and mechanisms of kamacite sulfurization were studied experimentally at tempera- tures and H2S/H2 ratios relevant to the solar nebula. Pieces of the Canyon Diablo meteorite were heated at 558 K, 613 K, and 643 K in 50 parts per million by volume (ppmv) H2S-H2 gas mixtures for up to one month. Optical microscopy and x-ray diffraction analyses show that the morphology and crystal orientation of the resulting sulfide layers vary with both time and temperature. Electron microprobe analyses reveal three distinct phases in the reaction products: monosulfide solid solution (mss), (Fe,Ni,Co)l,S, pentlandite (Fe,Ni,Co)g,Ss, and a P-rich phase. The bulk composition of the remnant metal was not significantly changed by sulfurization. Kamacite sulfurization at 558 K followed parabolic kinetics for the entire duration of the experiments. Sulfide layers that formed at 613 K grew linearly with time, while those that formed at 643 K initially grew linearly with time then switched to parabolic kinetics upon reaching a critical thickness. The experimental results suggest that a variety of thermodynamic, kinetic, and physical processes control the final composition and morphology of the sulfide layers. We combine morphological, x-ray diffraction, elec- tron microprobe, and kinetic data to produce a comprehensive model of sulfide formation in the solar nebula. Then, we present a set of criteria to assist in the identification of solar nebula condensate sulfides in primitive meteorites.

(Received 1997 September 8; accepted in revised form I998 March 3) Abstract-Literature data on major and trace elemental abundances and water contents of the shergottite, nakhlite, and chassigny (SNC) meteorites are compiled and evaluated. The individual members of the SNC group are relatively homogeneous, and representative average compositions for each meteorite can be com- puted from multiple data reported in the literature. Major element abundances are used to calculate norma- tive compositions and densities. The data survey shows that our knowledge of whole rock abundances in SNC meteorites is very limited for many elements and that more basic analytical work is needed. IN T R 0 DUCT I0 N The Mars Pathfinder mission and recent speculation about the existence of primitive life on Mars have intensified the interest in the shergottite, nakhlite and chassigny (SNC) meteorites, which are widely believed to originate from Mars. More than ten years have past since Treiman et al. (1986) summarized compositional data for SNC meteorites. Since then, four more SNC meteorites have been recognized and many more analytical data have become available in tainties) were computed for each element. Sometimes authors pub- lished the same data repeatedly, and these analyses were included only once in the computation of average values and standard devia- tions. For several elements, two bulk analyses are available for an individual meteorite. In these cases, the uncertainty reflects the range in values. In some cases (e.g., Na, K, In, or halogens), analyses reported in the older literature are apparently discordant and these data were excluded from the calculation of the mean concentrations. The bulk water abundances are derived from the published water the literature. This data survey provides an overview of available bulk elemental data and mineralogical compositions for the SNC meteorites. These data are useful for modeling parent body mantle composition and structure, core-mantle differentiation, radioactive heat sources, and thermal evolution on the Shergotty parent body

— The purpose of this survey is to establish reference bulk elemental abundances for the eucrites and thereby provide the basis to test core formation models as well as partial melting, fractional crystallization and magma ocean theories for the eucrite parent body. In order to evaluate bulk elemental abundances for the eucrites, 296 peer-reviewed articles, monographs, theses or books and 143 abstracts dating from 1938 to 1997 were surveyed. Of the 101 eucrites having at least one set of elemental abundance analyses reported in the literature, 20 were selected for in-depth examination. The selection criteria of our sample were based on the total number of analyses available for a given eucrite and the total number of elements for which data exist. The mean bulk elemental abundance, 1σ standard deviation, and the percent deviation were calculated for each element in a given eucrite. In order to evaluate the quality of the mean abundances, the elements were then grouped according to availability of data and percent deviations. Possible reasons for the different deviations in the different groups are briefly discussed. From the major element abundances, the normative (CIPW) composition, the molar compositions of pyroxene, olivine and plagioclase, and the bulk densities were calculated and compared to petrographic observations. The calculated norms for the noncumulates agree well with the observations while the norms for the cumulates do not. Possible reasons for this are discussed. Unfortunately, analyses of many elements are poorly represented in the literature and many bulk analyses suffer from unacceptable levels of uncertainty. Therefore, future work requires bulk elemental analyses for some of the more poorly characterized elements in eucrites, especially those of key elements used for planetary modeling.

bearing hydrous silicates are unstable because of sulfatization to anhydrite. Some Fe-free micas (e.g., eastonite, eastonite‐ water from Venus, via oxidation of the surface and hydrophlogopite micas), and some alkali amphiboles might be stable gen escape to space, has been important in influencing the on Venus’ surface, especially in the lower temperature high- oxidation state of the atmosphere and surface. At suffilands. We discuss hydrous mineral formation in the interior ciently high abundances, water vapor also participates in and on the surface of Venus. We review the literature on mica chemical weathering reactions leading to the formation of and amphibole thermal decomposition and find that dehydra- hydrous minerals on Venus’s surface. tion of phlogopitic micas and fibrous amphiboles produces Information about the presence (or absence) of hydrous (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of degrees minerals and OH-bearing nominally anhydrous minerals higher than the onset of dehydroxylation. These observations on Venus’s surface is important to understanding the presraise the possibility that anhydrides formed from hydrous sili- ent and past atmospheric‐lithospheric water cycle and wacates, which may have been present during a wetter period ter inventory on Venus, the petrology of convergent marin Venus’ history, may persist somewhere on Venus’ present gins of lithospheric blocks, the generation and physical surface. We discuss experiments that could be used on future properties of magmas, the atmospheric D/H ratio, the staspacecraft missions to detect hydroxyl in rocks and hydrous bility of Venus’s current climate and its evolution, the rate silicates on Venus. Finally, we review estimates of the amount of water outgassing from volcanism, the supply of water of water and OH (hydroxyl) in the Earth’s mantle. Based on this review, we suggest that even if no hydrous silicates are from cometary and asteroidal impacts, and the physical stable on Venus, significant amounts of water are plausibly properties (e.g., visible and infrared (IR) reflectance, elecpresent in surface rocks as OH in nominally anhydrous trical and thermal conductivity, dielectric properties, meminerals. © 1997 Academic Press chanical strength) of Venus’s lithosphere, surface rocks

We report results from an experimental study of sulfide chemistry during metamorphism on ordinary chondrite parent bodies. Artificial LL-chondrite material, composed of silicate, iron metal, and sulfide grains, was placed in sealed, evacuated silica tubes and heated to either 500°C or 900°C. These temperatures are representative peak metamorphic temperatures experienced by type 3 and type 6 ordinary chondrites, respectively. Rapid sulfur mobilization occurs during heating and results in the formation of sulfide rims around the metal grains and sulfur loss from the original sulfide crystals. The newly formed sulfide rims have two distinct layers that incorporate nearby silicate grains. We also observe narrow sulfide trails that follow silicate grain boundaries and connect separate sulfide-rimmed metal grains. The morphologies of the sulfide rims suggest that vapor transport is the main mechanism for sulfur mobilization. Sulfur loss is observed along cracks and crystal boundaries of the initial sulfide grains. After extensive reaction, patches of iron metal appear at the outer edges of the sulfide crystals and large pore spaces form throughout the original sulfides. The experimental metal-sulfide assemblages resemble those found in low metamorphic grade ordinary chondrites. This suggests metamorphism is mainly responsible for the observed metal-sulfide textures in these chondrites.

Equilibrium condensation calculations successfully explain the complementary trace element abundance patterns observed in carbon star atmospheres and circumstellar SiC grains found in meteorites. Fractional trace element condensation into SiC depletes the gas in refractory trace elements, while more volatile elements remain in the gas. The observed complementary patterns imply that dust forms relatively close to the star, possibly during the minimum light phase in stellar variability cycles. Once the gas falls back onto the star during stellar contraction, photospheric abundances become relatively enriched in more volatile elements. The complementary trace element abundances link circumstellar SiC grains from meteorites to carbon star atmospheres.

We derive the bulk chemical composition, physical properties, and trace element abundances of Mars from two assumptions: (1) Mars is the parent body for the Shergottite–Nakhlite–Chassignite (SNC) meteorites, and (2) the oxygen isotopic composition of Mars was determined by the oxygen isotopic compositions of the different types of nebular material that accreted to form Mars. We use oxygen isotopes to constrain planetary bulk compositions because oxygen is generally the most abundant element in rock, and is either the first or second (after iron) most abundant element in any terrestrial planet, the Moon, other rocky satellites, and the asteroids. The oxygen isotopic composition of Mars, calculated from oxygen isotopic analyses of the SNC meteorites, corresponds to the accretion of about 85% H-, 11% CV-, and 4% CI-chondritic material. (Unless noted otherwise, mass percentages are used in this paper.) The bulk composition of Mars follows from mass balance calculations using mean compositions for these chondrite groups. We predict that silicates (mantle + crust) comprise about 80% of Mars. The composition of the silicate fraction represents the composition of the primordial martian mantle prior to crustal formation. The FeO content of the mantle is 17.2%. A metal–sulfide core, containing about 10.6% S, makes up the remaining 20% of the planet. Our bulk composition is similar to those from other models. We calculate the abundances of siderophile (“metal-loving”) and chalcophile (“sulfide-loving”) elements in the martian mantle from the bulk composition using (metal–sulfide)/silicate partition coefficients. Our results generally agree with predictions of the SNC meteorite model of Wanke and Dreibus for the composition of Mars. However, we predict higher abundances for the alkalis and halogens than those derived from SNC meteorite models for Mars. The apparent discrepancy indicates that the alkalis and halogens were lost from the martian mantle by hydrothermal leaching and/or vaporization during accretion. Geochemical arguments suggest that vaporization was only a minor loss process for these elements. On the other hand, aqueous transport of the alkalis and halogens to the surface is supported by the terrestrial geochemistry of these elements and the high K, Rb, Cl, and Br abundances found by the Viking XRF and Phobos gamma ray experiments on the surface of Mars.

The chemistry of Be and B in the solar nebula is reinvestigated using thermodynamic equilibrium calculations. The dominant Be gases are monatomic Be at high temperatures and the hydroxides BeOH and Be(OH) 2 at lower temperatures. Beryllium condenses as gugiaite (Ca 2 BeSi 2 O 7 ) in solid solution with melilite with a 50% condensation temperature of 1490 K. If an ideal solid solution of chrysoberyl (BeAl 2 O 4 ) into spinel is assumed, most of the Be condenses into spinel, yielding a 50% condensation temperature of 1501 K. However, the difference in the crystal structures of spinel and chrysoberyl indicates that their solid solution may be non-ideal. At high temperatures the dominant B gases are BO, HBO, and HBO 2 , while NaBO 2 , KBO 2 , and LiBO 2 are dominant at lower temperatures. Boron is less refractory than Be and is calculated to condense into solid solution with feldspar. The majority of B condenses as danburite (CaB 2 Si 2 O 8 ) in solid solution with anorthite. At lower temperatures, when the feldspar composition is more albitic, the remaining B condenses as reedmergnerite (NaBSi 3 O 8 ). The 50% condensation temperature of B is 964 K. The 50% condensation temperature of B is similar to that of Na and much higher than that of S. Therefore, normalized B abundances in chondrites are expected to correlate with Na abundances. Be is predicted to be concentrated in melilite, a conclusion which is consistent with the few measurements of Be concentrations in calcium aluminum-rich inclusions (CAIs). Boron is predicted to be concentrated in feldspar, but no analytical data are available to test this prediction.

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3E9 km) extracted from the Murchison meteorite. The spherules have 12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission elec- tron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite struc- ture consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3E4 nm in diameter, with little graphitic layering order. These sheets prob- ably condensed as isolated particles that subsequently coalesced to form the cores, after which the sur- rounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5E200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these car- bides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evi- dently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermody- namics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures dynes cm~2), and/or condensable carbon number densities cm~3) are required. (Z0.1 (Z108 The graphite spherules with 12C/13C ratios less than the solar value may have originated in AGB stellar winds. However, in the spherically symmetric AGB atmospheres customarily assumed in models of stellar grain formation, pressures are much too low (by factors of to produce carbide crystals or Z102) graphite spherules of the sizes observed within plausible timescales. If some of the graphite spherules formed in the winds from such stars, it thus appears necessary to assume that the regions of grain forma- tion are density concentrations with length scales less than a stellar radius. Some of the spherules with both 12C/13C ratios greater than the solar value and 28Si excesses probably grew in the ejecta of super- novae. The isotopic compositions and growth constraints imply that they must have formed at high den- sities (e.g., with g cm~3) from mixtures of inner-shell material with material from the C-rich

— Partition coefficients of the rare-earth-elements (REE) between sulfides (FeS or CaS) and silicate melt were determined experimentally at 1200–1300 °C. The REE sulfide/silicate partition coefficients (D) are ≤1 under the experimental O and S fugacities, which demonstrates that the REE are mainly located in the silicate phase. Rare-earth-element partition coefficients in the FeS/silicate system decrease from light to heavy REE, while the opposite behavior is found for the CaS/silicate system, where partition coefficients increase from light to heavy REE. In both sulfide systems, Eu is preferentially incorporated into the sulfide phases, as also expected from thermodynamic calculations. The Eu sulfide/silicate partition coefficient is about a factor of ten higher than that of neighboring Sm and Gd, in accordance with thermodynamic predictions of REE sulfide/silicate partition coefficients. The low sulfide/silicate partition coefficients indicate that CaS (oldhamite) in enstatite achondrites (aubrites) cannot have gained its high REE concentrations during igneous differentiation processes. The high REE concentrations and the REE patterns in aubritic oldhamite are more plausibly explained by REE condensation into refractory CaS. The refractory nature of CaS prevented major exchange reactions of the oldhamite with other aubritic minerals during the short differentiation and metamorphism period on the aubrite parent body. Thus, oldhamite in aubrites may be relict condensates altered to different degrees during short heating events, as originally suggested by Lodders and Palme (1990).