Predicted Abundances of Carbon Compounds in Volcanic Gases on Io

We use chemical equilibrium calculations to model the speciation of carbon in volcanic gases on Io. The calculations cover wide temperature (500-2000 K), pressure (10^-8 to 10^+2 bars), and composition ranges (bulk O/S atomic ratios \~0 to 3), which overlap the nominal conditions at Pele (1760 K, 0.01 bar, O/S ~ 1.5). Bulk C/S atomic ratios ranging from 10^-6 to 10^-1 in volcanic gases are used with a nominal value of 10^-3 based upon upper limits from Voyager for carbon in the Loki plume on Io. Carbon monoxide and CO2 are the two major carbon gases under all conditions studied. Carbonyl sulfide and CS2 are orders of magnitude less abundant. Consideration of different loss processes (photolysis, condensation, kinetic reactions in the plume) indicates that photolysis is probably the major loss process for all gases. Both CO and CO2 should be observable in volcanic plumes and in Io's atmosphere at abundances of several hundred parts per million by volume for a bulk C/S ratio of 10^-3.Comment: 21 pages, 4 figures, 4 tables; accepted by Astrophysical Journa

A comparative study of experimental and meteoritic metal-sulfide assemblages

Sulfide formation via a gas-solid reaction between iron-nickel metal and H_2/H_2S gas mixtures was studied experimentally. This reaction produces distinctive chemical fractionations in both metal and sulfide that can help identify pristine nebular sulfide condensates in meteorites. The resulting sulfide morphology consists of a troilite scale divided into two distinct layers : an inner layer containing small, randomly oriented crystals and an outer layer consisting of large, columnar crystals. A thin band of metal surrounding the unreacted metal core and small metal blebs located in the inner sulfide layer are significantly enriched in nickel relative to the starting metal composition. The stoichiometry of the sulfide is nearly ideal {(Fe+Ni+Co)/S=1} at the metal-sulfide interface but the sulfur content increases with distance from the metal. A significant amount of nickel is present in the sulfide layer and increases in concentration across the sulfide layer. The nickel concentration gradient results from diffusion of nickel ions in the sulfide being faster than that of iron ions. Microprobe analyses on metal-sulfide assemblages in the LL3 unequilibrated ordinary chondrite Allan Hills-764 (ALH-764) do not show these chemical fractionations. Instead, textural and chemical evidence suggests that these meteoritic sulfides were altered during a post accretion heating event

Introducing the Author-ity Exporter, and a case study of geo-temporal movement of authors

We introduce a web service, Author-ity Exporter, that permits searching and exporting data from Author-ity -- a database that has PubMed author names disambiguated with a high degree of accuracy [1]. Each author is represented by a cluster of papers annotated by publication count, time-span, affiliations, topics, journals, co-authors, citations as well as imputed data from MapAffil [2], Genni [3], and Ethnea [4] and links to their NIH/NSF grants and USPTO patents; and we have plans for more. This service should enable and simplify new types of author-centered bibliometric analyses with a unique strength in funding, geography, and diversity (gender, ethnicity, and professional age). We also present an illustrative case study of modeling of authors’ career movements to and from a specific city based on data retrieved from Author-ity Exporter. The service (and the R code used in the case study) are available at http://abel.ischool.illinois.edu/cgi-bin/exporter/search.pl.NIH P01AG039347Ope

Application of an Equilibrium Vaporization Model to the Ablation of Chondritic and Achondritic Meteoroids

We modeled equilibrium vaporization of chondritic and achondritic materials using the MAGMA code. We calculated both instantaneous and integrated element abundances of Na, Mg, Ca, Al, Fe, Si, Ti, and K in chondritic and achondritic meteors. Our results are qualitatively consistent with observations of meteor spectra.Comment: 8 pages, 4 figures; in press, Earth, Moon, and Planets, Meteoroids 2004 conference proceeding

The kinetics and mechanism of iron sulfide formation in the solar nebula

We summarize an experimental study of the kinetics and mechanism of FeS formation by the reaction of H_2S-H_2 gas mixtures with iron metal. Characterization of the reacted samples by optical microscopy, X-ray diffraction, electron microprobe analyses, and gravimetric analyses provided detailed information on the Fe/S ratio, microstructure and morphology, and formation kinetics of the iron sulfide layers. The Fe/S ratios of the iron sulfide layers varied from Fe_S to FeS with temperature and gas composition, in agreement with models of gas-solid equilibrium. The morphology, microstructure, and growth orientation of the sulfide layers also varied with temperature and gas composition. Typically, sulfide layer growth proceeded by the development of a compact, uniformly oriented scale which later cracked when it could no longer plastically deform. Further reaction led to the growth of a finer grained, randomly oriented, highly porous inner layer between the metal and original sulfide scale. Initially sulfide layers grew linearly with time with the kinetics controlled by chemical reactions at the gas-solid interface. However, upon reaching a critical thickness, diffusion through the sulfide scale became the rate limiting step and layer growth followed parabolic kinetics. The linear and parabolic rate constants for iron sulfide growth were determined and then used to constrain FeS formation in the solar nebula. FeS formation is rapid compared to estimated nebular lifetimes of 1-10 million years. Our results also imply that the variations in the sulfur content of chondritic material are due to removal of metal grains from contact with the gas (e. g., by accretion into larger bodies) at temperatures above 400K, where complete sulfur condensation occurs, rather than by kinetic inhibition of gas-solid equilibrium between H_2S gas and iron metal grains

Constraints on stellar grain formation from presolar graphite in the murchison meteorite

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3-9 μm) 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 electron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite structure 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 3-4 nm in diameter, with little graphitic layering order. These sheets probably condensed as isolated particles that subsequently coalesced to form the cores, after which the surrounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5-200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these carbides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evidently 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 thermodynamics 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 (≳ 0.1 dynes cm-2), and/or condensable carbon number densities (≳108 cm-3) are required. 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 ≳102) to produce carbide crystals or 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 formation are density concentrations with length scales less than a stellar radius. Some of the spherules with both12C/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 densities (e.g., with p≳10-12 g cm-3) from mixtures of inner-shell material with material from the C-rich outer zones

Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres

We present a comprehensive description of the theory and practice of opacity calculations from the infrared to the ultraviolet needed to generate models of the atmospheres of brown dwarfs and extrasolar giant planets. Methods for using existing line lists and spectroscopic databases in disparate formats are presented and plots of the resulting absorptive opacities versus wavelength for the most important molecules and atoms at representative temperature/pressure points are provided. Electronic, ro-vibrational, bound-free, bound-bound, free-free, and collision-induced transitions and monochromatic opacities are derived, discussed, and analyzed. The species addressed include the alkali metals, iron, heavy metal oxides, metal hydrides, $H_2$, $H_2O$, $CH_4$, $CO$, $NH_3$, $H_2S$, $PH_3$, and representative grains. [Abridged]Comment: 28 pages of text, plus 22 figures, accepted to the Astrophysical Journal Supplement Series, replaced with more compact emulateapj versio

Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs

M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric CO is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectroscopic detection of CO in Gliese 229B and existing M band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analysis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47, log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range 45-69 M_Jup.Comment: 36 pages incl. 12 figures and 3 tables, accepted by Ap

Injection of meteoric phosphorus into planetary atmospheres

This study explores the delivery of phosphorus to the upper atmospheres of Earth, Mars, and Venus via the ablation of cosmic dust particles. Micron-size meteoritic particles were flash heated to temperatures as high as 2900 ​K in a Meteor Ablation Simulator (MASI), and the ablation of PO and Ca recorded simultaneously by laser induced fluorescence. Apatite grains were also ablated as a reference. The speciation of P in anhydrous chondritic porous Interplanetary Dust Particles was made by K-edge X-ray absorption near edge structure (XANES) spectroscopy, demonstrating that P mainly occurs in phosphate-like domains. A thermodynamic model of P in a silicate melt was then developed for inclusion in the Leeds Chemical Ablation Model (CABMOD). A Regular Solution model used to describe the distribution of P between molten stainless steel and a multicomponent slag is shown to provide the most accurate solution for a chondritic-composition, and reproduces satisfactorily the PO ablation profiles observed in the MASI. Meteoritic P is moderately volatile and ablates before refractory metals such as Ca; its ablation efficiency in the upper atmosphere is similar to Ni and Fe. The speciation of evaporated P depends significantly on the oxygen fugacity, and P should mainly be injected into planetary upper atmospheres as PO2, which will then likely undergo dissociation to PO (and possibly P) through hyperthermal collisions with air molecules. The global P ablation rates are estimated to be 0.017 ​t ​d−1 (tonnes per Earth day), 1.15 ​× ​10−3 ​t ​d−1 and 0.024 ​t ​d−1 for Earth, Mars and Venus, respectively