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Bruce Fegley Jr

Publications and source records attributed to Bruce Fegley Jr.

15 recordsLinked to original sources
arXiv2023

Solar System Abundances and Condensation Temperatures of the Halogens Fluorine, Chlorine, Bromine, and Iodine

We review a large body of literature for concentrations of halogens in chondrites and stellar halogen data used for solar system abundances (i.e., representative abundances of the solar system at the time of its formation) and associated analytical problems. Claims of lower solar system chlorine, bromine and iodine abundances from recent analyses of CI-chondrites are untenable because of incompatibility of such low values with nuclear abundance systematics and measurements of halogens in the sun and other stars. We suspect analytical problems associated with these peculiar rock types caused lower analytical results in several studies. Mass concentrations in CI-chondrites are F=92+-20 ppm, Cl=717+-110 ppm, Br=3.77+-0.90 ppm, and I=0.77+-0.31 ppm, and abundances normalized to N(Si) =10^6 atoms are N(F)=1270+-270, N(Cl)=5290+-810, N(Br)=12.3+-2.9, and N(I)=1.59+-0.64. Meteoritic values scaled to present-day photospheric abundances with log N(H)=12 are A(F)=4.61+-0.09, A(Cl)=5.23+-0.06, A(Br)=2.60+-0.09, and A(I)=1.71+-0.15. These recommended present-day solar system abundances compare to the sunspot values of N(F)=776+-260, A(F)=4.40+-0.25, and N(Cl)=5500+-810, A(Cl)=5.25+-0.12 and are consistent with F and Cl abundance ratios in other stars and other astronomical environments. The chlorine abundance of 776+-21 ppm by Yokoyama et al. (2022) for the CI-chondrite-like asteroid Ryugu is consistent with the chlorine abundance evaluated for CI-chondrites here. Updated equilibrium 50% condensation temperatures from our previous work (Lodders 2003, Fegley & Schaefer 2010, Fegley & Lodders 2018) considering solid-solution and kinetic inhibition effects are 713K (F), 427K (Cl), 392K (Br) and 312K (I) at 10^-4 bar total pressure. Condensation temperatures computed with lower halogen abundances do not represent the correct condensation temperatures from a solar composition gas. (abridged)

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arXiv2019

Thermodynamics of Element Volatility and its Application to Planetary Processes

Despite its importance in geological sciences, our understanding of interactions between gas and condensed phases (comprising solids and liquids) remains clouded by the fact that, often, only indirect evidence remains for their occurrence. This arises from the tendency for the vapour phase to escape from the condensed phase with which it interacts, owing to its much lower density and thus greater volume. For a gas that is sufficiently tenuous that interactions do not occur between its constituent molecules, this relationship is quantified in the ideal gas law (Clapeyron 1834): $PV=nRT$ (1) where $P$ is the total pressure exerted by the gas, $V$ its volume, $n$ is the number of moles, $R$ the gas constant 8.3145 $Jmol^{-1}K^{-1}$, Horstmann, 1873) and $T$ the absolute temperature. One mole of an ideal gas at 273.15 $K$ and $10^5$ $Pa$ (standard temperature and pressure for gases) has a molar volume of 22,711 $cm^3/mol$, $10^3$ x greater than typical silicate liquids or minerals. As a result, vaporisation processes in nature are often informed by chemical and textural evidence remaining in the condensed phase.

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arXiv2016

Solubility of Rock in Steam Atmospheres of Planets

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.

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arXiv2012

Chemistry of the Earth's Earliest Atmosphere

In this chapter we describe chemistry of the early atmosphere of the Earth during and shortly after its formation where there is little if any geological record. We review the arguments for a secondary origin of the terrestrial atmosphere, that is by outgassing during and/or after accretion rather than by capture of solar nebula gas. Then we discuss sources of volatiles accreted by the Earth using meteorites as analogs for the material present in the solar nebula. The next section reviews heating during accretion of the Earth. Subsequently we describe chemistry of the silicate vapor, steam, and gaseous stages of atmospheric evolution on the early Earth. We close with a summary of the key questions that remain unresolved.

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arXiv2012

Vaporization of the Earth: Application to Exoplanet Atmospheres

Currently, there are about 3 dozen known super-Earth (M < 10 MEarth), of which 8 are transiting planets suitable for atmospheric follow-up observations. Some of the planets are exposed to extreme temperatures as they orbit close to their host stars, e.g., CoRot-7b, and all of these planets have equilibrium temperatures significantly hotter than the Earth. Such planets can develop atmospheres through (partial) vaporization of their crustal and/or mantle silicates. We investigated the chemical equilibrium composition of such heated systems from 500 - 4000 K and total pressures from 10-6 to 10+2 bars. The major gases are H2O and CO2 over broad temperature and pressure ranges, and Na, K, O2, SiO, and O at high temperatures and low pressures. We discuss the differences in atmospheric composition arising from vaporization of SiO2-rich (i.e., felsic) silicates (like Earth's continental crust) and MgO-, FeO-rich (i.e., mafic) silicates like the bulk silicate Earth. The computational results will be useful in planning spectroscopic studies of the atmospheres of Earth-like exoplanets.

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arXiv2010

Atmospheric Chemistry of Venus-like Exoplanets

We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibira between the atmosphere and lithosphere, as on Venus. The results of our calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the surface. These results will be useful in planning future observations of the atmospheres of terrestrial-sized exoplanets by current and proposed space observatories such as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope (JWST), Terrestrial Planet Finder, and Darwin.

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arXiv2010

Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars III. Iron, Magnesium, and Silicon

We use thermochemical equilibrium calculations to model iron, magnesium, and silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-, and Si-bearing gases and condensates is determined as a function of temperature, pressure, and metallicity. Our results are thus independent of any particular model atmosphere. The condensation of Fe metal strongly affects iron chemistry by efficiently removing Fe-bearing species from the gas phase. Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Mg- and Si-bearing gases are effectively removed from the atmosphere by forsterite (Mg2SiO4) and enstatite (MgSiO3) cloud formation. Monatomic Mg is the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO) is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and EGPs, whereas SiH4 is dominant in the deep atmosphere of Jupiter and other gas giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly important with decreasing effective temperature. In principle, a number of Fe, Mg, and Si gases are potential tracers of weather or diagnostic of temperature in substellar atmospheres.

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arXiv2009

Chemistry of Silicate Atmospheres of Evaporating Super-Earths

We model the formation of silicate atmospheres on hot volatile-free super-Earths. Our calculations assume that all volatile elements such as H, C, N, S, and Cl have been lost from the planet. We find that the atmospheres are composed primarily of Na, O2, O, and SiO gas, in order of decreasing abundance. The atmospheric composition may be altered by fractional vaporization, cloud condensation, photoionization, and reaction with any residual volatile elements remaining in the atmosphere. Cloud condensation reduces the abundance of all elements in the atmosphere except Na and K. We speculate that large Na and K clouds such as those observed around Mercury and Io may surround hot super-Earths. These clouds would occult much larger fractions of the parent star than a closely bound atmosphere, and may be observable through currently available methods.

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arXiv2008

Volatile Element Chemistry during Metamorphism of Ordinary Chondritic Material and Some of its Implications for the Composition of Asteroids

We used chemical equilibrium calculations to model thermal metamorphism of ordinary chondritic material as a function of temperature, pressure, and trace element abundance and use our results to discuss volatile mobilization during thermal metamorphism of ordinary chondrite parent bodies. The calculations include ~1,700 solids and gases of 40 elements. We compiled trace element abundances in H-, L-, and LL-chondrites for the elements Ag, As, Au, Bi, Cd, Cs, Cu, Ga, Ge, In, Pb, Rb, Sb, Se, Sn, Te, Tl, and Zn, and identified abundance trends as a function of petrographic type within each class. We found that abundance patterns within the H- and L- chondrites are consistent with mobilization of volatile elements in an onionshell-type parent body. LL-chondrites have more complex abundance patterns that may support a rubble-pile model for the LL-chondrite parent body. We calculated volatility sequences for the trace elements in the ordinary chondritic material, which differs significantly from the solar nebula volatility sequence.

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arXiv2006

Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars II. Sulfur and Phosphorus

Thermochemical equilibrium and kinetic calculations are used to model sulfur and phosphorus chemistry in giant planets, brown dwarfs, and extrasolar giant planets (EGPs). The chemical behavior of individual S- and P-bearing gases and condensates is determined as a function of pressure, temperature, and metallicity. The results are independent of particular model atmospheres and, in principle, the equilibrium composition along the pressure-temperature profile of any object can be determined. Hydrogen sulfide (H2S) is the dominant S-bearing gas throughout substellar atmospheres and approximately represents the atmospheric sulfur inventory. Silicon sulfide (SiS) is a potential tracer of weather in substellar atmospheres. Disequilibrium abundances of phosphine (PH3) approximately representative of the total atmospheric phosphorus inventory are expected to be mixed upward into the observable atmospheres of giant planets and T dwarfs. In hotter objects, several P-bearing gases (e.g., P2, PH3, PH2, PH, HCP) become increasingly important at high temperatures.

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arXiv2005

Silicon Tetrafluoride on Io

Silicon tetrafluoride (SiF4) is observed in terrestrial volcanic gases and is predicted to be the major F - bearing species in low temperature volcanic gases on Io (Schaefer and Fegley, 2005b). SiF4 gas is also a potential indicator of silica-rich crust on Io. We used F/S ratios in terrestrial and extraterrestrial basalts, and gas/lava enrichment factors for F and S measured at terrestrial volcanoes to calculate equilibrium SiF4/SO2 ratios in volcanic gases on Io. We conclude that SiF4 can be produced at levels comparable to the observed NaCl/SO2 gas ratio. We also considered potential loss processes for SiF4 in volcanic plumes and in Io's atmosphere including ion-molecule reactions, electron chemistry, photochemistry, reactions with the major atmospheric constituents, and condensation. Photochemical destruction (tchem ~ 266 days) and/or condensation as Na2SiF6 (s) appear to be the major sinks for SiF4. We recommend searching for SiF4 with infrared spectroscopy using its 9.7 micron band as done on Earth.

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arXiv2005

Chemical Constraints on the Water and Total Oxygen Abundances in the Deep Atmosphere of Saturn

Thermochemical equilibrium and kinetic calculations for the trace gases CO, PH3, and SiH4 give three independent constraints on the water and total oxygen abundances of Saturn's deep atmosphere. A lower limit to the water abundance of H2O/H2 > 1.7 x 10^-3 is given by CO chemistry while an upper limit of H2O/H2 < 5.5 x 10^-3 is given by PH3 chemistry. A combination of the CO and PH3 constraints indicates a water enrichment on Saturn of 1.9 to 6.1 times the solar system abundance (H2O/H2 = 8.96 x 10^-4). The total oxygen abundance must be at least 1.7 times the solar system abundance (O/H2 = 1.16 x 10^-3) in order for the SiH4 to remain below a detection limit of SiH4/H2 < 2 x 10^-10. A combination of the CO, PH3, and SiH4 constraints suggests that the total oxygen abundance on Saturn is 3.2 to 6.4 times the solar system abundance. Our results indicate that oxygen on Saturn is less enriched than other heavy elements (such as C and P) relative to a solar system composition. This work was supported by NASA NAG5-11958.

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arXiv2004

Alkali and Halogen Chemistry in Volcanic Gases on Io

We use chemical equilibrium calculations to model the speciation of alkalis and halogens in volcanic gases emitted on Io. The calculations cover wide temperature (500-2000 K) and pressure (10^-6 to 10^+1 bars) ranges, which overlap the nominal conditions at Pele (T = 1760 K, P = 0.01 bars). About 230 compounds of 11 elements (O, S, Li, Na, K, Rb, Cs, F, Cl, Br, I) are considered. We predict the major alkali and halogen species in a Pele-like volcanic gas and the major alklai and halogen condensates. We also model disequilibrium chemistry of the alkalis and halogens in the volcanic plume. Based on this work and our prior modeling for Na, K, and Cl in a volcanic plume, we predict the major loss processes for the alkali halide gases are photolysis and/or condensation onto grains. On the basis of elemental abundances and photochemical lifetimes, we recommend searching for gaseous KCl, NaF, LiF, LiCl, RbF, RbCl, CsF, and CsCl around volcanic vents during eruptions. Based on abundance considerations and observations of brown dwarfs, we also recommend a search of Io's extended atmosphere and the Io plasma torus for neutral and ionized Li, Cs, Rb, and F.

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arXiv2004

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.

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