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It is now understood that the accretion of terrestrial planets naturally involves giant collisions, the moon-forming impact being a well-known example. In the aftermath of such collisions, the surface of the surviving planet is very hot and potentially detectable. Here we explore the atmospheric chemistry, photochemistry, and spectral signatures of post-giant-impact terrestrial planets enveloped by thick atmospheres consisting predominantly of CO2 and H2O. The atmospheric chemistry and structure are computed self-consistently for atmospheres in equilibrium with hot surfaces with composition reflecting either the bulk silicate Earth (which includes the crust, mantle, atmosphere, and oceans) or Earth's continental crust. We account for all major molecular and atomic opacity sources including collision-induced absorption. We find that these atmospheres are dominated by H2O and CO2, while the formation of CH4 and NH3 is quenched because of short dynamical timescales. Other important constituents are HF, HCl, NaCl, and SO2. These are apparent in the emerging spectra and can be indicative that an impact has occurred. The use of comprehensive opacities results in spectra that are a factor of two lower brightness temperature in the spectral windows than predicted by previous models. The estimated luminosities show that the hottest post-giant-impact planets will be detectable with near-infrared coronagraphs on the planned 30 m class telescopes. The 1-4 μm will be most favorable for such detections, offering bright features and better contrast between the planet and a potential debris disk. We derive cooling timescales on the order of 105-6 yr on the basis of the modeled effective temperatures. This leads to the possibility of discovering tens of such planets in future surveys. © 2014. The American Astronomical Society. All rights reserved..

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.09.002 Byline: Laura Schaefer (a)(c), Bruce Fegley (a)(b)(c) Keywords: Atmospheres; evolution; Atmospheres; chemistry; Asteroids; Meteorites; Terrestrial planets; Satellites; atmospheres Abstract: We used chemical equilibrium calculations to model thermal outgassing of ordinary chondritic material as a function of temperature, pressure, and bulk composition and use our results to discuss outgassing on asteroids and the early Earth. The calculations include [approximately equal to]1000 solids and gases of the elements Al, C, Ca, Cl, Co, Cr, F, Fe, H, K, Mg, Mn, N, Na, Ni, O, P, S, Si, and Ti. The major outgassed volatiles from ordinary chondritic material are CH.sub.4, H.sub.2, H.sub.2O, N.sub.2, and NH.sub.3 (the latter at conditions where hydrous minerals form). Contrary to widely held assumptions, CO is never the major C-bearing gas during ordinary chondrite metamorphism. The calculated oxygen fugacity (partial pressure) of ordinary chondritic material is close to that of the quartz-fayalite-iron (QFI) buffer. Our results are insensitive to variable total pressure, variable volatile element abundances, and kinetic inhibition of C and N dissolution in Fe metal. Our results predict that Earth's early atmosphere contained CH.sub.4, H.sub.2, H.sub.2O, N.sub.2, and NH.sub.3; similar to that used in Miller -- Urey synthesis of organic compounds. Author Affiliation: (a) Planetary Chemistry Laboratory, Washington University, One Brookings Drive, Campus Box 1169, St. Louis, MO 63130-4899, USA (b) McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, Campus Box 1169, St. Louis, MO 63130-4899, USA (c) Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, Campus Box 1169, St. Louis, MO 63130-4899, USA Article History: Received 27 June 2006; Revised 31 August 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.05.020 Byline: Laura Schaefer, Bruce Fegley Keywords: Io; Volcanic gases; Silicon; Fluorine; Silicon tetrafluoride; Condensates; Geochemistry; Atmospheric chemistry Abstract: Silicon tetrafluoride (SiF.sub.4) is observed in terrestrial volcanic gases and is predicted to be the major F-bearing species in low-temperature volcanic gases on Io [Schaefer, L., Fegley Jr., B., 2005b. Alkali and halogen chemistry in volcanic gases on Io. Icarus 173, 454-468]. SiF.sub.4 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 SiF.sub.4/SO.sub.2 ratios in volcanic gases on Io. We conclude that SiF.sub.4 can be produced at levels comparable to the observed NaCl/SO.sub.2 gas ratio. We also considered potential loss processes for SiF.sub.4 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 (t.sub.chem [approximately equal to]266 days) and/or condensation as Na.sub.2SiF.sub.6 (s) appear to be the major sinks for SiF.sub.4. We recommend searching for SiF.sub.4 with infrared spectroscopy using its 9.7 [mu]m band as done on Earth. Author Affiliation: Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences, Campus Box 1169, Washington University, One Brookings Dr., St. Louis, MO 63130-4899, USA Article History: Received 7 February 2005; Revised 17 May 2005

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.sup.-6] to [10.sup.+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. The elemental abundances for O, S, Na, K, and Cl are based upon observations. CI chondritic elemental abundances relative to sulfur are used for the other alkalis and halogens (as yet unobserved on Io). We predict the major alkali species in Pele-like volcanic gases and the percentage distribution of each alkali are LiCI (73%), LiF (27%); NaCl (81%), Na (16%), NaF (3%); KCl (91%), K (5%), KF (4%); RbCl (93%), Rb (4%), RbF (3%); CsCl (92%), CsF (6%), Cs (2%). Likewise the major halogen species and the percentage distribution of each halogen are NaF (88%), KF (10%), LiF (2%); NaCl (89%), KCl (11%); NaBr (89%), KBr (10%), Br (1%); NaI (61%), I (30%), KI (9%). We predict the major halogen condensates and their condensation temperatures at P = 0.01 bar are NaF (1115 K), LiF (970 K); NaCl (1050 K), KCl (950 K); KBr (750 K), RbBr (730 K), CsBr (645 K); and solid 12 (200 K). 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. Their estimated photochemical lifetimes range from a few minutes for alkali iodides to a few hours for alkali fluorides. Condensation is apparently the only loss process for elemental iodine. On the basis of elemental abundances and photochemical lifetimes, we recommend searching for gaseous KCl, NaF, LiE 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. Keywords: Io; Volcanic gases; Volcanic condensates; Volcanism; Alkalis; Sodium; Potassium; Lithium; Rubidium; Cesium; Fluorine; Chlorine; Bromine; Iodine; Sulfur

We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the O-Na-K-Fe-Si-Mg Ca-Al-Ti system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10 [+ or -] 3 reported by Brown (2001, Icarus 151, 190-195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including Mg[Cl.sub.2], MgS, MgCl, Fe[Cl.sub.2], FeS, FeCl, and SiS. Keywords: Io; Volcanism; Geochemistry; Lava; Magma: Vaporization; Vapor pressure; High temperature; Alkalis: Sodium; Potassium; Silicon; Iron; Magnesium; Calcium; Aluminum; Titanium; Silicon monoxide; Basalt; Komatiite: Dunite: Tholeiite; CAI

Chemical equilibrium calculations of volatile metal geochemistry on Venus show that high dielectric constant compounds of lead and bismuth such as PbS (galena), B[i.sub.2][S.sub.3] (bismuthite) or Pb-Bi sulfosalts condense in the venusian highlands and may be responsible for the low radar emissivities observed by Magellan and Pioneer Venus. Our calculations also show that elemental tellurium is unstable on Venus' surface and will not condense below 46.6 km. This is over 30 km higher than Maxwell Montes, the highest point on Venus' surface. Elemental analyses of Venus' highlands surface by laser induced breakdown spectroscopy (LIBS) and/or X-ray fluorescence (XRF) can verify the identity of the heavy metal frost on Venus. The Pb-Pb age of Venus could be determined by mass spectrometric measurements of the p[b.sup.207]/P[b.sup.204] and P[b.sup.206[/P[b.sup.204] isotopic ratios in Pb-bearing frosts. All of these measurements are technologically feasible now. Keywords: Venus: Geochemistry; Trace elements; Te; Pb; Bi; Dielectric constant

There is a general belief that hydrous minerals cannot exist on Venus under current surface conditions. This view was challenged when Johnson and Fegley (2000, Icarus 146, 301-306) showed that tremolite ([Ca.sub.2][Mg.sub.5][Si.sub.8][0.sub.22][(OH).sub.2]), a hydrous mineral, is stable against thermal decomposition at current Venus surface temperatures, e.g., 50% decomposition in 4 Ga at 740 K. To further explore hydrous mineral thermal stability on Venus, we experimentally determined the thermal decomposition kinetics of fluorine-bearing tremolite. Fluor-tremolite is thermodynamically more stable than OH-tremolite and should decompose more slowly. However how much slower was unknown. We measured the decomposition rate of fluorine-bearing tremolite and show that its decomposition is several times to greater than ten times slower than that of OH-tremolite. We also show that F-bearing tremolite is depleted in fluorine after decomposition and that fluorine is lost as a volatile species such as HF gas. If tremolite ever formed on Venus, it would probably also contain fluorine. The exceptional stability of F-bearing tremolite strengthens our conclusions that if hydrous minerals ever formed on Venus, they could still be there today. Keywords: Venus; Venus, surface; Tremolite; Fluorine; Mineralogy

We present revised tremolite powder thermal decomposition kinetics using previous and newly acquired data from longer time (years instead of months) and lower temperature experiments (< 1073 K). We also present kinetic results for decomposition of millimeter- to centimeter-sized tremolite grains. Natural tremolite samples were heated at ambient pressure in flowing C[O.sub.2] or [N.sub.2] gas from 1023-1238 K. The tremolite decomposition products are a physical mixture of two pyroxene solid solutions (with the bulk composition [Dp.sub.59][En.sub.41]), a silica polymorph, and water vapor. Decomposition rates were calculated by using the mass loss of the heated samples. Tremolite crystals and crystalline powder decompositions follow different but related Avrami-Erofe'ev (nucleation and growth) kinetic models. The rate equations for thermal decomposition of tremolite crystalline powder and the larger crystal grains are [log.sub.10] [k.sub.powder] ([h.sup.-1]) = 18.69([+ or -] 0.19) - 23,845([+ or -] 833)/T and [log.sub.10] [k.sub.crystal] ([h.sup.-1]) = 19.82([+ or -] 0.07) - 25,670([+ or -] 916)/T. The associated apparent activation energies are 456([+ or -] 16) kJ [mol.sup.-1] and 491 ([+ or -] 18) kJ [mol.sup.-1], respectively. We propose a decomposition mechanism and suggest that decomposition and dehydroxylation occur simultaneously. The rate-limiting step is proposed to be structural rearrangement of the amphibole structure to the two pyroxenes and silica. This step and the overall decomposition rate are predicted to be independent of pressure from 1 to 100 bars. These kinetic analyses strengthen our previous conclusion (Johnson and Fegley, 2000, Icarus 146, 301-306) that if hydrous minerals, such as tremolite, formed on Venus during a wetter past, then these minerals could still exist at current conditions on Venus' surface today. Keywords: Venus; Mineralogy; Tremolite; Kinetics; Mechanism

Observations of the Io plasma torus and neutral clouds indicate that the extended ionian atmosphere must contain sodium, potassium, and chlorine in, atomic and/or molecular form. Models that consider sublimation of pure sulfur dioxide frost as the sole mechanism for generating an atmosphere on Io cannot explain the presence of alkali and halogen species in the atmosphere--active volcanoes or surface sputtering must also be considered, or the alkali and halide species must be discharged along with the S[O.sub.2] as the frost sublimates. To determine how volcanic outgassing can affect the chemistry of Io's atmosphere, we have developed a one-dimensional photochemical model in which active volcanoes release a rich suite of S-, O-, Na-, K-, and Cl-bearing vapor and in which photolysis, chemical reactions, condensation, and vertical eddy and molecular diffusion affect the subsequent evolution of the volcanic gases. Observations of Pele plume constituents, along with thermochemical equilibrium calculations of the composition of volcanic gases exsolved from high-temperature silicate magmas on Io, are used to constrain the composition of the volcanic vapor. We find that NaCl, Na, Cl, KCl, and K will be the dominant alkali and chlorine gases in atmospheres generated from Pele-like plume eruptions on Io. Although the relative abundances of these species will depend on uncertain model parameters and initial conditions, these five species remain dominant for a wide variety of realistic conditions. Other sodium and chlorine molecules such as NaS, NaO, [Na.sub.2], NA[S.sub.2], Na[O.sub.2], NaOS, NaS[O.sub.2], SCl, ClO, [Cl.sub.2], [S.sub.2]Cl, and S[O.sub.2][Cl.sub.2] will be only minor constituents in the ionian atmosphere because of their low volcanic emission rates and their efficient photochemical destruction mechanisms. Our modeling has implications for the general appearance, properties, and variability of the neutral sodium clouds and jets observed near Io. The neutral NaCl molecules present at high altitudes in atmospheres generated by active volcanoes might provide the Na[X.sup.+] ion needed to help explain the morphology of the high-velocity sodium 'stream' feature observed near Io. Key Words: Io; photochemistry; atmospheres, composition; volcanism; geochemistry; alkalis; halogens.

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus 141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science 288, 1208-1210) suggest that [S.sub.2] may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with S[O.sub.2] frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, [S.sub.2], [S.sub.3], [S.sub.4], SO, and [S.sub.2]O and a decreased abundance of O and [O.sub.2] in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/S[O.sub.2] ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO 'sticking' probabilities at Io's surface; in that case, the SO/S[O.sub.2] ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., [S.sub.2], [S.sub.3], [S.sub.4], and [S.sub.2]O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by [S.sub.4] radicals that are created and temporarily preserved when sulfur vapor (predominantly [S.sub.2]) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity--during periods when volcanic outgassing of [S.sub.2] (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased. Key Words: Io; photochemistry; atmospheres, composition; volcanism; geochemistry.

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 C[H.sub.4], CO, N[H.sub.3], [N.sub.2], and [H.sub.2]O 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 C[O.sub.2] 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 C[H.sub.2]O. 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. Key Words: Jupiter; giant planets; extrasolar planets; brown dwarfs; Gliese 229B; T dwarfs; L dwarfs; M dwarfs; atmospheric chemistry; carbon; nitrogen; oxygen; methane; carbon monoxide; ammonia; thermo-chemical equilibrium.

We use chemical equilibrium calculations to model [S.sub.2]O formation by high temperature thermochemical reactions in volcanic gases on Io. The calculations predict formation of 1-6% [S.sub.2]O gas at observed hot spot temperatures in S[O.sub.2]-[S.sub.2] gas mixtures at pressures of 1-100 bars in volcanoes on Io. The [S.sub.2]O abundance increases with increasing pressure (up to [approximately]6% at 100 bars) and is relatively insensitive to temperature and the bulk O/S ratio over wide ranges. Condensation of volcanic [S.sub.2]O provides a plausible explanation for the solid [S.sub.2]O that has been suggested to be present on Io's surface (e.g., around the Pele volcano). Key Words: Io; volcanic gases; Pele volcano; disulfur monoxide; sulfur oxides; sulfur dioxide; sulfur; geochemistry; volcanism.