Channon Visscher

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, PH 2, PH, and HCP) become increasingly important at high temperatures.

Quenching of Carbon Monoxide and Methane in the Atmospheres of Cool Brown Dwarfs and Hot Jupiters

We explore quench kinetics in the atmospheres of substellar objects using updated timescale arguments, as suggested by a thermochemical kinetics and diffusion model that transitions from the thermochemical-equilibrium regime in the deep atmosphere to a quench-chemical regime at higher altitudes. More specifically, we examine CO quench chemistry on the T dwarf Gliese 229B and CH4 quench chemistry on the hot-Jupiter HDxa0189733b. We describe a method for correctly calculating reverse rate coefficients for chemical reactions, discuss the predominant pathways for interconversion as indicated by the model, and demonstrate that a simple timescale approach can be used to accurately describe the behavior of quenched species when updated reaction kinetics and mixing-length-scale assumptions are used. Proper treatment of quench kinetics has important implications for estimates of molecular abundances and/or vertical mixing rates in the atmospheres of substellar objects. Our model results indicate significantly higher Kzz values than previously estimated near the CO quench level on Gliese 229B, whereas current-model-data comparisons using CH4 permit a wide range of Kzz values on HDxa0189733b. We also use updated reaction kinetics to revise previous estimates of the Jovian water abundance, based upon the observed abundance and chemical behavior of carbon monoxide. The CO chemical/observational constraint, along with Galileo entry probe data, suggests a water abundance of approximately 0.51-2.6 × solar (for a solar value of H2O/H2 = 9.61 × 10–4) in Jupiters troposphere, assuming vertical mixing from the deep atmosphere is the only source of tropospheric CO.

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 equilibrium 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.

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 Saturns deep atmosphere. A lower limit to the water abundance of H2O/H2 ≥ (1.7) × 10-3 is given by CO chemistry, whereas an upper limit of H2O/H2 ≤ (5.5) × 10-3 is given by PH3 chemistry. A combination of the CO and PH3 constraints indicates a water enrichment on Saturn of 1.9-6.1 times the solar system abundance (H2O/H2 = 8.96 × 10-4). The total oxygen abundance must be at least 1.7 times the solar system abundance (O/H2 = 1.16 × 10-3) in order for SiH4 to remain below the detection limit of SiH4/H2 < 2 × 10-10. A combination of the CO, PH3, and SiH4 constraints suggests that the total oxygen abundance on Saturn is 3.2-6.4 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 the solar system composition.

The deep water abundance on Jupiter: New constraints from thermochemical kinetics and diffusion modeling

Abstract We have developed a one-dimensional thermochemical kinetics and diffusion model for Jupiter’s atmosphere that accurately describes the transition from the thermochemical regime in the deep troposphere (where chemical equilibrium is established) to the quenched regime in the upper troposphere (where chemical equilibrium is disrupted). The model is used to calculate chemical abundances of tropospheric constituents and to identify important chemical pathways for CO–CH 4 interconversion in hydrogen-dominated atmospheres. In particular, the observed mole fraction and chemical behavior of CO is used to indirectly constrain the jovian water inventory. Our model can reproduce the observed tropospheric CO abundance provided that the water mole fraction lies in the range (0.25–6.0)xa0×xa010 −3 in Jupiter’s deep troposphere, corresponding to an enrichment of 0.3–7.3 times the protosolar abundance (assumed to be H 2 O/H 2 xa0=xa09.61xa0×xa010 −4 ). Our results suggest that Jupiter’s oxygen enrichment is roughly similar to that for carbon, nitrogen, and other heavy elements, and we conclude that formation scenarios that require very large (>8× solar) enrichments in water can be ruled out. We also evaluate and refine the simple time-constant arguments currently used to predict the quenched CO abundance on Jupiter, other giant planets, and brown dwarfs.

On the abundance of non-cometary HCN on Jupiter

Using one-dimensional thermochemical/photochemical kinetics and transport models, we examine the chemistry of nitrogen-bearing species in the Jovian troposphere in an attempt to explain the low observational upper limit for HCN. We track the dominant mechanisms for interconversion of N2-NH3 and HCN-NH3 in the deep, high-temperature troposphere and predict the rate-limiting step for the quenching of HCN at cooler tropospheric altitudes. Consistent with some other investigations that were based solely on time-scale arguments, our models suggest that transport-induced quenching of thermochemically derived HCN leads to very small predicted mole fractions of hydrogen cyanide in Jupiters upper troposphere. By the same token, photochemical production of HCN is ineffective in Jupiters troposphere: CH4-NH3 coupling is inhibited by the physical separation of the CH4 photolysis region in the upper stratosphere from the NH3 photolysis and condensation region in the troposphere, and C2H2-NH3 coupling is inhibited by the low tropospheric abundance of C2H2. The upper limits from infrared and submillimetre observations can be used to place constraints on the production of HCN and other species from lightning and thundershock sources.