Theoretical survey of the NH+CH3 potential energy surface in relation to Titan atmospheric chemistry

This paper looks at the possibilities opened by reaction of NH with methyl radical. A general survey of the potential surface by ab initio Möller-Plesset (MP2) and density functional theory has been performed, including determination of transition states on the pathways considered and vibrational analysis of all stationary points. Energetical data have been obtained using coupled cluster molecular orbital methods (CCSD(T)). It is shown that HCN is not formed from these starting reactants, but that CH₂NH, a possible precursor of tholins and of prebiotic compounds through hydration is the major product.     

Reactions of nitriles in ices relevant to Titan, comets, and the interstellar medium: formation of cyanate ion, ketenimines, and isonitriles

Motivated by detections of nitriles in Titan's atmosphere, cometary comae, and the interstellar medium, we report laboratory investigations of the low-temperature chemistry of acetonitrile, propionitrile, acrylonitrile, cyanoacetylene, and cyanogen (CH₃CN, CH₃CH₂CN, CH₂CHCN, HCCCN, and NCCN, respectively). A few experiments were also done on isobutyronitrile and trimethylacetonitrile ((CH₃)₂CHCN and (CH₃)₃CCN, respectively). Trends were sought, and found, in the photo- and radiation chemical products of these molecules at 12-25 K. In the absence of water, all of these molecules isomerized to isonitriles, and CH₃CN, CH₃CH₂CN, and (CH₃)₂CHCN also formed ketenimines. In the presence of H₂O, no isonitriles were detected but rather the cyanate ion (OCN⁻) was seen in all cases. Although isonitriles, ketenimines, and OCN⁻ were the main focus of our work, we also describe cases of hydrogen loss, to make smaller nitriles, and hydrogen addition (reduction), to make larger nitriles. HCN formation also was seen in most experiments. The results are presented in terms of nitrile ice chemistry on Titan, in cometary ice, and in the interstellar medium. Possible connections to prebiotic chemistry are briefly discussed.     

Key reactions in the photochemistry of hydrocarbons in Neptune's stratosphere

We studied the propagation of uncertainties carried by the reaction rate coefficients in the photochemistry of Neptune's stratosphere. We showed that the uncertainties on the mole fractions of main hydrocarbons are equal to or larger than the estimated uncertainties on abundances gathered from observations. From a global sensitivity analysis study, we determined a list of 26 key reactions and discussed the 7 main key reactions that should be studied in priority to lower the uncertainties in the mole fractions computed from a photochemical model. This methodology is essential to improve the predictivity of photochemical models and, consequently, to better understand the physical and chemical processes that govern the composition of giant planet atmospheres.     

The role of organic haze in Titan's atmospheric chemistry: II. Effect of heterogeneous reaction to the hydrogen budget and chemical composition of the atmosphere

One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186-200] in a one-dimensional photochemical model. Our results indicate that 60-75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C₄H₂ and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH₄ due to an episodic CH₄ release into Titan's atmosphere.     

Virial treatment of the speed of sound in cold, dense atmospheres and application to Titan

The speed of sound in a gas can be used to identify its composition, as has been done on the Earth. We show that, unlike in terrestrial applications, the third virial coefficient cannot be neglected in cold and dense atmospheres. We derive a model for the speed of sound of pure gases and gas mixtures at low temperatures and high pressures, based on the virial equation. After comparing the results of our model to measured data, we apply our model to the atmosphere of Titan. The difference between our third-order virial expansion and the commonly used second-order expansion is significant, showing that the third virial coefficient needs to be taken into account when accurate speed-of-sound measurements are used to derive atmospheric properties under Titan conditions.     

An experimental study of the reaction kinetics of C2(XΣg) with hydrocarbons (CH4, C2H2, C2H4, C2H6 and C3H8) over the temperature range 24-300 K: Implications for the atmospheres of Titan and the Giant Planets

The reactivity of C₂(X¹Σ⁺_g) with simple saturated (CH₄, C₂H₆ and C₃H₈) and unsaturated (C₂H₂ and C₂H₄) hydrocarbons has been studied in the gas phase over the temperature range 24-300 K using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique. All reactions have been found to be very rapid in this temperature range and the rate coefficients are typically ?10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ with the exception of methane for which the rate coefficient is one order of magnitude lower: ∼10⁻¹¹ cm³ molecule⁻¹ s⁻¹. These results have been analyzed in terms of potential destruction sources of C₂(X¹Σ⁺_g) in the atmospheres of Titan and the Giant Planets. It appears that the rate coefficient of the reaction ¹C₂ + CH₄ should be updated with our new data and that reactions with C₂H₂, C₂H₄ and C₂H₆ should also be included in the existing photochemical models.     

Sensitivity of the orbital content of a model stellar system to the potential approximation used to describe it

We have classified orbits in a stationary triaxial stellar system created from a cold dissipationless collapse of 100,000 particles. In order to integrate the orbits, two potential approximations with different fitting functions were used in turn. We found that the relative amount of chaotic versus regular orbits does depend on the chosen approximation of potential, even though both models resulted in very good fits of the underlying exact potential. On the other hand, the content of regular orbits, i.e., its distribution among main families, does not strongly depend of the potential approximation, being therefore a more robust signature of the gravitational system under study.