The Deep Water Abundance on Jupiter: New Constraints from Thermochemical Kinetics and Diffusion Modeling

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-CH4 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) x 10^-3 in Jupiter's deep troposphere, corresponding to an enrichment of 0.3 to 7.3 times the protosolar abundance (assumed to be H2O/H2 = 9.61 x 10^-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 times 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.Comment: 42 pages, 7 figures, 4 tables, with note added in proof. Accepted for publication in Icarus [in press

A proposed chemical scheme for HCCO formation in cold dense clouds

The ketenyl radical (HCCO) has recently been discovered in two cold dense clouds with a non-negligible abundance of a few 10-11 (compared to H2). Until now, no chemical network has been able to reproduce this observation. We propose here a chemical scheme that can reproduce HCCO abundances together with HCO, H2CCO and CH3CHO in the dark clouds Lupus-1A and L486. The main formation pathway for HCCO is the OH + CCH → HCCO + H reaction as suggested by Agúndez et al. but with a much larger rate coefficient than used in current models. Since this reaction has never been studied experimentally or theoretically, this larger value is based on a comparison with other similar systems

Under what Conditions does (SiO)N Nucleation Occur? A Bottom-up Kinetic Modelling Evaluation

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable

An Experimental and Theoretical Study of the Reactions OIO + NO and OIO + OH

The kinetics of the reaction OIO + NO were studied by pulsed laser photolysis/time-resolved cavity ring-down spectroscopy, yielding k(235-320 K) = 7.6+4.0-3.1 × 10-13 exp[(607±128)/T] cm3 molecule-1 s-1. Quantum calculations on the OIO + NO potential-energy surface show that the reactants form a weakly bound OIONO intermediate, which then dissociates to the products IO + NO2. Rice-Ramsberger-Kassel-Markus (RRKM) calculations on this surface are in good accord with the experimental result. The most stable potential product, IONO2, cannot form because of the significant rearrangement of OIONO that would be required. The reaction OIO + OH was then investigated by quantum calculations of the relevant stationary points on its potential-energy surface. The very stable HOIO2 molecule can form by direct recombination, but the bimolecular reaction channels to HO2 + IO and HOI + O2 are closed because of significant energy barriers. RRKM calculations of the HOIO2 recombination rate coefficient yield krec,0 = 1.5 × 10-27 (T/300 K)-3.93 cm6 molecule-2 s-1, krec,8 = 5.5 × 10-10 exp(46/T) cm3 molecule-1 s-1, and Fc = 0.30. The rate coefficients of both reactions are fast enough around 290 K and 1 atm pressure for these reactions to play a potentially important role in the gas phase and aerosol chemistry in the marine boundary layer of the atmosphere