HCN production in Titan's Atmosphere: Coupling quantum chemistry and disequilibrium atmospheric modeling

Abstract

Hydrogen cyanide (HCN) is a critical reactive source of nitrogen for building key biomolecules relevant for the origin of life. Still, many HCN reactions remain uncharacterized by experiments and theory, and the complete picture of HCN production in planetary atmospheres is not fully understood. To improve this situation, we develop a novel technique making use of computational quantum chemistry, experimental data, and atmospheric numerical simulations. First, we use quantum chemistry simulations to explore the entire field of possible reactions for a list of primary species in N2-, CH4-, and H2-dominated atmospheres. In this process, we discover 33 new reactions with no previously known rate coefficients. From here, we develop a consistent reduced atmospheric hybrid chemical network (CRAHCN) containing experimental values when available, and our calculated rate coefficients otherwise. Next, we couple CRAHCN to a 1D chemical kinetic model (ChemKM) to compute the HCN abundance as a function of atmospheric depth on Titan. Our simulated atmospheric HCN profile agrees very well with the Cassini observations. CRAHCN contains 104 reactions however nearly all of the simulated atmospheric HCN profile can be obtained using a scaled down network of only 19 dominant reactions. From here, we form a complete picture of HCN chemistry in Titan's atmosphere, from the dissociation of the main atmospheric species, down to the direct production of HCN along 4 major channels. One of these channels was first discovered and characterized in Pearce et al. (2019) and this work.

23 pages (+41 supplementary pages), 5 figures, 5 tables (+11 supplementary tables), Accepted for publication in ApJ

References in corpus (15)

• Fingering convection and cloudless models for cool brown dwarf atmospheres
• A chemical model for the atmosphere of hot Jupiters
• VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
• The Effects of Consistent Chemical Kinetics Calculations on the Pressure-Temperature Profiles and Emission Spectra of Hot Jupiters
• Pseudo 2D chemical model of hot Jupiter atmospheres: application to HD 209458b and HD 189733b
• Coupled cluster theory in materials science
• Hydrogen Cyanide in Nitrogen-Rich Atmospheres of Rocky Exoplanets
• New chemical scheme for studying carbon-rich exoplanet atmospheres
• A new astrobiological model of the atmosphere of Titan
• On the Composition of Young, Directly Imaged Giant Planets
• Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres
• From cold to hot irradiated gaseous exoplanets: Fingerprints of chemical disequilibrium in atmospheric spectra
• A reduced chemical scheme for modelling warm to hot hydrogen-dominated atmospheres
• A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients
• Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b II. Mapping the effects of gas kinetics

Cited by in corpus (8)

• The Composition and Chemistry of Titan's Atmosphere
• Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds
• An experimental and theoretical investigation of HCN production in the Hadean Earth atmosphere
• Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations
• CRAHCN-O: A Consistent Reduced Atmospheric Hybrid Chemical Network Oxygen Extension for Hydrogen Cyanide and Formaldehyde Chemistry in CO2-, N2-, H2O-, CH4-, and H2-Dominated Atmospheres
• Towards Prebiotic Chemistry on Titan: Impact experiments on organic haze particles
• Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres
• Deep Mantle-Atmosphere Coupling and Carbonaceous Bombardment: Options for Biomolecule Formation on an Oxidized Early Earth