Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

E Hébrard,K M Hickson,J C Loison,M Dobrijevic,A Bergeat,F Caralp

doi:10.1051/0004-6361/201220686

Abstract

The description of C3 hydrocarbon chemistry in current photochemical models of Titan’s atmosphere is found to be far from complete. We have carefully investigated the photochemistry involving C3 Hp compounds in the atmosphere of Titan (considering both photolysis and neutral reactions), which considerably impacts the abundances of many other hydrocarbon species (including C2 compounds). Model results indicate that three species (C3 , c-C3 H2 and C3 H3 ) could be abundant enough to be present in the Cassini /INMS data. Because the error bars on predicted C3 -hydrocarbon abundances are considerably larger than those of the observational data, new experimental and theoretical studies targeting the measurement of low-temperature reaction rate constants and product branching ratios are required to reduce current model uncertainties. In particular, we highlight 30 “key reactions”, the uncertainty factors of which should be lowered to improve the quality of photochemical models involving C3 Hp molecules.

Highlights

The latest results obtained by the Cassini-Huygens mission confirmed that the dissociation and ionization of methane of CH4 and nitrogen N2 in the high stratosphere of Titan initiates a complex photochemistry of hydrocarbons and nitrogen compounds through molecule-molecule and ion-molecule reactions, which leads to the production of CnHp hydrocarbons
Model results indicate that three species (C3, Cyclic C3H2 (c−C3H2) and C3H3) could be abundant enough to be present in the Cassini/Ion and Neutral Mass Spectrometer (INMS) data
Because the error bars on predicted C3-hydrocarbon abundances are considerably larger than those of the observational data, new experimental and theoretical studies targeting the measurement of low-temperature reaction rate constants and product branching ratios are required to reduce current model uncertainties

Summary

Introduction

The latest results obtained by the Cassini-Huygens mission confirmed that the dissociation and ionization of methane of CH4 and nitrogen N2 in the high stratosphere of Titan initiates a complex photochemistry of hydrocarbons and nitrogen compounds through molecule-molecule and ion-molecule reactions, which leads to the production of CnHp hydrocarbons (up to n = 6 at least; the heaviest hydrocarbon clearly detected is benzene, see for instance Brown et al 2009, for a comprehensive review of our current knowledge about Titan) The modeling of such an atmospheric system is limited by the lack of kinetic and photolytic data at low temperature (T ∈ [100, 150] K) especially for heavy hydrocarbons. Hébrard et al (2012) updated the chemical scheme of C2-hydrocarbons to provide a solid basis for the study of HCN and HNC neutral productions In this previous model, we modified various rate constants and branching ratios for reactions involving C and CH. This is for example the case for benzene C6H6, which has been detected in abundance with the INMS instrument, the production of which is assumed to involve ion chemistry, and notably dissociative recombinations (Westlake et al 2012; Plessis et al 2012)

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