title: Laboratory studies of molecular growth in the Titan ionosphere.
abstract: Experimental simulations of the initial steps of the ion-molecule reactions occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Italy. The measurements consisted of irradiating gas mixtures with a monochromatic photon beam, from the methane ionization threshold at 12.6 eV, up to and beyond the molecular nitrogen dissociative ionization threshold at 24.3 eV. Three gas mixtures of increasing complexity were used: N(2)/CH(4) (0.96/0.04), N(2)/CH(4)/C(2)H(2) (0.96/0.04/0.001), and N(2)/CH(4)/C(2)H(2)/C(2)H(4) (0.96/0.04/0.001/0.001). The resulting ions were detected with a high-resolution (1 T) FT-ICR mass spectrometer as a function of time and VUV photon energy. In order to interpret the experimental results, a Titan ionospheric model was adapted to the laboratory conditions. This model had previously allowed the identification of the ions detected in the Titan upper atmosphere by the ion neutral mass spectrometer (INMS) onboard the Cassini spacecraft. Comparison between observed and modeled ion densities validates the kinetic model (reactions, rate constants, product branching ratios) for the primary steps of molecular growth. It also reveals differences that we attribute to an intense surface chemistry. This result implies that heterogeneous chemistry on aerosols might efficiently produce HCN and NH(3) in the Titan upper atmosphere.
authors.affiliation: Laboratoire de Planétologie de Grenoble, CNRS, Université J Fourier, UMR 5109, Grenoble, France.
authors.forename: Roland
authors.initials: R
authors.lastname: Thissen
authors.forename: Veronique
authors.initials: V
authors.lastname: Vuitton
authors.forename: Panayotis
authors.initials: P
authors.lastname: Lavvas
authors.forename: Joel
authors.initials: J
authors.lastname: Lemaire
authors.forename: Christophe
authors.initials: C
authors.lastname: Dehon
authors.forename: Odile
authors.initials: O
authors.lastname: Dutuit
authors.forename: Mark A
authors.initials: MA
authors.lastname: Smith
authors.forename: Stefano
authors.initials: S
authors.lastname: Turchini
authors.forename: Daniele
authors.initials: D
authors.lastname: Catone
authors.forename: Roger V
authors.initials: RV
authors.lastname: Yelle
authors.forename: Pascal
authors.initials: P
authors.lastname: Pernot
authors.forename: Arpad
authors.initials: A
authors.lastname: Somogyi
authors.forename: Marcello
authors.initials: M
authors.lastname: Coreno