Titan's atmosphere simulation experiment using continuum UV‐VUV synchrotron radiation

Abstract

[1] A new reactor, named APSIS for Atmospheric Photochemistry Simulated by Synchrotron, is designed for simulating the reactivity occurring in planetary upper atmospheres. In this reactor, a gas mixture roughly reproducing Titan's main atmosphere composition (N 2 /CH 4 = 90/10) is irradiated by a continuous spectrum in the 60À350 nm range, provided by the DISCO beamline at the SOLEIL synchrotron radiation facility. This spectral range enables the dissociation and ionization of N 2 and CH 4 , as observed in plasma reactors and Titan's ionosphere. The neutral products are detected in situ by quadrupole mass spectrometry and collected with a cryogenic trap for ex situ analysis by gas chromatography-mass spectrometry. The detected reaction products include C2, C3, C4, and probably C5 organic compounds, with important amounts of nitrogen-bearing species: HCN, CH 3 CN, and C 2 N 2 . Neutral mass spectra obtained with APSIS are compared with Ion and Neutral Mass Spectrometer experiments of the Cassini space probe in the upper Titan atmosphere and with other results of current Titan atmosphere chemistry laboratory simulations.

Dissociative recombination exalts molecular growth in N2/CH4plasmas

“…"Titan" P /Pa [14,15], with a specific focus on synchrotron-based experimental setups designed to mimic Titan ionospheric chemistry [17,18], and notably the APSIS reactor [19]. Of particular interest is the role of ion chemistry on the formation of heavy N-bearing molecules in ionospheric conditions [16].…”

“…A photochemical-transport model has been designed to represent different approximations of the APSIS reactor [19], from 0D (1-cell model) to 2D (cylindrical symmetry). In order to be able to manage the computational cost of the probabilistic tree representation of dissociative recombination BR sets [11], we use here a 1-cell geometry.…”

“…The model assumes no transport and a uniform spatial distribution of all species, but takes into account the gas inflow (injection) and outflow (pumping) and the radiative transfer. Details of the model are presented elsewhere [14,15].…”

“…In both cases a 90:10 N 2 /CH 4 mixture is considered. The two scenarii differ in several parameters, as reported in Table. 1: (i) the synchrotron DISCO flux at SOLEIL synchrotron has a rather flat wavelength distribution, in contrast with the solar spectrum mainly peaked at Lyman- [19]; (ii) APSIS experiments are led at room temperature, whereas a reference temperature of 150 K is used to represent Titan's ionospheric conditions; and (iii) a large (10 6 ) ratio between the APSIS and "Titan" pressure conditions, which is the major source of difference in the products distributions and abundances [14,15].…”

“…The model implements the ion chemistry database established by Carrasco et al [24] and Plessis et al [11], involving C, H and N. Oxygen chemistry has been excluded because it does not play a significant role in APSIS photochemistry [19].…”