Comparative Experimental Investigation of Titan's Atmospheric Chemistry Driven by Solar EUV Radiation and Energetic Electron Precipitation

Details Comparative Experimental Investigation of Titan's Atmospheric Chemistry Driven by Solar EUV Radiation and Energetic Electron Precipitation Comparative Experimental Investigation of Titan's Atmospheric Chemistry Driven by Solar EUV Radiation and Energetic Electron Precipitation

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.3610i&link_type=abstract

American Astronomical Society, DPS meeting #42, #36.10; Bulletin of the American Astronomical Society, Vol. 42, p.1066

Astronomy and Astrophysics

Astronomy

Scientific paper

The observations by the Cassini Ion Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) clearly demonstrate the importance of complex organic chemistry in the upper atmosphere of Titan; a complex coupling of neutral and ion chemistry for organic aerosol generation induced by EUV photons and Saturn's magnetospheric charged particles. To understand the dominant energy source for aerosol formation and its formation chemistry, we comparatively investigate the chemical mechanism in N2/CH4 gas mixtures resulting from EUV-VUV synchrotron radiation (50-150 nm) and tunable mono-energetic electron beam irradiation (5 eV - 2000 eV). These excitation energy sources cover the dominant energy source available in Titan's upper atmosphere.
Our previous study of the EUV-VUV photolysis of N2/CH4 gas mixtures revealed the unique role of nitrogen photoionization in the catalytic formation of complex hydrocarbons and in the major nitrogen fixation process in Titan's upper atmosphere (Imanaka and Smith, 2007, 2009, 2010). However, relative roles of ion-molecule reactions and radical/neutral reactions in such complex chemistry remain to be determined. We characterized the electron energy distribution by conducting the Langmuir probe measurements. Degradation of the primary photoelectron from N2 photoionization at 20.6 eV photons is clearly observed, and the electron density rapidly decreases down to 109-10 cm-3, which suggests the complex coupling of ion-molecular reactions and dissociative ion-electron recombination reactions for the observed development of complex organic molecules.
The electron beam irradiation experiments at energy larger than 200 eV shows distinct gaseous product distribution with nitrogenated gaseous species from those with EUV irradiation products. The generation of secondary electrons and multiple inelastic collisions of fast electrons might increases the nitrogen fixation efficiency. The much less stringent spin selection rules could provide many additional chemical pathways not open to optical absorption processes.
We acknowledge support from the NASA grants NNX08AO13G, NNX09AM95G, NNX10AF08G, and the NAI program.

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