Previous study of the vacuum UV photolysis of N2/CH4 gas mixtures indicates that photoionization of N2 by EUV radiation plays a major role in initiating the production of complex organic molecules such as benzene and toluene (Imanaka and Smith, 2007). In this study, we further attempt to understand the role of EUV-VUV light in the nitrogen fixation and haze formation mechanisms in the ionosphere of Titan.Gas mixtures of N2/CH4 in a windowless photocell are irradiated with EUV-VUV light (50 nm to 150 nm) using a synchrotron radiation source at the Advanced Light Source. Various 15N and 13C labeled gas mixtures are irradiated to assess the nitrogen incorporation chemistry and the range of nitrogenous products. Comparison of mass spectra of neutral gas products obtained from 15N2/CH4 and 14N2/CH4 gas mixtures reveals that CxHy chemistry proceeds at much faster rates than nitrogen incorporation chemistry and no significant nitrogeneous gaseous species are observed at UV wavelengths longer than 60 nm. The effective quantum yield of benzene (= produced benzene molecules/total absorbed photons by N2/CH4) at 60 nm is in the order of 10-3, which almost explains the benzene abundance of 1 ppm observed with the Cassini INMS (Wait et al., DPS 2006). This suggests that benzene production with EUV radiation is able to balance against the large photolysis destruction rate at wavelengths longer than 150 nm in Titan's ionosphere. Our laboratory detection of larger molecules, such as xylenes and naphthalene, as well as brownish solid aerosol products implies the production of larger PAHs and haze particles in Titan's ionosphere. We acknowledge support from the NASA Exobiology program (grant NNG05GO58G). Reference: Imanaka, H., Smith, M. A., Role of photoionization in the formation of complex organic molecules in Titan's upper atmosphere, Geophys. Res. Lett.,34, L02204, doi:10.1029/2006GL028317, 2007.
AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007