Photochemical activity of Titan's low-altitude condensed haze
Titan, the largest moon of Saturn and similar to Earth in many aspects, has unique orange-yellow colour that comes from its atmospheric haze, whose formation and dynamics are far from well understood. Present models assume that Titan's tholin-like haze formation occurs high in atmosphere through gas-phase chemical reactions initiated by high-energy solar radiation. Here we address an important question: Is the lower atmosphere of Titan photochemically active or inert? We demonstrate that indeed tholin-like haze formation could occur on condensed aerosols throughout the atmospheric column of Titan. Detected in Titan's atmosphere, dicyanoacetylene (C_4N_2) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that C_4N_2 ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan's atmosphere, almost close to the surface.
Additional Information© 2013 Macmillan Publishers Limited. Received 26 June 2012; Accepted 26 February 2013; Published 03 April 2013. The Jet Propulsion Laboratory (JPL) part of the work is partly supported by several of the following funding sources: NASA Astrobiology Institute team 'Titan as a Prebiotic Chemical System', the Jet Propulsion Laboratory Director's Research and Development Fund and the JPL Research and Technology Development funding for the infrastructure of the Ice Spectroscopy Laboratory (ISL) and Titan organic aerosol spectroscopy and chemistry (TOAST) laboratory at JPL. The University of Provence part of the work was funded by the French national program Environnements Planétaires et Origines de la Vie (EPOV). This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Author contributions: M.S.G. and M.A. conceived the research ideas and wrote a significant part of the publication. M.S.G. coordinated the research activity, put the team together, involved in conducting the experiments, data analysis and data interpretation. R.J. and A.L. contributed to build infrastructure of the laboratory needed to synthesize the materials, conducted synthesis under the guidance of I.C. and majority of experiments described in this publication under the guidance of M.G. I.C. conducted the research in Marseille, France, as well as guided the synthesis and a part of photochemical investigations at JPL. R.J. performed the work as a NASA Post Doctoral Fellow at the Jet Propulsion Laboratory. A.L. performed a part of this work at the Jet Propulsion Laboratory during Academy of Finland fellowship.
Supplemental Material - ncomms2649-s1.pdf