Titan’s Oxygen Chemistry and its Impact on Haze Formation

International audienceThough Titan’s atmosphere is reducing with its 98% N2, 2% CH4 and 0.1% H2, CO is the fourth most abundant molecule with a uniform mixing ratio of ∼50 ppm. Two other oxygen bearing molecules have also been observed in Titan’s atmosphere: CO2 and H2O, with a mixing ratio of ∼15 and ∼1 ppb, respectively. The physical and chemical processes that determine the abundances of these species on Titan have been at the centre of a long-standing debate as they place constraints on the origin and evolution of its atmosphere [1]. Moreover, laboratory experiments have shown that oxygen can be incorporated into complex molecules, some of which are building blocks of life [2]. Finally, the presence of CO modifies the production rate and size of tholins [3,4], which transposed to Titan’s haze may have some strong repercussions on the temperature structure and dynamics of the atmosphere.We present here our current understanding of Titan’s oxygen chemistry and of its impact on the chemical com- position of the haze. We base our discussion on a photochemical model that describes the first steps of the chemistry and on state-of-the-art laboratory experiments for the synthesis and chemical analysis of aerosol analogues. We used a very-high resolution mass spectrometer (LTQ-Orbitrap XL instrument) to characterize the soluble part of tholin samples generated from N2/CH4/CO mixtures at different mixing ratios and with two different laboratory set-ups. These composition measurements provide some understanding of the chemical mechanisms by which CO affects par- ticle formation and growth. Our final objective is to obtain a global picture of the fate and impact of oxygen on Titan, from its origin to prebiotic molecules to haze particles to material deposited on the surface