Titan's Organic Residue from Benzene Ices: Role of Atmospheric Pressure

Benzene has been detected in Titan's atmosphere, the Saturn's largest moon, and it is known to condense when it reaches the stratosphere to form ice, which is submitted to low ultraviolet (UV) radiation. In a previous work, we put in evidence that the irradiation of benzene ice at lambda > 230 nm induces the formation of volatile photoproducts, in particular, fulvene (a benzene isomer) and a solid residue. In this work, we study the influence of pressure on the residue formation after benzene ice's irradiation and its characterization using infrared (IR), Raman, and gas chromatography-mass spectrometry (GC-MS) methods. We show that the residue obtained during the increase of temperature performed at 1 mbar is dominated by CH sp(2) groups, whereas the one obtained at 10(-6) mbar contains an equivalent proportion of CH sp(2) and sp(3). Polyphenyls with up to three rings are unambiguously identified by GC-MS in all of the residues formed.