Chemical structure and optical properties of Titan's tholins. Implications for the analysis of spectral data of Cassini-Huygens mission

Abstract : Laboratory experiments have been frequently carried out to simulate Titan s atmospheric chemistry by irradiating initial mixtures of gases present in Titan s atmosphere using photon excitation or plasma discharges Solid brownish particles - often named tholins are produced and are used as analogues of Titan s aerosols The more efficient devices for the tholins production are cold plasma discharges The chemical structure of these compounds was up to now not elucidated as well as the identification of the parameters which do control their optical properties We present a new study which provides new clues on the chemical structure of these compounds We discuss some implication for the interpretation of spectral data of Titan and other icy satellites possibly covered by complex organics Tholins produced by two different plasma low pressure cold plasma discharges are studied a Radio Frequency discharge in Service d A e ronomie Verri e res-le-buisson - France and a Direct Current discharge in LISA Cr e teil France Samples produced by these two plasmas are analysed by UV Raman spectroscopy High Resolution Transmission Electron Microscopy infrared spectroscopy and their reflectance in the visible IR range are measured Our study shows that the apparent colour varies from dark brown to yellow when the methane percentage increases All samples are amorphous no polymeric structures They exhibit similar sp 2 structures this latter being the key factor controlling the optical properties in the visible and near infrared spectral