ABSOLUTE ABSORPTION-COEFFICIENT OF C6H2 IN THE MID-UV RANGE AT LOW-TEMPERATURE - IMPLICATIONS FOR THE INTERPRETATION OF TITAN ATMOSPHERIC SPECTRA

The interpretation of mid-UV albedo spectra of planetary atmospheres, especially that of Titan, is the main goal of the SIPAT (Spectroscopie uv d'Interet Prebiologique dans l'Atmosphere de Titan) research progr am. This laboratory experiment has been developed in order to systemat ically determine the absorption coefficients of molecular compounds wh ich are potential absorbers of scattered sunlight in planetary atmosph eres, with high spectral resolution, and at various temperatures below room temperature. From photochemical modelling and experimental simul ations, we may expect triacetylene (C6H2) to be present in the atmosph ere of Titan, even though it has not yet been detected. We present her e the first determination of the absolute absorption coefficient of th at compound in the 200-300 nm range and at two temperatures (296 K and 233 K). The temperature dependence of the C6H2 absorption coefficient in that wavelength range is compared to that previously observed in t he case of cyanoacetylene (HC3N). We then discuss the implications of the present results for the interpretation of Titan UV spectra, where it appears that large uncertainties can be introduced either by the pr esence of trace impurities in laboratory samples or by the variations of absorption coefficients with temperature.