Study of Titan's fall southern stratospheric polar cloud composition with Cassini/CIRS: Detection of benzene ice

摘要

We report the detection of a spectral signature observed at 682 cm (-1) by the Cassini Composite Infrared Spectrometer (CIRS) in nadir and limb geometry observations of Titan's southern stratospheric polar region in the middle of southern fall, while stratospheric temperatures are the coldest since the beginning of the Cassini mission In the same period, many gases observed in CIRS spectra (C2H2, HCN, C4H2, C3H4, HC3N and C6H6) are highly enriched in the stratosphere at high southern latitude due to the air subsidence of the global atmospheric circulation and some of these molecules condense at much higher altitude than usually observed for other latitudes The 682 cm(-1) signature, which is only observed below an altitude of 300 km, is at least partly attributed to the benzene (C6H6) ice nu(4) C-H bending mode While we first observed it in CIRS nadir spectra of the southern polar region in early 2013, we focus here on the study of nadir data acquired in May 2013, which have a more favorable observation geometry We derived the C6H6 ice mass mixing ratio in 5 degrees latitude bins from the south pole to 65 degrees S and infer the C6H6 cloud top altitude to be located deeper with increasing distance from the pole We additionally analyzed limb data acquired in March 2015, which were the first limb dataset available after the May 2013 nadir observation, in order to infer a vertical profile of its mass mixing ratio in the 0.1-1 mbar region (250-170 km) We derive an upper limit of similar to 15 mu m for the equivalent radius of pure C6H6 ice particles from the shape of the observed emission band, which is consistent with our estimation of the ice particle size from condensation growth and sedimentation timescales We compared the ice mass mixing ratio with the haze mass mixing ratio inferred in the same region from the continuum emission of CIRS spectra, and derived that the haze mass mixing ratios are similar to 30 times larger than the C6H6 ice mass mixing ratios for all