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

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 ν4 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° latitude bins from the south pole to 65°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 ∼1.5 µm 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 ∼30 times larger than the C6H6 ice mass mixing ratios for all observations. Several other unidentified signatures are observed near 687 and 702 cm−1 and possibly 695 cm−1, which could also be due to ice spectral signatures as they are observed in the deep stratosphere at pressure levels similar to the C6H6 ice ones. We could not reproduce these signatures with pure nitrile ices (HCN, HC3N, CH3CN, C2H5CN and C2N2) spectra available in the literature except the 695 cm−1 feature that could possibly be due to C2H3CN ice. From this tentative detection, we derive the corresponding C2H3CN ice mass mixing ratio profile and also inferred an upper limit of its gas volume mixing ratio of 2  ×  10−7 at 0.01 mbar at 79°S in March 2015.