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We report the detection of a spectral signature observed at 682 cm$^{-1}$ bythe Cassini Composite Infrared Spectrometer (CIRS) in nadir and limb geometryobservations of Titan's southern stratospheric polar region in the middle ofsouthern fall, while stratospheric temperatures are the coldest since thebeginning of the Cassini mission. The 682 cm$^{-1}$ signature, which is onlyobserved below an altitude of 300-km, is at least partly attributed to thebenzene (C$_6$H$_6$) ice $\nu_{4}$ C-H bending mode. While we first observed itin CIRS nadir spectra of the southern polar region in early 2013, we focus hereon the study of nadir data acquired in May 2013, which have a more favorableobservation geometry. We derived the C$_6$H$_6$ ice mass mixing ratio in5{\deg}S latitude bins from the south pole to 65{\deg}S and infer theC$_6$H$_6$ cloud top altitude to be located deeper with increasing distancefrom the pole. We additionally analyzed limb data acquired in March 2015, whichwere the first limb dataset available after the May 2013 nadir observation, inorder to infer a vertical profile of its mass mixing ratio in the 0.1 - 1 mbarregion (250 - 170 km). We derive an upper limit of $\sim$1.5 $\mu$m for theequivalent radius of pure C$_6$H$_6$ ice particles from the shape of theobserved emission band. Several other unidentified signatures are observed near687 and 702 cm$^{-1}$ and possibly 695 cm$^{-1}$, which could also be due toice spectral signatures as they are observed in the deep stratosphere atpressure levels similar to the C$_6$H$_6$ ice ones. We could not reproducethese signatures with pure nitrile ice (HCN, HC$_3$N,CH$_3$CN, C$_2$H$_5$CN andC$_2$N$_2$) spectra available in the literature except the 695 cm$^{-1}$feature that could possibly be due to C$_2$H$_3$CN ice.

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