Evolution and search for new molecules in Titan's stratosphere from Cassini CIRS observations

International audienceWe observe the onset and enhancement at Titan's south pole in several trace species, such as complex hydrocarbons like C6H6 and nitriles like HC3N, observed only at high northern latitudes before equinox. We analyze Cassini/CIRS nadir spectra taken from 2012 to 2015 at high resolution at several latitudes from 70°S to 70°N after the Southern Autumnal Equinox [1-4]. In the more recent dates, most molecules show dramatic increases in the south. The 70°S and 50°S or mid-latitudes show different behavior demonstrating that they are subject to different dynamical processes in and out of the polar vortex region. For most species, we find higher abundances at 50°N compared to 50°S, with the exception of C3H8, CO2, C6H6 and HC3N, which arrive at similar mixing ratios after mid-2013 [3,4]. While the 70°N data show generally no change with a trend rather to a small decrease for most species within 2014, the 70°S results indicate a strong enhancement in trace stratospheric gases after 2012. In particular, HC3N, HCN and C6H6 have increased by 3 orders of magnitude over the past 3-4 years while other molecules, including C2H4, C3H4 and C4H2, have increased less sharply (by 1-2 orders of magnitude). This is a strong indication of the rapid and sudden buildup of the gaseous inventory in the southern stratosphere during 2013-2014, as expected as the pole moves deeper into winter shadow. Subsidence gases that accumulate in the absence of ultraviolet sunlight, evidently increased quickly since 2012 and some of them may be responsible also for the reported haze decrease in the north and its appearance in the south at the same time [5]. We also look for the appearance of new molecules (complex hydrocarbons and nitriles) in large averages of CIRS spectra, based on model predictions.References[1] Coustenis, et al., Icarus 207, 461, 2010 ; [2] Bampasidis et al., ApJ 760, 144, 8 p., 2012; [3] Coustenis et al., Astrophys. J. 799, 177, 9p, 2013 ; [4] Coustenis, A., et al., Icarus 270, 409, 2016; [5] Jennings, D. E., et al., ApJ Lett., 804, L34, 2015