Titan's atmosphere evolution during the Cassini mission and comparison with the Voyager data

We have monitored Titan's seasonal evolution in the neutral atmosphere, during the Cassini-Huygens mission. We present here an analysis of nadir spectra acquired by Cassini/CIRS (Jennings et al., 2017, App. Opt. 56, 5274) at high resolution in the past years and describe the temperature and composition variations. During the first years of the mission, Titan's stratosphere was still enhanced in the North. The equatorial and mid-latitudes do not show a lot of variation. In 2010, we have returned to the same season as for the Voyager 1 encounter with Titan in 1980. We will discuss the data comparison between CIRS and V1/IRIS. Since 2010, with CIRS, we have observed at Titan's south pole a strong temperature decrease and the onset of a strong enhancement of several trace species such as complex hydrocarbons and nitriles (Coustenis et al. 2016,Icarus 270, 409 and references therein). This is due to the transition of Titan's seasons from northern winter in 2002 to summer in 2017 and the advent of winter in the south pole. During this transition period species with longer chemical lifetimes linger in the north undergoing slow photochemical destruction, while those with shorter lifetimes decrease and reappear in the south. An opposite effect was expected in the north, but was not observed until recently. From 2013 until 2016, the northern temperature increased by 10 K, while the south has shown a more significant decrease (up to 25 K). While the south polar region is continuously enhanced since about 2012, the chemical content in the north is finally showing a clear depletion for most molecules only since 2015 (Coustenis et al., 2018, submitted). This can set constraints on photochemical and GCM models.